Research and analysis

Working paper 4. Physical activity and exposure to air pollution

Published 18 March 2025

Summary

The behavioural advice that accompanies the current Daily Air Quality Index (DAQI) indicates, depending on the air pollution band, whether individuals should “consider reducing”, “reduce” or “avoid” strenuous physical exertion. One of the considerations of the Committee on the Medical Effects of Air Pollutants’ (COMEAP) (Air Quality Information System (AQIS) Sub-group is whether this advice regarding physical activity is still appropriate.

The available evidence indicates that, for most people, the long-term benefits of regular physical activity outweigh the possible risks of experiencing adverse symptoms from undertaking physical activity during short-term periods of elevated air pollution. A loss of the benefits of physical activity, for both the general population and susceptible groups, is one of the possible unintended consequences of the current DAQI advice. Advice could therefore focus on reducing personal exposure to outdoor air pollution, rather than on discouraging physical activity. Specific advice, during a pollution episode, on where and when air pollution is most likely to be high would help individuals reduce exposure.

Effects of exercising during elevated air pollution are likely to vary between individuals. Information that enables individuals to respond appropriately for their individual circumstances might be more appropriate than providing the same, directive, advice to everyone. This could include advice for individuals to monitor their symptoms and to respond according to plans developed with their clinicians.

Evidence suggests that health outcomes may be affected for at least a week after short-term exposure to air pollution. Therefore, advising individuals to monitor their symptoms for several days following exposure is appropriate.

Other considerations relevant to the current advice include the potential impacts of indoor air pollution, if individuals are encouraged to avoid outdoor air pollution, and other environmental triggers such as pollen, if they choose to conduct physical activity in green spaces.

For mass participation sporting events, organisers should be advised to raise awareness of the potential health impacts of short-term exposure to air pollution during air pollution episodes. This would enable participants to make informed choices regarding participation, based on their individual circumstances, and allow them to monitor symptoms.

Introduction

The DAQI is used by Defra to provide information to the public about current and forecast levels of air pollution. Depending on whether pollution is regarded as “Low”, “Moderate”, “High” or “Very High”, accompanying advice indicates, whether (and how) individuals should consider changing their behaviour to avoid the likelihood of experiencing adverse health effects. The current advice in the DAQI focuses on the level of physical activity or exertion, particularly outdoors. Depending on the pollution band, people are advised to “consider reducing”, “reduce” or “avoid” strenuous physical exertion. Separate advice is provided for those regarded as being particularly ‘at-risk’ of experiencing adverse health effects following short-term exposure to elevated levels of pollution and for the general population. The advice for those who are potentially at-risk is more precautionary than for the general population (see Annexe A).

The current DAQI is based on recommendations made by COMEAP during its last review of the AQI (COMEAP, 2011). As part of the current project to review the AQIS, it was considered important to review the behavioural and health advice that accompanies the DAQI, to determine whether it reflects current evidence and understanding. As the current advice largely concerns reducing physical activity outdoors when pollution levels are elevated, the evidence relating to the effects of physical activity when air pollution is elevated is relevant.

Early discussions of the AQIS Steering Group highlighted the need to avoid potential unintended health consequences of the DAQI advice. The UK public health community is concerned about low levels of physical activity in the UK, and related consequences for health. In the UK, it is recommended that adults undertake at least 150 minutes of moderate-intensity or 75 minutes of vigorous-intensity activity each week. Daily physical activity or planned exercise is encouraged, as opposed to infrequent, long sessions (NHS, 2021a). It is estimated that physical inactivity costs the UK £7.4 billion annually and is associated with 1 in 6 deaths (Office for Health Improvement & Disparities, 2022).

Physiological effects of physical activity

The immediate response to physical activity is increased breathing rate and tidal volume, increased heart rate and stroke volume (the volume of blood pumped out of the left ventricle of the heart during each cardiac contraction) (Burton and co-authors, 2004; Cerqueira and co-authors, 2019), and an inflammatory response if the intensity of exercise is moderate to high (Brown and co-authors, 2015).

Importantly, this acute inflammation should not always be interpreted as a negative response to physical activity, as inflammation is a very broad, multifactorial concept. In healthy individuals, an inflammatory response to physical activity is associated with an adaptive and protective upregulation in anti-inflammatory and antioxidant defences and is short-lived (Docherty and co-authors, 2022). However, some evidence suggests that this adaptive capacity is compromised in those with pre-existing conditions (such as COPD) and in aging (Mercado and co-authors, 2015; Tu and co-authors, 2019; Zhang and co-authors, 2015) meaning that the inflammation might remain unresolved.

Moderate or vigorous physical activities, such as those named in the NHS Physical Activity Guidelines (NHS 2021a; b), will result in transient increases in inflammatory markers. These increases are seen as beneficial (Brown and co-authors, 2015), and are associated with increased metabolism of glucose and lipids (Pedersen and co-authors, 2003; Glund and Krook, 2008) and enhanced cell signalling and blood flow (Butterfield and co-authors, 2006), for example. The same inflammatory markers are often involved in both muscle and lung tissue physiology and pathophysiology, and have been found to be increased following pollution exposure. Given some systemic inflammatory responses to physical activity and air pollution may present similarly, differentiating what either physical activity or air pollution are separately responsible for is not straightforward.

The long-term benefits of regular physical activity will typically include a reduction in chronic inflammation (Beavers and co-authors, 2010), as observed in a variety of healthy adult (Monteiro-Junior and co-authors, 2018; Rose and co-authors, 2021) and chronic condition (You and co-authors, 2013) groups. Other important, general benefits include weight management and reduced risk of a range of health conditions including, but not limited to, cardiovascular disease, type 2 diabetes, cancer, osteoarthritis, mental health conditions, and cognitive impairment (Garcia and co-authors, 2023).

Some acute benefits of physical activity, such as bronchodilation, are short-lived and would stop shortly after physical activity (Haverkamp and Mickleborough, 2016). The long-term benefits of physical activity are a result of short-term habitual behaviours and an accumulation of the beneficial effects of physical activity sessions. As well as physiological benefits, physical activity has both immediate and longer-term benefits for mental health (Herbert and co-authors, 2020; Singh and co-authors, 2023; World Health Organization, 2022). For some individuals, not undertaking physical activity for a day or two might have very detrimental effects on their health and wellbeing.

Consideration of definitions of physical activity

Within this report the term physical activity is intended to cover the broad variety of activities that may contribute towards national recommendations for daily or weekly physical activity levels for children and adults (NHS, 2021a, b). In this report, “physical activity” or “exercise” is based on the likely physiological demand of the activity or exercise being performed. When reviewing advice for the DAQI, it may be important to consider how physical activity is characterised. The NHS defines moderate physical activity as being able to talk, but not sing, when exercising. In contrast, vigorous activity is characterised by breathing hard and fast, and not being able to say more than a few words whilst exercising (NHS, 2021a).

We intend to be inclusive regarding the varied modes of activity or exercise named in existing guidelines. However, activities that would ordinarily be performed in an indoor environment (for example, swimming or spinning classes) will not be as relevant, as the focus of this report is on ambient outdoor environments. Otherwise, this report is intended to align with existing national guidelines for physical activity.

Current DAQI advice is to reduce or avoid strenuous physical activity during elevated air pollution (this differs between at-risk individuals and the general population). In this report, we are considering “strenuous” (as defined in the DAQI) and “vigorous” (as defined in NHS advice), to be interchangeable terms. What constitutes light activity, moderate activity and strenuous or vigorous activity will differ between individuals, and also between the general population and individuals that may be classified as ‘at-risk’, and people may interpret these terms differently (NHS, 2021a). Other definitions of physical activity also include a consideration of heart rate levels; however, this requires people to know their heart rate, which they may not be able to monitor in real-time, and might complicate guidance.

Tainio and co-authors (2021) suggest that more research is needed on how to tailor communication and engagement on the relations between air pollution and physical activity for different target audiences. Advice may need to differ to accommodate for different physical ability, fitness levels, and differences in duration, intensity and frequency of exercise.

Questions addressed by the Sub-group

In this working paper, the COMEAP AQIS Sub-group addresses several questions by examining the evidence on air pollution and physical activity.

The research questions the Sub-group considered were:

1. To what extent does the available evidence support the current DAQI advice to reduce or avoid physical activity, particularly outdoors, when air pollution concentrations are elevated:
   • For the general population?
   • For susceptible groups?
2. What are the possible unintended consequences of the current advice?
3. How should these factors be taken into account when considering whether or how the current advice regarding physical activity should be revised or refined?
4. Is there specific advice that could/should be provided to those that regularly achieve more than the recommended minimum amount of physical activity or to organisers of public sporting events?

In this working paper, the Sub-group has drawn on a report on Physical Activity and Air Pollution by the Centre for Environmental Health and Sustainability from the University of Leicester, which was commissioned by UKHSA to inform discussions of the AQIS Steering Group (Panayi and co-authors, 2023). This working paper also reviews evidence relevant to assessing the appropriateness of advice not to exercise, or to reduce exercise, outdoors during short-term air pollution episodes. A narrative review of the literature was conducted using PubMed. Keywords: “physical activity” or “exercise” and “air pollution” and “children” or “elderly” or “older people”; all years and publication types were included. This working paper also discusses other aspects relevant to refining the advice associated with the DAQI. This includes mention of indoor air quality (as some people might choose to exercise indoors if pollution outdoors is elevated) and advice relevant to public participation in sports events. The Committee’s focus is advising on public health. Therefore, evidence relevant to athletic performance, or advice relevant to elite competitions or professional athletes, was not considered.

To what extent does the available evidence support the current DAQI advice to reduce or avoid physical activity, particularly outdoors?

The University of Leicester Centre for Environmental Health and Sustainability report on physical activity and exposure to air pollution

The University of Leicester Centre for Environmental Health and Sustainability (CEHS) conducted a review on physical activity and exposure to air pollution, which was commissioned by UKHSA to inform discussions by the AQIS Steering Group (Panayi and co-authors, 2023).

The review identified only a small number of studies (DeMeo and co-authors, 2004; Giles and co-authors, 2018; Kubesch and co-authors, 2015a; Kubesch and co-authors, 2015b; Matt and co-authors, 2016; Marmett and co-authors, 2022; Nwokoro and co-authors, 2012) that looked at differences in physiological biomarkers while engaged in physical activity, compared to when undertaking no physical activity, in the same environmental conditions. Investigating the responses to rest and physical activity in the same environmental conditions is valuable for gaining a more valid insight into the combined impact of physical activity and air pollution exposure. The authors summarise that the number of participants included in most of these studies was between 18 to 30 participants, but one study included 120 participants. Participants in 3 studies were in the 18 to 60 years age range and the other 4 studies included adults between 20 to 40 years old. Physiological endpoints examined included pulmonary and systemic inflammatory responses, blood pressure, forced expiratory volume (FEV), peak expiratory flow (PEF), oxygen saturation, self-reported throat and chest symptoms, and biomarkers and inflammatory markers including airway macrophage carbon and TNF-a, respectively. Only one of these studies (DeMeo and co-authors, 2004) included individuals who would be regarded as being in susceptible groups (COPD, asthma, cardiovascular conditions (heart attack, angina, heart failure, hypertension)); the other 6 studies were in healthy individuals. The report summarised the findings of these studies as indicating that, compared to participants at rest, physical activity had beneficial effects on pulmonary function and attenuated a reported increase in systolic blood pressure associated with short-term exposure to traffic-related air pollution. One study, which examined exposure to black carbon (a constituent of particulate matter (PM) associated with combustion sources) found an increase in short-term airway inflammation markers associated with high intensity physical activity (cycling vs a non-cycling control group).

Due to the limited number of studies identified that included a control group (undertaking no physical activity in the same environmental conditions), a further 17 studies were included that did not have a control group. Out of these 17 studies, 10 examined effects in healthy individuals (Liu and co-authors, 2015; Weichenthal and co-authors, 2014; Rundell and co-authors, 2007; Bos and co-authors, 2011; Frampton and co-authors, 2015; Strak and co-authors, 2009; Brauner and co-authors, 2008; Cruz and co-authors, 2022; Tainio and co-authors, 2016; Pasqua and co-authors, 2018). The report summarises that most of these studies suggested that exposure to particulate matter whilst undertaking physical activity was associated with adverse effects.

The report concludes that studies of this type that examined physiological effects in those with chronic diseases (asthma, COPD, ischaemic heart disease, patients fitted with an implanted cardioverter defibrillator, coronary artery disease) (McCreanor and co-authors, 2007; Sinharay and co-authors, 2018; Syed and co-authors, 2021; Liu and co-authors, 2019; Mills and co-authors, 2007; Pekkanen and co-authors, 2002; Lanki and co-authors, 2006), found that exercising in higher air pollution concentrations could have greater adverse effects than in healthy individuals. Nonetheless, it was noted by the report’s authors that the evidence is limited, and results are mixed. They also point out that it is unclear whether the effects seen were due to a combination of exposure to air pollution and physical activity, or just air pollution exposure, as there is no control group not undertaking physical activity.

The studies examined in the report were conducted in a range of locations with different pollutant concentrations. It is not possible to compare these concentrations directly with the DAQI bands, because of the different averaging times. Nonetheless, in order to give an indication of the likely relevance to concentrations experienced in the UK, the authors of the CEHS report have indicated how the concentrations in the studies compare with the DAQI pollution bands. This can be found in Figure 1. Although concentrations in many of the studies were undertaken in air pollution concentrations corresponding to ‘High’ or ‘Very High’ in the DAQI, concentrations in a similar number of studies corresponded to ‘Low’ or ‘Moderate’ bands.

Figure 1. Studies included in the review conducted by The University of Leicester Centre for Environmental Health and Sustainability (Panayi and co-authors, 2023; Panayi and co-authors, in preparation)

Image reproduced with permission of authors

Figure 1 explanatory text

A table containing a list of studies included in the review conducted by The University of Leicester Centre for Environmental Health and Sustainability. The table also contains a list of pollutants that have been included within each study. Coloured boxes show the comparison of the DAQI pollution bands (1, air pollution is low; 10, air pollution is very high) with the concentrations reported in the studies. Ticks and crosses represent whether pollutants are associated with adverse health effects by each study.

Most of the studies identified by the literature search focused on particulate matter, with very few studies available for nitrogen dioxide and ozone. Therefore, the authors also conducted an informal review of some chamber studies examining the effect of air pollution (ozone, particles, sulphur dioxide, nitrogen dioxide) exposure during physical activity (Mudway and Kelly, 2004; Delvin and co-authors, 2012; Frampton and co-authors, 2015; Kim and co-authors, 2011; Gong and co-authors, 2003; 2004; 2005). The findings of these suggest that there are adverse health effects of air pollution whilst undertaking physical activity. However, most studies used very high levels of air pollution. They may therefore not be directly applicable to the DAQI, although these findings do emphasise the association of inhaled dose and magnitude of the physiologic response.

To add context to their review, the authors also considered concentrations of pollutants indoors relative to outdoor concentrations. The limited number of studies examined suggested that exercising indoors could lead to higher exposure to air pollution, depending on room ventilation and particulate matter resuspension. It was noted that pollutants in the indoor air could come both from the indoor sources and from penetration of outdoor pollutants. It was recognised that further examination was needed on the indoor/outdoor air pollution relationship and physical activity.

DeFlorio-Barker and co-authors (2020)

A recent review by DeFlorio-Barker and co-authors (2020) examined the acute effects of short-term exposure to air pollution while being physically active. The review found 25 studies from 2000 to 2020, which included scenarios: at rest in clean air, physical activity in clean air, at rest in polluted air, and physical activity in polluted air. Studies were mainly based in North America and Europe, these include: 6 studies from the United States, 5 in Canada, 4 in Spain, 3 in Denmark, 2 in the United Kingdom and 2 studies were conducted in 3 different European cities (Antwerp, Belgium; Barcelona, Spain; London, United Kingdom).The articles included a range of health endpoints, including lung function, platelet activation, blood pressure, heart rate variability, brain-derived neurotrophic factor, inflammation, cardiac stress test measurements, oxidative stress-induced DNA damage, vascular effects including microvascular function, blood-gas permeability, and endothelial function.

9 out of 25 articles identified evidence of a statistical interaction between air pollution exposure and physical activity (DeFlorio-Barker and co-authors, 2020). The review indicated that physical activity was generally beneficial for healthy adults. However, the authors suggested that, particularly among those with pre-existing conditions, and even in low levels of air pollution, low-intensity activities (such as walking) may intensify the negative impacts of air pollution. However, it should be noted that this review included only 3 studies examining susceptible groups (Bennett and co-authors, 2016; Girardot and co-authors, 2006; Sinharay and co-authors, 2018) and only one of these found an interaction between air pollution and physical activity (Sinharay and co-authors, 2018). Pollutant levels, physical activity intensity, and the population studied appeared to influence the statistical interaction (DeFlorio-Barker and co-authors, 2020).

Hung and co-authors (2021)

Hung and co-authors (2021) conducted a review on the acute effects of exercising in air pollution. Randomised Controlled Trials exploring the acute health- or exercise-related markers of short-term air pollution exposure, during moderate to vigorous physical activity were eligible for inclusion. Studies examining healthy individuals were considered, and physical activity was defined as exercising for at least half of the duration of exposure and exercising at an average intensity equivalent to 3 or more metabolic equivalents of task, as defined in the American College of Sport Medicine’s definition of moderate-intensity physical activity. The authors used the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach to assess the certainty of evidence. The health endpoints assessed were pulmonary function, symptom responses, cardiovascular function, cognitive function, systemic inflammation and exercise response.

The majority of evidence was related to ozone; out of a total of 53 studies, 34 studies examined the effects of ozone. Annexe B provides Figure 1 of the review, which shows harvest plots of the association for the different pollutants, which demonstrates the weight of the evidence for ozone. It was found that ozone exposure during moderate-to-vigorous physical activity has an adverse effect on pulmonary function (Annexe B), but health effects for exposure to other pollutants is uncertain due to the limited evidence.

The REVIHAAP project

A review of evidence on the health aspects of air pollution, ‘the REVIHAAP project’ (WHO, 2013), answered 24 questions relevant to reviewing European policies on air pollution, and addressing the health aspects of these policies. This included some consideration of effects of short-term exposure to air pollution. It was found that evidence from experimental studies suggests that ozone causes transient decrements in lung function (McDonnell and co-authors, 2012; Schelegle and co-authors, 2012 cited in WHO, 2013). However, experimental studies of ozone highlight the considerable individual variation in responses to ozone (McDonnell and co-authors, 2012; Schelegle and co-authors, 2012 cited in WHO, 2013). It is important, though, to recognise that ozone is part of a complex photochemical mixture of air pollutants that people are exposed to.

Potentially susceptible groups

This section of the report focuses on ‘at-risk’ groups identified in the current DAQI. The Sub-group has considered other potential ‘at-risk’ groups that may need to be included in an air quality information system in working paper 1. The Sub-group examined additional evidence for children and older people, as little evidence was examined for these in the report by Leicester University. Two recent reviews (one focusing on children and young people, and the other on older people) are discussed below. These studies on children and older people add to the evidence base but effects are mixed.

Comment on the report by The University of Leicester Centre for Environmental Health and Sustainability

The report on exercise and physical activity by CEHS looked at studies that examine the effect of air pollution and physical activity on specific susceptible groups, including those with cardiovascular and respiratory health conditions. However, evidence was limited, with only 8 studies looking at physical activity and air pollution in susceptible groups. Of these studies, 7 out of the 8 studies did not have a control group (no non-physical activity group). Little evidence was examined for other susceptibility factors considered in the current DAQI, for example, children with respiratory conditions such as asthma, or older people.

Children

DeFlorio-Barker and co-authors (2022) conducted a systematic review of 10 studies which examined effects in children and adolescents less than 21 years of age. Five studies were based in China, 3 based in the United States, and 1 each in Indonesia and Germany. The articles included a range of endpoints including insulin resistance, fasting plasma glucose, metabolic syndrome, obesity, cardiopulmonary fitness, executive function, methylation of the promotor for Forkhead Box P3 (FOXP3) (a protein involved in immune system responses), airway inflammation and incident asthma. They found mixed evidence in children undertaking physical activity in air pollution, with adverse, apparently protective and null effects reported.

As the DAQI focuses on short-term air pollution concentrations, further detail on the studies reviewed by DeFlorio-Barker and co-authors (2022) which used short-term air pollution averages in the analysis of health effects, is discussed here. A number of studies using long-term pollution averages were also included in this review, further detail of these studies has been provided in Annexe C.

Two studies looked at short-term air pollution exposure. Lovinsky-Desir and co-authors (2016) examined airway inflammation while children were wearing devices that measured black carbon during at least 60 minutes moderate to vigorous activity. They found that higher exposure to black carbon attenuated the protective effect of physical activity on airway inflammation. Conversely, Lovinsky-Desir and co-authors (2017) identified physical activity was associated with lower FOXP3 promotor methylation under conditions of high exposure to black carbon. Reduced FOXP3 promoter methylation was associated with higher lung function. Children in this study were wearing monitoring devices during at least 60 minutes moderate to vigorous activity.

Older people

For older people, a review by D’Oliveira and co-authors (2023) found 21 studies that examined the impact of air pollution on the health of older adults during physical activity and sedentary behaviour. Older people were defined as 60 years or older. A total of 235,074 people participated in the studies, ranging from 60 to 97 years, 53% were women and 47% were men. The health endpoints assessed were heart disease, COPD, depression, and cognitive decline. They found mixed evidence; in 11 studies, they found the beneficial effects of physical activity were attenuated by exposure to air pollution, or exposure to air pollution negatively impacted the health of older adults. However, in the other 10 studies, the effects of physical activity were greater than the negative effects of air pollutants in older adults. The selected studies were worldwide, including the United Kingdom (2 studies), Brazil (1 study), Australia (1 study), China (8 studies), United States (5 studies), Chile (1 study), South Korea (2 studies), and Japan (2 studies). However, most studies were based in China, and therefore may not be directly applicable to UK conditions, given the different environmental climates and air pollution levels.

As the DAQI focuses on short-term air pollution concentrations, further detail on the studies reviewed by D’Oliveira and co-authors (2023), which used short-term air pollution averages in the analysis of health effects, are discussed here. A number of studies using long-term pollution averages were included in this review, further detail of these studies has been provided in Annexe C.

Sinharay and co-authors (2018) measured shortness of breath, cough, sputum, wheezing, sweat, exhaled nitric oxide, Forced Expiratory Volume (FEV1) and Forced Vital Capacity (FVC), and pulse wave velocity in adults 60 years and over with COPD, ischaemic heart disease or no disease. They found that participants with COPD reported more cough, sputum, shortness of breath, and wheeze when exposed to short-term levels of air pollution. All adults, irrespective of disease status, had increased lung function and decreased pulse wave velocity when walking in an area away from air pollution. This suggests that short-term exposure to air pollution attenuated the beneficial cardiopulmonary benefits of walking in adults aged 60 years and older. PM_2.5, PM₁₀, ultrafine particles and black carbon were measured using handheld aerosol monitors, NO₂ was measured using stationary monitoring sites.

In Brazil, Cassilhas and co-authors (2022) examined the effect of short-term exposure to particulate matter, measured using a handheld monitoring device, while groups of volunteers aged 60 or older exercised in indoor and outdoor environments. There were 48 participants in total, split into 4 groups, control indoor and outdoor, exercise indoor and outdoor. They found that participants were exposed to higher levels of particles in the outdoor environment, and that exercise indoors promoted better blood glucose control and increases in brain-derived neurotrophic factor levels.

Delfino and co-authors (2010) found that exposure to air pollutants was associated with increased ambulatory blood pressure in older people (64 participants) aged 65 and over, that had a history of coronary heart disease. Ambulatory blood pressure was taken at every waking hour (14 hours per day), real-time activity was measured using actigraphs and participants were asked to fill out diaries to report perceived physical exertion. Short-term (hourly) air pollution measurements of PM_2.5, O₃, CO, NOx, and organic carbon were taken at the home address. A positive association was found between 1 to 8-hour average air pollution and blood pressure. The association was stronger when moderate to strenuous physical exertion was reported but there was no association when actigraphs reported higher activity.

Gold and co-authors (2005) evaluated effects on older people (24 participants, 61 to 88 years) with a tendency to develop ST-segment depression. Participants were monitored during a protocol of 5 minutes rest, 5 minutes of standing, 5 minutes of exercise outdoors, including one climb on an incline, 5 minutes of recovery and 20 cycles of paced breathing (coached by a technician). Air pollution measurements (PM_2.5, black carbon, carbon monoxide) were recorded at a central site within 0.5km of the home address of the subjects, which was on a high-traffic street. SO₂, O₃ and NO₂ measurements were obtained from state monitoring sites. For the statistical analyses, the ST-segment level was treated as a continuous outcome for each part of the protocol, therefore participants were included whose mean ST-segment values were negative at least twice for that part of the protocol. The study found mean black carbon level in the previous 12 hours, and the black carbon level 5 hours before testing predicted ST-segment depression in most portions of the protocol. During post-exercise rest, an elevated black carbon level was associated with −0.1 mm ST-segment depression in the continuous models. Elevated black carbon also predicted increased risk of ST-segment depression of at least 0.5 mm among those with at least one episode of that level of ST-segment depression (Gold and co-authors, 2005).

In a study involving 12 healthy non-smoking men and women (60 to 79 years), the effect of O₃ was examined on pulmonary function using 4 experiments. First, a 1-hour continuous exercise protocol, second, a 2-hour intermittent exercise protocol, each performed while exposed to filtered air, and to 0.45 ppm O₃, resulting in different effective doses of O₃ (Drechsler-Parks and co-authors, 1990). It was found that concentration of O₃ was the most important factor of the 3 components of the effective dose (O₃ concentration, mean minute ventilation, and duration of exposure). The authors noted that the results are consistent with studies on younger adults (Drechsler-Parks and co-authors, 1990).

Health advice for physical activity in susceptible groups

When developing advice on physical activity and air pollution, it is important to consider the existing limitations and barriers to exercise of those in susceptible groups, regardless of air pollution exposure. This should include a consideration of what physical activity is manageable for those with chronic health conditions, and the possible benefits physical activity may bring to disease management.

Therefore, as part of this working paper, general health advice (without consideration of air pollution) for physical activity for ‘at-risk’ individuals (as defined in the current DAQI) has been examined. Internet searches (Google) using the terms “advice”, “physical activity” and either “respiratory disease”, “cardiovascular disease”, “children” or “older people” were conducted to identify general health advice for these groups. From these searches, information was collected from specific sources, including non-governmental organisations and NHS websites. These websites were the focus, in the light of findings from commissioned public insight research by Verian from the AQIS project. This had found that research panel participants, looking at sources of air quality information, had some preferred sources for health-related information. Preferred messengers included the NHS and charities such as Asthma and Lung UK, and the British Heart Foundation.

Respiratory conditions

General advice for those with a lung condition is to keep active. Advice is centred around living and managing a lung condition and promotes physical activity to increase strength of the muscles around the lung and other general health benefits (Asthma and Lung UK, 2023). This advice also provides information on types of physical activity for aerobic health, muscle strengthening, flexibility, and how to increase movement safely with a lung condition. This includes ensuring that people using a reliever inhaler have it with them when exercising, and encouragement of consulting respiratory nurses if oxygen is used. Advice from Asthma and Lung UK considers pollen and mould (relevant to indoor air) as triggers (Asthma and Lung UK, 2023).

The NHS provides general information on asthma, and promotes regular exercise (NHS, 2021c). Furthermore, some hospital trusts explain the ‘cycle of inactivity’ in pulmonary rehabilitation leaflets to promote physical activity in those with respiratory conditions. Advice also recommends to stop exercising and seek medical attention if individuals feel they have a sudden shortness of breath, breathing does not go back to normal after physical activity, the chest feels tight or heavy, there is pain that spreads to the arms, back, neck and jaw, or feel sick or are being sick (West Suffolk NHS Foundation Trust, 2020; Kent Community Hospital Foundation Trust, 2021; Calderdale and Huddersfield NHS Foundation Trust, undated).

Cardiovascular conditions

Similarly, the British Heart Foundation encourages physical activity for those with cardiovascular conditions. However, the advice recommends that individuals should speak to their doctor before physical activity is undertaken, and gives some factors that should be taken into consideration (certain heart conditions, medications, pacemakers, and weather). Much of the advice centres around specific types of physical activity, rehabilitation after surgery and how to exercise safely if you have symptoms (British Heart Foundation, 2023). The World Heart Federation also provides points on physical activity for people with various severities of cardiovascular disease including cardiovascular disease (CVD) risk factors, people living with heart disease and cardiac rehabilitation (World Heart Federation, 2023).

Some hospital trusts also offer advice on exercising with cardiovascular conditions, explaining that, for many, physical activity is a high-benefit/low-risk intervention (Guy’s and St Thomas’ Hospitals, 2021). Advice is given to avoid some physical activity that pose particular risk for some conditions, for example patients with enlarged aortas should avoid heavy straining such as weightlifting, or those on medications that reduce clotting ability should avoid activities that carry a high risk of injuries that might cause bleeding (Guy’s and St Thomas’ Hospitals, 2021).

Children and Older People

Children with lung problems and older people are also mentioned as ‘at-risk’ in the DAQI (further information can be found in working paper 1). As for adults with asthma, children are encouraged to receive guidance on inhaler use, and to learn techniques for breathing (Guy’s and St Thomas’ Hospital Trust, 2022).

Current health advice accompanying the DAQI states that children need not be kept from school or prevented from taking part in games. It explains that children with asthma may notice that they need to increase their use of reliever medication on days when levels of air pollution are higher than average. Accompanying health advice on the short-term effects of air pollution with the DAQI can be found in Annexe D.

The NHS also recommends that older adults do some type of physical activity every day, including aerobic activities and activities that improve strength, balance, flexibility, and to reduce time spent sitting or lying down (NHS, 2021b).

Advice for exercising outdoors

In all cases, advice that promotes physical activity for ‘at-risk’ groups promotes moderate exercise and gives examples of exercises outdoors, including, gardening, walking to the shops (instead of taking a bus or car), sporting events such as parkrun™ or participating in (typically) outdoor team sports.

Section summary

Currently, the DAQI recommends reducing and avoiding physical activity when air pollution is elevated. However, the report conducted by CEHS found that undertaking moderate physical activity in polluted environments had beneficial effects on pulmonary function in healthy individuals compared to no physical activity, in the 7 studies that included a control group undertaking no physical activity (Panayi, and co-authors, 2023). Studies without control groups suggested that short-term exposure to traffic pollution may offset some of the short-term cardiopulmonary benefits of physical activity, with some evidence that those in susceptible groups may experience larger adverse effects. The DAQI mentions children with asthma as a susceptible group, the review discussed above (DeFlorio-Barker and co-authors, 2022) is not informative about children with asthma, though evidence for children in general is mixed. Additional evidence from a review by D’Oliveira and co-authors (2023) on older people is also mixed.

Moreover, advice from specialist support networks for those that may be more susceptible to air pollution, encourages physical activity to promote management of the condition and a healthy lifestyle. Some evidence suggests that those with a condition may already experience fear or anxiety around physical activity, due to symptoms such as breathlessness and physical sensations associated with exertion (Farris and co-authors, 2019; McCormack and co-authors, 2021).

What are the possible unintended consequences of the current advice?

In this section, we discuss some possible unintended consequences that should be considered when developing behavioural and public health advice for air pollution episodes.

Providing advice to reduce or avoid physical activity in air pollution may confuse some individuals in groups that are encouraged to exercise, or worsen fears of exercising altogether. Children with asthma are identified by the DAQI as a susceptible group, and consideration should be given to the need to avoid advice which risks stigmatising these children.

Providing advice to reduce physical activity, not only for susceptible groups but also the general population, could lead to people exercising less often, which could have detrimental impacts on long-term health and wellbeing. As well as benefits for physical health, physical activity also has beneficial effects on mental health (Sharma and co-authors, 2006), and therefore advice to reduce physical activity would need to carefully consider the potential detrimental impacts on mental health and wellbeing too.

Mitigation methods to reduce air pollution exposure include avoiding locations, or times, when outdoor pollution is particularly high. However, choosing to exercise in an area where there are fewer obvious sources of anthropogenic air pollution (for example, vehicles) might not always be beneficial, if levels of pollen or ozone are higher (for example, in rural areas or green spaces). Similarly, telling people to avoid or reduce exercising outdoors when air pollution is elevated could encourage them to stay or exercise indoors. This could expose them to indoor pollutants, either from indoor sources or penetration of outdoor pollutants.

Indoor air quality

If individuals might prefer to exercise indoors during an outdoor air pollution episode, consideration should be given to indoor air quality and thermal comfort. Buildings have different types of ventilation. The extent of infiltration of a pollutant will depend upon factors such as the building’s air permeability (uncontrolled ingress of outdoor air indoors through gaps and cracks in the building envelope) and ventilation (controlled air exchange through windows and ventilation systems) (AQEG, 2022). Where only natural ventilation is available, closing windows will reduce air pollution ingress from outdoors. Where mechanical ventilation, portable air cleaners or air conditioning with filtration are available, these can remove particles from the outdoor air to some extent. However, the extent to which particles are removed depends upon the efficiency and the maintenance of the filter (how frequently it is changed) (Sublett and co-authors, 2010). In the case of a prolonged air pollution episode, the indoor concentrations arising from infiltration would be expected to increase over the duration of the episode, to follow eventually the variation of the outdoor levels but at a lower concentration, depending on the physical characteristics of the pollutants.

People may choose to exercise at home, which could lead to exposure to multiple indoor sources of air pollution (for example, see Figure 2 below). Individuals should be aware of and mitigate these, if possible.

Figure 2. Some sources and types of air pollutants within homes

Image reproduced with permission from Royal College of Physicians (Every breath we take: the lifelong impact of air pollution. Report of a working party. London: RCP, 2016.)

Figure 2 explanatory text

A house with labelled sources of indoor air pollutants associated with different rooms within the house.

Bedroom: dust and dust mites, bacteria and viruses, pet dander, VOCs from personal care products.

Bathroom: Mould and mildew, bacteria, VOCs and other chemicals from cleaning products.

Kitchen: CO, NO₂ and particulates from gas cookers or stoves, VOCs from household cleaning products.

Attic: Man-made mineral fibres, asbestos, formaldehyde, dust.

Living areas: Radon from soil/bedrock, CO and NO₂ from fires and wood-burning stoves, VOCs and formaldehyde from carpets, paints, glues, furniture and air fresheners, tobacco smoke, pet dander.

Garage: CO from car exhaust, mould and mildew, VOCs from stored paints and solvents, pesticides and herbicides.

Both the National Institute for Health and Care Excellence (NICE, 2020) and The Royal College of Paediatrics and Child Health (RCPCH) and Royal College of Physicians (RCP) (RCPCH/RCP, 2020) have made recommendations for reducing or mitigating against air pollution in homes. Both provide specific guidance for those more susceptible to the health impacts of indoor air pollution exposure including people who have pre-existing conditions (including asthma, other respiratory and cardiovascular conditions), pregnant women, those who have recently given birth, young children and older people. Advice from NICE (2020) is outlined in Figure 3 below.

Figure 3. Advice on Improving Indoor Air Quality

Image reproduced with permissions from NICE.

Figure 3 explanatory text

A NICE (2020) 2-page visual summary for indoor air quality at home (first page presented here).

Text with actions for local authorities and actions for healthcare professionals relating to advice on improving indoor air quality. Actions for local authorities include providing advice to reduce damp and mould and increase ventilation. Actions for health care professionals include providing advice for people with breathing or heart problems, advice for people allergic to house dust mites, advice for pregnant women and babies under 12 months.

While this advice mainly relates to exposure in the home, it is also important to consider sources of indoor air pollution in indoor exercise environments, such as gyms and sports halls.

It is recognised that there is currently little evidence on indoor air quality in gyms, although air pollutants are present in these environments. Examples include volatile organic compounds from cleaning products, and resuspension of particulate matter due to increased movement (Finewax and co-authors, 2020; Szulc and co-authors, 2023; Slezakova and co-authors, 2019). Occupancy levels may also affect pollutant concentrations (Slezakova and co-authors, 2018; Peixoto and co-authors, 2023). Microplastics are also present in indoor environments, which could include gyms, but the toxicological effect of these particles is unknown (Allen and co-authors, 2022; Kacprzak and Tijing, 2022). However, work on understanding these risks is ongoing, WHO have outlined the research required to improve exposure assessment for microplastics (WHO, 2022a) and the Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment (COT) have produced a statement examining the potential risks from exposure to microplastics (COT, 2024). COMEAP are also working on a statement on the current state of science available to inform an assessment of the health risks from the inhalation of microplastics.

Nevertheless, it is important to ensure that people choosing gyms to meet their physical activity needs are not discouraged from using them, as this could cause a barrier to meeting the recommended physical activity targets. However, from an equity and accessibility standpoint, it may be pertinent to avoid specifically recommending using gyms or indoor sports environments in DAQI advice, as not everyone can afford gym or sports facility membership costs.

Consideration of the lagged effects of short-term exposure to air pollution in relation to physical activity advice

In a separate working paper (working paper 2), the Sub-group has looked at the lagged effects of short-term exposure to air pollution. The current literature indicates that adverse health impacts from short-term exposure to air pollution can persist for at least a week after the day of the elevated air pollution event.

If the role of physical activity is primarily viewed as increasing exposure to air pollution (by increasing the volume of air inhaled) then it would be necessary to reduce activity only during the period when air pollution is elevated. However, if the physiological effects of physical activity after exposure increased the risk of experiencing lagged health effects, then it might be appropriate to advise a reduction in physical activity during the following days. 

Some clinical effects of exposure take several days to manifest. In people with asthma, bronchospasm is a fairly immediate response to exposure. In contrast, inflammatory responses leading to asthma exacerbations develop over a period of days. Therefore, careful monitoring of asthma control should continue for a period of time after exposure. Peak flow monitoring could be useful to identify early warning signs before an individual becomes symptomatic.

The need to change physical activity behaviour might be very dependent on individual characteristics. Therefore, we suggest that public guidance should focus on individuals monitoring their own symptoms and adapting their behaviour if necessary. The type of physical activity might be important and it might be more appropriate to encourage changing the type of physical activity to reduce risk, rather than reducing physical activity altogether.

Importantly, if susceptible individuals are advised to consider behavioural modifications, for not just the day of high pollution but also the week following, it will be important to ensure that the impact on other aspects of daily life is considered to be proportionate and manageable for high-risk individuals.

Is there specific advice that could/should be provided to those that regularly achieve more than the recommended physical activity target and organisers of public sporting events?

Ensuring individuals are aware of air pollution (including after an air pollution event has passed), and to monitor their symptoms is particularly relevant for those that regularly achieve more than the minimum recommended physical activity target, as they could be more exposed to air pollution. These individuals may have increased exposure when training at high intensity, or for extended duration, in environments with elevated air pollution levels. In addition, there is a high prevalence of asthma among endurance athletes (Rasmussen and co-authors 2022 a, b). At the time of writing, the minimum target is 150 minutes of moderate-intensity physical activity per week for adults (NHS, 2021a), upwards of 300 minutes of moderate-intensity physical activity per week for adults is considered above average (WHO, 2022b).

Research suggests air pollution exposure advice may need to be provided to organisers of outdoor mass sporting events, for example, marathons and triathlons (Tainio and co-authors, 2021). Environmental factors may need to be considered for sporting events, for example, temperature and air pollution. Clear communication of the potential risks, allowing individuals to make informed decisions on whether they should take part, is needed. Some advice on pollution for event organisers is available, for example the ‘Pollution Protocol’ by Clean Air in London. Among other recommendations, this advises event organisers to issue updates and health advice for runners and spectators when air pollution is ‘Moderate’ (as defined in the DAQI) and increase alerts when air pollution is ‘High’ (as defined in the DAQI). In some countries, for example Canada, there is consideration of sporting events in the Air Quality Health Index within the additional advice pages of the index. Advice includes sports event organisers to consider reducing intensity or rescheduling the event if the air pollution risk is moderate or high (as defined in the Canadian Air Quality Health Index) (Government of Canada, 2016). Consideration of the type of physical activity is needed, both in terms of how individuals could be exposed to environmental pollution and how the particular physical activity impacts the body (Rundell, 2011).

Evidence on lag structures (see working paper 2) suggest that adverse health impacts of exposure to short-term air pollution persist for over 24 hours. There may need to be a consideration of this factor in communication to sporting event participants, prior to a sporting event, so that participants with pre-existing conditions can decide whether or not they should take part, alter the management of their disease, or monitor their symptoms.

Conclusions

In this working paper, the Sub-group have examined evidence (a report from CEHS on Physical Activity and exposure to air pollution, and some additional evidence and guidance) relevant to addressing the following questions:

1. To what extent does the available evidence support the current DAQI advice to reduce or avoid physical activity, particularly outdoors, when air pollution concentrations are elevated:
   • For the general population?
   • For susceptible groups?
2. What are the possible unintended consequences of the current advice?

The available evidence indicates that, for most people, the long-term benefits of regular physical activity outweigh the possible risks of experiencing adverse symptoms from undertaking physical activity during short-term periods of elevated air pollution. Nonetheless, some people, particularly those with chronic cardiovascular or respiratory conditions, could experience adverse effects. 

Currently, DAQI advice recommends reducing or avoiding physical activity when exposure is elevated. However, general health advice for susceptible groups often encourages individuals to partake in physical activity, in order to maintain a healthy lifestyle and manage their condition.

Potentially unintended consequences of current DAQI advice are that it may discourage individuals to exercise, which may be more detrimental to health than exposure to air pollution at the levels usually found in the UK. Avoiding obvious sources of anthropogenic air pollution (for example, vehicles) might not always be beneficial, if levels of pollen or ozone are higher in alternative spaces, for example, green spaces and rural areas. Moreover, people may respond to the advice by deciding to exercise indoors; as such, it is important to consider factors that may affect indoor air pollution levels. 

Evidence on lag effects (see working paper 2) suggests the impact of exposure to short-term air pollution may affect health outcomes several days after the exposure. However, the need to change exercise behaviour may be very dependent on individual characteristics, and behavioural modifications (considering possible lagged effects) need to be proportionate and manageable.

Recommendations

Our recommendations are intended to address questions 3 and 4 below.

Question 3: how should these factors be taken into account when considering whether or how the current advice regarding physical activity should be revised or refined?

Question 4: is there specific advice that could/should be provided to those that regularly achieve more than the recommended minimum amount of physical activity or to organisers of public sporting events?

Evidence suggests that the long-term benefits of regular physical activity outweigh the possible risks of exercising during short-term periods of elevated air pollution (hours or days), for most people. Therefore, individuals should not be discouraged from exercising; instead, advice could be provided on reducing personal exposure to outdoor air pollution by adjusting regular habits and behaviours to avoid travelling through areas, or during periods of the day, where and when air pollution levels are likely to be higher.

So that the potential health benefits associated with physical activity are not sacrificed, at-risk individuals concerned about their symptoms may wish to consider adapting their level of physical activity, rather than avoiding physical activity. To overcome some of the possible unintended consequences of the current advice, and to align better with generic health advice for susceptible groups, it may be more appropriate to advise people to monitor their symptoms on days when air pollution is elevated, and respond in accordance with their clinician’s advice, rather than advising people to reduce their physical activity.

Advice should also refer to other triggers and environmental exposures (for example, indoor air pollution and pollen) which may exacerbate symptoms, to avoid possible unintended consequences. Specific advice on where and when air pollution is most likely to be elevated would help individuals reduce exposure.

Advice may also need to consider that short-term exposure (hours or days) to air pollution could affect health outcomes several days after exposure. This could include advice to individuals that are more susceptible to air pollution to monitor their symptoms during physical activity, and for at least a week after physical activity, and adjust management of their condition in accordance with their clinician’s advice.

For those who regularly achieve more than the minimum recommended physical activity target per week, advice could focus on the timing, intensity and location of the physical activity. For example, advice could recommend performing at least some of their planned high intensity physical activity in an environment with lower concentrations of air pollution, or on days when lower concentrations are expected.

For mass participation sporting events, advice should raise awareness of the potential health impacts of short-term exposure (hours or days) to air pollution. This would enable sporting event participants to make informed choices on whether they should participate based on their individual circumstances. This advice may also need to take in consideration possible lagged effects of air pollution so that individuals can monitor symptoms after the event.

COMEAP’s role has been to consider the scientific evidence and how this might inform the advice that should be given. Further consideration, and research, will be needed regarding how best to word and communicate the advice to the various target audiences.

Recommendations for research

As a result of this working paper, a number of recommendations for further research have been identified.

Future research looking at the effects of physical activity during air pollution episodes needs to assess defined clinical outcomes (for example, PEF, reliever inhaler usage and exacerbations in people with asthma). Many of the studies within this working paper examine surrogate endpoints (for example, inflammation markers) and therefore are not of direct clinical relevance.

While it is acknowledged that they can be resource intensive, the inclusion of multiple control groups in future human experimental studies would help the interpretation of their findings. For example, a group undertaking physical activity in polluted air, a control group undertaking physical activity but not exposed to polluted air, a control group not undertaking physical activity and not exposed to polluted air, and a control group exposed to polluted air but not undertaking physical activity.

Further research into the relationship between acute air pollution-induced responses and physical activity is needed, which focuses on dosimetry in vulnerable groups and an understanding of underlying physiological, immunological, antioxidant and xenobiotic responses in children, older people, and in individuals with chronic respiratory and cardiovascular conditions.

Further research is needed to determine the level of exposure to indoor air pollutants in gyms and indoor sporting environments and, the effect of physical activity on the health effects of exposure to indoor air pollutants. General advice on reducing indoor air pollution has been highlighted in this document; this mainly relates to the home. Some literature has been identified that examines indoor air quality in gyms and other indoor exercise environments; however, it is recognised that this area of research is relatively unexplored.

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Annexe A. Current advice accompanying the UK Daily Air Quality Index (DAQI)

Air pollution banding	Value	Accompanying health messages for at-risk individuals [note 1]	Accompanying health messages for the general population
Low	1 - 3	Enjoy your usual outdoor activities	Enjoy your usual outdoor activities
Moderate	4 - 6	Adults and children with lung problems, and adults with heart problems, who experience symptoms should consider reducing strenuous physical activity, particularly outdoors.	Enjoy your usual physical activities.
High	7 - 9	Adults and children with lung problems, and adults with heart problems, should reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms. People with asthma may find they need to use their reliever inhaler more often. Older people should also reduce physical exertion.	Anyone experiencing discomfort such as sore eyes, cough or sore throat should consider reducing activity, particularly outdoors.
Very high	10	Adults and children with lung problems, adults with heart problems and older people, should avoid strenuous physical activity. People with asthma may find they need to use their reliever inhaler more often.	Reduce physical exertion, particularly outdoors, especially if you experience symptoms such as cough or sore throat.

Note 1: Adults and children with heart or lung problems are at greater risk of symptoms. Follow your doctor’s usual advice about exercising and managing your condition. It is possible that very sensitive individuals may experience health effects even on ‘Low’ air pollution days. Anyone experiencing symptoms should follow the guidance provided.

Table from the Daily Air Quality Index - Defra, UK (Accessed 3 November 2023).

Annexe B: Figure 1 reproduced from Hung and co-authors (2021)

Figure B1. Harvest plots of the association between short-term exposure to A ozone, B carbon monoxide, C nitrogen dioxide, D small engine exhaust, E diesel exhaust, or F traffic-related air pollution exposure during moderate to vigorous physical activity and acute health- and exercise-related outcomes

Reproduced from Hung and co-authors (2021)

The figure highlights the pollutants and health outcomes that were most frequently investigated in the literature that was reviewed. It should be noted that the bars indicate where an effect of air pollution was observed during exercise, but not necessarily if there was an additive or synergistic effect between air pollution and exercise.

Annexe C. Further detail on the studies reviewed by DeFlorio-Barker and co-authors (2022) and D’Oliveira and co-authors (2023), which use long-term pollution averages in the analysis of health effects

DeFlorio-Barker and co-authors (2022)

The following studies evaluate effects of longer-term exposure and health effects of physical activity in children, and therefore may not be directly relevant to DAQI advice.

Gao and co-authors (2013) and Yu and co-authors (2004) examined VO₂ max, (the maximum rate of oxygen the body is able to use during physical exertion), and annual mean air pollution concentrations measured using fixed site monitoring of particulate matter (PM₁₀), nitrogen dioxide (NO₂), sulphur dioxide (SO₂), and ozone (O₃). Both studies used the multi-stage fitness test to predict VO₂ max. Both studies found cardiopulmonary fitness in self-reported physically active children increased in low air pollution districts (SO₂: 11.8 to 13.4µg/m³, NO₂: 42.9 to 48.4µg/m³, PM₁₀: 44.9 to 48.9µg/m³, O₃: 38.6µg/m³), but cardiopulmonary fitness did not improve with increased physical activity in high air pollution districts (SO₂: 18.6 to 22.8µg/m³, NO₂: 57.6 to 58.5 µg/m³, PM₁₀: 57.6µg/m³, O₃: 25.8µg/m³).

McConnell and co-authors (2002) found that the odds of incident asthma increased in the most active children in areas of high ozone compared to low ozone. This study used 4-year mean concentrations of ozone for each community examined.

Yu and co-authors (2020) examined the effect of long-term exposure to PM_2.5 on fasting plasma glucose. They found that the risk in elevated fasting plasma glucose (high blood sugar) was higher for those with higher physical activity levels compared to those with low activity. Physical activity measurements were measured in participants with overweight or obesity and the authors state that further work needs to be carried to out to compare results of participants without overweight or obesity. Therefore, these results explaining the impacts of physical activity on the association between PM_2.5 and fasting plasma glucose should be treated with caution.

Gui and co-authors (2020) looked at exposure to ambient air pollution and executive function (including working memory, inhibitory control, behavioural regulation and metacognition) among Chinese children. They reported no statistically significant interactions between annual averages of air pollution (PM_2.5, PM₁₀, NO₂, O₃, SO₂), physical activity and executive function. However, stratified analyses suggested increased physical activity may be beneficial for improving executive function in children, even among those with higher exposures to environmental pollution (DeFlorio-Barker and co-authors, 2022).

Thiering and co-authors (2016), Zhang and co-authors (2021a) and (2021b) found no effect of air pollution on the beneficial effects of physical activity. These all used long-term modelled air pollution data, and looked at long-term endpoints including insulin resistance, obesity and metabolic syndrome, respectively.

D’Oliveira and co-authors (2023)

The following studies evaluate effects of longer-term exposure and health effects of physical activity in older people, and therefore may not be directly relevant to DAQI advice. Wu and co-authors (2022), using the UK Biobank dataset, found that physical activity could weaken the relationship between PM_2.5 and major depressive disorder among older people. PM_2.5 and PM₁₀ concentrations were estimated for the year of 2010 using a land use regression model. Physical activity was measured using the International Physical Activity Questionnaire.

Using population data from the China Health and Retirement Longitudinal Study (CHARLS) (n = 24,623), Wang and co-authors (2020) found that depression symptoms were associated with annual modelled estimates of PM_2.5. However, physical activity, measured in hours per week, reduced depression symptoms, but this reduction decreased with increasing PM_2.5. Similarly in the United States, roadway distance was found to be associated with PM_2.5 and NO₂ exposure, and depression in a population of community-dwelling older individuals (57 to 85 years old), as part of the National Social Life, Health and Aging Project (NSHAP). While not significantly different, the association between roadway distance and depression was stronger in those that were less physically active.

Deng and co-authors (2021) found that physical activity modified the association between biomass fuels and cognitive ability in stratified analyses. They found those with poor cognitive ability engaged in irregular exercise. Furthermore, physical activity attenuated associations of high liver enzyme levels with short-term exposure (daily concentrations of PM_2.5, NO₂, O₃, CO and SO₂) in a study involving 545 individuals (Kim and co-authors, 2015).

In the United States, it was found that exposure to modelled estimates of annual average ozone increases the risk of diabetes among older (60 years or older) Mexican-Americans, particularly those with higher levels of outdoor physical activity (Yu and co-authors, 2021). Physical activity was measured using questionnaires and activities were then assigned a metabolic equivalent of task (MET) according to the Compendium of Physical Activities (Ainsworth and co-authors, 2000).

Zhang and co-authors (2021c) found long-term exposure to PM_2.5 (modelled estimates) had a significant detrimental impact on weight status. The authors used random sub-samples of the sample population (9,064 participants) to ask questions about physical activity. This was a binary variable in the linear mixed-effects models, ‘1’ was entered if no physical activity was undertaken and ‘0’ if moderate or vigorous physical activity was undertaken for at least 10 minutes a week. Physical activity was not found as a significant mediator of the detrimental effects of PM_2.5.

Sun and co-authors (2020) found that physical activity can decrease the risk of mortality in both areas of relatively poor and good air quality (PM_2.5). This study used a prospective cohort of 57,775 participants aged 65 years or over without dementia, enrolled during 1998-2001 and followed up until 2011. Participants were provided a questionnaire on physical activity volumes and types. Using the same cohort, Ran and co-authors (2021) estimated annual mean PM_2.5 exposures at residential address using a satellite-based model. The authors found no evidence of an interaction between habitual physical activity (volume and type) and PM_2.5 inhalation on the risk of incident dementia.

Cognition and cardiovascular markers were examined in groups of older women exposed to annual averages of PM_2.5, PM₁₀, O₃, NO₂ and SO₂ in Chile (Molina-Sotomayor and co-authors, 2019). 4 groups, including an active/clean air group, active/polluted air group, sedentary/clean air group, and sedentary/polluted air group were assessed over 2 years using the Mini Mental State Examination, VO₂ max, heart rate and oxygen saturation. Significant differences were found between the active/clean air group and the sedentary/polluted air group; however differences between the active/clean air group and the active/polluted group were not significant.

Some studies found in the review did not directly examine physical activity status and the effect of air pollution exposure. Bagheri and co-authors (2021) found a one standard deviation increase in long-term neighbourhood NO₂ and walkability score were both associated with a with 10% higher odds of dementia. Yu and co-authors (2017) found that air pollution significantly discouraged older people to engage in activity. They found that a one standard deviation increase of average PM_2.5 concentration was associated with a reduction in weekly total hours of walking. In Japan, Tanaka and co-authors (2014) compared individuals that were diagnosed with a pollution-related illness who were exposed to pollution more than 50 years ago (34 individuals), and age-matched patients with COPD (24 individuals) (all aged over 65 years). They found that pulmonary function in those with pollution-related diseases was better than that of the COPD group, but physical function (muscle strength and performance incremental walking test) was significantly worse than individuals with COPD.

Annexe D: Accompanying advice on the short-term effects of air pollution on health

The text below has been reproduced from Defra: Short-term effects of air pollution on health.

Short-term effects

Air pollution has a range of effects on health. However, air pollution in the UK on a day-to-day basis is not expected to rise to levels at which people need to make major changes to their habits to avoid exposure; Nobody need fear going outdoors, but they may experience some noticeable symptoms depending on which of the following population groups they are in:

• Adults and Children with lung or heart conditions - It is known that, when levels of air pollutants rise, adults suffering from heart conditions, and adults and children with lung conditions, are at increased risk of becoming ill and needing treatment. Only a minority of those who suffer from these conditions are likely to be affected and it is not possible to predict in advance who will be affected. Some people are aware that air pollution affects their health: adults and children with asthma may notice that they need to increase their use of inhaled reliever medication on days when levels of air pollution are higher than average.

• Older people - Older people are more likely to suffer from heart and lung conditions than young people and so it makes good sense for them to be aware of current air pollution conditions.

• The general population - At Very High levels of air pollution, some people may experience a sore or dry throat, sore eyes or, in some cases, a tickly cough even in healthy individuals.

• Children - Children need not be kept from school or prevented from taking part in games. Children with asthma may notice that they need to increase their use of reliever medication on days when levels of air pollution are higher than average.

Note: Adults and children with heart or lung problems are at greater risk of symptoms. Follow your doctor’s usual advice about exercising and managing your condition. It is possible that very sensitive individuals may experience health effects even on Low air pollution days. Anyone experiencing symptoms should follow the guidance provided.