Guidance

Air emissions risk assessment for your environmental permit

How to complete an air emissions risk assessment, including how to calculate the impact of your emissions and the standards you must meet.

From:
Environment Agency and Department for Environment, Food & Rural Affairs
Published
1 February 2016
Last updated
21 May 2024 — See all updates

Applies to England

Before you start this risk assessment

Read the following guides before you start this risk assessment:

  • the risk assessment overview – this explains the other steps to take in risk assessment and whether you need to do an air emissions risk assessment
  • best available techniques (BAT) from the European Commission – you may need to apply, or in some cases exceed, BAT depending on how harmful your emissions could be to the environment

How this risk assessment works

You need to compare the impact of your emissions to air to the following environment standards:

  • Air Quality Standards Regulations 2010 Limit Values and Target Values
  • UK Air Quality Strategy Objectives
  • Environmental Assessment Levels

Find the environmental standards.

Steps to complete this risk assessment

To complete an air emissions risk assessment you need to follow these steps.

  1. Calculate the environmental concentration of each substance you release into the air – known as the process contribution (PC).

  2. Identify PCs with insignificant environmental impact so that they can be ‘screened out’ – this means that you do not have to assess them any further.

  3. For substances not screened out in step 2, calculate the predicted environmental concentration (PEC) for each substance you release to air – the PEC is the PC plus the concentration of the substance already present in the environment.

  4. Identify emissions that have insignificant environmental impact – these can be screened out.

  5. Get ‘detailed modelling’ (also known as detailed assessment or computer modelling) done for the emissions you cannot screen out.

  6. For each substance you’ve released to air, compare the PC and PEC with the relevant environmental standard and summarise your results.

  7. Check if you need to take further action.

  8. Check if you need to do any other risk assessments.

The Environment Agency sometimes refers to the following stages of air emissions risk assessment:

  • ‘stage 1’ – this is steps 1 and 2
  • ‘stage 2’ – this is steps 3 and 4

Risk assessment tool

You should use the Environment Agency’s risk assessment tool to complete your risk assessment. The only stage you cannot use it for is to screen out PCs or PECs of substances in protected conservation areas.

The figures the tool gives you are ‘worst case’ estimates. So the figures you get may be higher than if you calculate PCs or PECs using other methods, for example dispersion modelling software (which analyses how air pollutants disperse in the atmosphere).

This guide explains the steps to complete if you’re not using the risk assessment tool.

Calculate PC to air

You must calculate both your short term and long term PC to air for each substance. PC to air is measured in micrograms per cubic metre.

To calculate the PC to air, multiply the dispersion factor, in micrograms per cubic metre per gram per second, by the release rate, in grams per second.

If you do not have existing data

Use estimates if you do not have existing data (for example if your activity is new).

Where possible, use estimates based on similar operations elsewhere or from trials. Otherwise, use worst-case estimates.

State what assumptions you’ve made for these estimates.

Grouping air emissions

If you release volatile organic compounds into the air you should provide details of all emissions.  If you cannot identify what all the substances in them are, treat the unknowns as 100% benzene in your risk assessment. If you want to treat them as something else, you’ll need to explain why.

Nitrogen oxides (also known as oxides of nitrogen)

Emissions of nitrogen oxides should be recorded as nitrogen dioxide in your risk assessment (as nitrogen oxide converts to nitrogen dioxide over time):

  • for short term PCs and PECs, assume only 50% of emissions of oxides of nitrogen convert to nitrogen dioxide in the environment
  • for long term PCs and PECs, assume 100% of emissions of oxides of nitrogen convert to nitrogen dioxide

When using the risk assessment tool, and entering your nitrogen oxides emissions as nitrogen dioxide, it will do the conversion for you.

When your site does not operate all the time

Adjust your figures down, based on the percentage of the year that your site is not operating. For example, a site that only operates January to June should reduce its PC figures by 50%. This only applies to long term annual mean assessment. It does not apply to any other long term or short term assessment averaging periods.

When using the risk assessment tool, you can enter the percentage into ‘operating mode (%)’ and it will do the calculation for you.

PC: dispersion factor

The risk assessment tool calculates intermediate dispersion factors where the effective height is between given values.

If you’re not using the tool, this table shows the dispersion factors you can use. These factors are based on the point at which the substance is effectively released into the air. This is known as the ‘effective height of release’.

You must use different dispersion factors if your site has landfill gas engines, landfill gas flares or capped areas.

All dispersion factors are shown in micrograms per cubic metre per gram per second.

Effective height of release in metres Long term dispersion factor Monthly dispersion factor Hourly dispersion factor
0 148 529 3,900
10 32 33.7 580
20 4.6 6.2 161
30 1.7 2.3 77
50 0.52 0.68 31
70 0.24 0.31 16
100 0.11 0.13 8.6
150 0.048 0.052 4
200 0.023 0.026 2.3

Effective height of release: impact of nearby buildings

Treat the effective height of release as 0 metres when the emission is actually released at a point that’s either:

  • less than 3 metres above the ground or building on which the stack is located
  • more than 3 metres above the ground or the building, but less than the height of the tallest building within a distance that’s 5 times ‘L’

‘L’ is the lowest of either:

  • the height of the building
  • the greatest width between 2 points at the same height of the building (for example between 2 opposing corners of a roof)

When the effective height of release is more than 3 metres above the ground or building, but less than 2.5 times the building’s height, estimate it by following these steps.

  1. Take the actual height of release.
  2. Subtract the height of the tallest building within a distance 5 times L (this can be the building where the emissions are coming from, if it’s the tallest).
  3. Multiply the figure that’s left by 1.66.

When the actual stack height is more than 2.5 times the building height, the actual stack height can be treated as the effective height of release.

Dispersion factor: landfill gas engines, flares or capped areas

Dispersion factors for landfill gas engines, flares or capped areas are based on the shortest distance from the gas engine to whichever of these is nearest:

  • the site boundary
  • the nearest sensitive receptor

You can download the following dispersion factors, shown in micrograms per cubic metre per gram per second.

PC: release rate

Calculate the release rate by taking the substance’s actual gas flow in cubic metres per second.

Multiply this number by the substance’s concentration (in milligrams per cubic metre) divided by 1,000.

When a substance is released from more than one point (for example from several chimneys from a factory), you must add up the substance’s PC from each source (for example a chimney) to get the total PC for the substance. The risk assessment tool will do this calculation for you.

You should also describe:

  • how the concentration of an emission varies over the time of day or year
  • if you’re generating power, the energy demand when a release happens, for example whether it’s average demand or peak demand

Calculating averaging periods

You should use the appropriate averaging period when you compare the impact of your emissions against environmental standards.

Most long term standards are expressed over an annual mean averaging time. Some long term standards are now expressed as 24 hour mean long term standards, labelled ‘24 hour mean (long term)’. For these the 24 hour mean concentrations averaged over a longer time period should not exceed the standard. For these the long term dispersion factor is used without the ‘operating mode (%)’ adjustment applied. There are also some standards expressed over monthly and weekly mean averaging periods. These are also considered long term standards in air emissions assessment.

Most short term standards are expressed over an hourly averaging period. But sometimes the short term environmental standard is measured using different averaging times (for example, 15 minutes, 30 minutes, 8 hours or 24 hours). For these short term standards the mean concentration over these averaging times should not exceed the standard, or are only allowed to exceed the standard for a specified number of times per year.

PCs calculated on an hourly mean basis can be multiplied by the following factors to convert them to other less common averaging times:

  • 1.34 to convert it into a short term 15 minute mean
  • 1.3 to convert it into a short term 30 minute mean
  • 0.7 to convert it into an short term 8 hour mean
  • 0.59 to convert it to a short term 24 hour mean
  • 0.31 to convert it to a long term weekly mean

When using the risk assessment tool, it will do the these conversions for you.

For sulphur dioxide, the ‘short term’ periods are 15 minutes, 1 hour and 24 hours. Multiply the hourly dispersion factor by 1.34 to get the 15 minute dispersion factor. Multiply the hourly dispersion factor by 0.59 to get the 24 hour average.

Calculate PC for substance deposition

The following substances require you to calculate the impact they have when absorbed by soil and leaves (known as ‘deposition’):

  • arsenic
  • cadmium
  • chromium
  • copper
  • fluoride
  • lead
  • mercury
  • molybdenum
  • nickel
  • selenium
  • zinc

The impact on the soil is known as ‘PC to ground’. You calculate this as follows.

  1. Calculate the ‘PC to air’ by multiplying the long term dispersion factor by the release rate and multiplying by the ‘operating mode (%)’.
  2. Do this calculation: PC to air × 0.01 × 3 × 86,400.
  3. Divide the number you get by 1,000.

The number you’re left with is the PC to ground, in milligrams per square metre per day.

Screen out insignificant PCs

To screen out a PC for any substance so that you do not need to do any further assessment of it, the PC must meet both of the following criteria:

  • the short term PC is less than 10% of the short term environmental standard
  • the long term PC is less than 1% of the long term environmental standard

If you meet both of these criteria you do not need to do any further assessment of the substance.

If you do not meet them you need to carry out a second stage of screening to determine the impact of the PEC. Record the PCs for your insignificant emissions in your risk assessment.

Assess insignificant PCs to ground

The following are PC to ground limits in milligrams per square metre per day:

  • arsenic – 0.02
  • cadmium – 0.009
  • chromium – 1.5
  • copper – 0.25
  • fluoride – 2.1
  • lead – 1.1
  • mercury – 0.004
  • molybdenum – 0.016
  • nickel – 0.11
  • selenium – 0.012
  • zinc – 0.48

If the PC to ground for any of these substances is below 1% of the limit it’s insignificant.

If the PC to ground is 1% of the limit or greater, you may need to do further assessment such as detailed modelling. You should contact the Environment Agency if you think you may need to do further assessment.

Calculate PEC

You must calculate the short and long term PECs of PCs to air that were not screened out in the first stage.

To calculate the short and long term PECs of PCs to air, combine the following:

  • each substance’s PC to air
  • the concentration of the substance that’s already present in the environment - the ‘background concentration’

Record these figures in your risk assessment.

You can find out about background concentrations from:

This information will usually be shown as a long term (annual) average concentration.

Background concentrations may already include PCs from your site. To avoid your PCs being double-counted, use a background concentration from a source that is not affected by the direction that the wind predominantly blows from (that is the prevailing wind direction). For example, if the prevailing wind comes from the west, do not use a background concentration from a source to your east.

When you calculate background concentration, you can assume that the short term background concentration of a substance is twice its long term concentration.

Screen out PECs from detailed modelling

In the second stage of screening if you meet both of the following requirements you do not need to do any further assessment of that substance. You’ll need to do detailed modelling of emissions that do not meet both of the following requirements:

Screening for protected conservation areas

You must consider the impact of your site on protected conservation areas. Complete the nature and heritage conservation screening form to find out if any are near your site.

The screening process for protected conservation areas is limited to the emissions and emission periods in these environmental standards for protected conservation areas.

Substance Environmental standard Averaging time
Ammonia 1 microgram per cubic metre where lichens or bryophytes (including mosses, liverworts and hornwarts) are present, 3 micrograms per cubic metre where they’re not present Annual mean
Hydrogen fluoride 0.5 micrograms per cubic metre Weekly mean
Hydrogen fluoride 5 micrograms per cubic metre Daily mean
Nitrogen oxides (expressed as nitrogen dioxide) 30 micrograms per cubic metre Annual mean
Nitrogen oxides (expressed as nitrogen dioxide) 75 micrograms per cubic metre, 200 micrograms per cubic metre (but only for detailed assessments where the ozone is below the AOT40 critical level and sulphur dioxide is below the lower critical level of 10 micrograms per cubic metre) Daily mean
Ozone (used for detailed daily nitrogen oxides assessment) AOT40 of 6000 microgram per cubic metre calculated from accumulated hourly ozone concentrations – AOT40 means the sum of the difference between each hourly daytime (08:00 to 20:00 Central European Time) ozone concentration greater than 80 micrograms per cubic metre (40 parts per billion) and 80 micrograms per cubic metre, for the period between 01 May and 31 July Period between May and July
Sulphur dioxide 10 micrograms per cubic metre where lichens or bryophytes are present, 20 micrograms per cubic metre where they’re not present Annual mean
Acidity deposition Depends on location – use APIS to check it Annual mean
Nutrient nitrogen deposition Depends on location – use APIS to check it Annual mean

Check if there are any of the following within 10km of your site:

  • special protection areas (SPAs)
  • special areas of conservation (SACs)
  • Ramsar sites (protected wetlands)

Check if there are any of the following within 2km of your site:

  • sites of special scientific interest (SSSIs)
  • local nature sites (ancient woods, local wildlife sites and national and local nature reserves)

Some larger (greater than 50 megawatt) emitters with certain fuel types may be required to use increased screening distances of:

  • 15km for SACs, SPAs and Ramsar sites
  • 10km or 15km for SSSIs

You should increase the screening distance for air emissions on protected conservation areas to 15km for the following:

  • natural gas (or fuels with a similarly low sulphur content) fired combustion plants, with more than 500 megawatt thermal input
  • larger combustion plants using more sulphurous fuels with more than 50 megawatt thermal input

You should check the relevant screening distances at the pre-application stage.

When there are SPAs, SACs, Ramsar sites and SSSIs within the specified distance

If your emissions that affect SPAs, SACs, Ramsar sites or SSSIs meet both of the following criteria, they’re insignificant - you do not need to assess them any further:

  • the short term PC is less than 10% of the short term environmental standard for protected conservation areas
  • the long term PC is less than 1% of the long term environmental standard for protected conservation areas

If you do not meet these requirements you need to calculate the PEC and check the PEC against the standard for protected conservation areas.

You do not need to calculate PEC for short term targets.

If your short term PC exceeds the screening criteria of 10%, you need to do detailed modelling.

If your long term PC is greater than 1% and your PEC is less than 70% of the long term environmental standard, the emissions are insignificant – you do not need to assess them any further.

If your PEC is greater than 70% of the long term environmental standard, you need to do detailed modelling.

For SPAs, SACs and Ramsar sites, you need to consider the ‘in combination’ (combined) impact of all permissions, plans or projects that could also affect these sites. Contact the Environment Agency for further guidance on in-combination assessments.

When there are local nature sites within the specified distance

If your emissions meet both of the following criteria they’re insignificant – you do not need to assess them any further:

  • the short term PC is less than 100% of the short term environmental standard for protected conservation areas
  • the long term PC is less than 100% of the long term environmental standard for protected conservation areas

You do not need to calculate PEC for local nature sites. If your PC exceeds the screening criteria you need to do detailed modelling.

You cannot use the risk assessment tool to check how significant a PC or PEC is for deposition of nutrient nitrogen or acidity. This is because nutrient nitrogen and acidity targets vary depending on location. The APIS site-relevant critical load tool will tell you the standard that you need to compare the PC or PEC against.

Record the PCs and PECs and the nitrogen and acidity critical load values you used for your insignificant emissions in your risk assessment.

There are different rules about what’s insignificant in air emissions from intensive farming.

Contact the Environment Agency for more information about modelling and screening for protected conservation areas.

Detailed modelling

You must do detailed modelling for any PECs not screened out as insignificant.

To do detailed modelling, you need to use computer software that models the passage of a substance as it travels through the atmosphere until it reaches the ground.

Detailed modelling requires specialist knowledge. You can find a consultant to do it for you. They’ll charge for their services. Contact the Environment Agency if you want to do your own detailed modelling.

For information on detailed modelling for environmental permitting applications see Environmental permitting: air dispersion modelling reports.

Air Quality Management Areas

Unless your process contribution (PC) is insignificant, you must have detailed modelling done if both of the following apply:

  • your emissions affect an Air Quality Management Area (AQMA)
  • restrictions apply for any substance you emit in this area

Check if your site is in an AQMA.

More accurate data

You can have detailed modelling done if you’ve used the risk assessment tool to do your risk assessment but you want to provide data that’s:

  • more accurate – the tool does not include the plume rise (a factor that affects the effective height of release) of your emissions in its calculations
  • less pessimistic – for example if you want to show that your emissions are a lower risk than the risk assessment tool’s estimates

Varying emission rates

The risk assessment tool assumes a constant emission rate for each substance over a year. You may need to do detailed modelling if your site’s output varies a lot, for example the output from a chemical factory or a power station can vary a lot from day to day. Check with the Environment Agency if you’re not sure.

Compare and summarise your results

In your application you need to include all of the following:

  • the PC
  • the PEC
  • the substances you’ve screened out
  • the substances that have had a detailed assessment
  • the relevant environmental standards that you referred to when evaluating your emissions
  • any additional action that you think you need to take, for example a cost benefit analysis

Check if you need to take further action

Your pre-application discussions with the Environment Agency may have already shown that you need to take further action, such as a cost benefit analysis of your proposals.

Your risk assessment may also show that you need to take further action.

When you do not need to take further action

You do not need to take further action if your assessment has shown that both of the following apply:

When you need to take further action

You’ll need to do a cost benefit analysis if any of the following apply:

  • your PCs could cause a PEC to exceed an environmental standard (unless the PC is insignificant compared to other contributors – if you think this is the case contact the Environment Agency)
  • the PC is not insignificant and the PEC is already exceeding an environmental standard
  • your activity or part of it is not covered by a ‘BAT reference document’ (BREF)
  • your proposals do not comply with BAT AELs - in this case you’ll need to make a request for an exception (‘derogation’) that includes a cost benefit analysis of your proposals
  • you’ve been asked to do a BAT assessment

When you need to contact the Environment Agency

In all other cases or if you’re not sure whether you need to take further action, contact the Environment Agency.

Cost benefit analysis tool

The Environment Agency has produced a cost benefit analysis tool to help you. Contact the Environment Agency for this tool.

Check if you need to do other risk assessments

You’ll need to check if you need to do any other risk assessments

Once you’ve done all the required risk assessments, submit them with your permit application. You can also use the risk assessment tool to submit this risk assessment.

Environmental standards for air emissions

Compare the impact of your air emissions against the following environmental standards when you do your air emissions risk assessment.

Air Quality Standards Regulations 2010 Limit Values

The Environment Agency must make sure your proposals do not exceed the Air Quality Standards Regulations 2010 Limit Values. You should check if you need to take further action if either:

  • a Limit Value is already exceeded at your location
  • a Limit Value could be exceeded by your proposed activity
Substance Averaging time Concentration Environmental standard Exceedances (number of times a year that you can exceed the limit)
Benzene Annual mean 5 micrograms per cubic metre Limit Value None
Carbon monoxide Maximum 8 hour running mean in any daily period 10 milligrams per cubic metre Limit Value None
Lead Annual mean 0.5 micrograms per cubic metre Limit Value None
Nitrogen dioxide 1 hour mean 200 micrograms per cubic metre Limit Value Up to 18 1-hour periods
Nitrogen dioxide Annual mean 40 micrograms per cubic metre Limit Value None
Particulates (PM10) 24 hour mean 50 micrograms per cubic metre Limit Value Up to 35 times a year
Particulates (PM10) Annual mean 40 micrograms per cubic metre Limit Value None
Particulates (PM2.5) Annual 20 micrograms per cubic metre Limit Value None
Sulphur dioxide 1 hour mean 350 micrograms per cubic metre Limit Value Up to 24 1-hour periods
Sulphur dioxide 24 hour mean 125 micrograms per cubic metre Limit Value Up to 3 24-hour periods

Air Quality Standards Regulations 2010 Target Values and UK Air Quality Strategy Objectives

Under the law, you will not usually have to go further than BAT to comply with either of the following standards:

  • Air Quality Standards Regulations 2010 Target Values
  • UK Air Quality Strategy (AQS) Objectives

As substances covered by these standards could still damage the environment, the Environment Agency may decide that you need to take further action if your emissions of a substance will be significant in relation to these standards.

The Environment Agency will decide this on a case by case basis. It will then let you know if you need to take further action, for example carrying out a cost benefit analysis.

Where a substance has both a Target Value and a UK AQS Objective over the same averaging time with different concentrations, you must use the lower concentration.

Substance Averaging time Concentration Environmental standard Exceedances (number of times a year you’re allowed to exceed the target)
1,3-butadiene Running annual mean 2.25 micrograms per cubic metre Objective None
Arsenic Annual mean 6 nanograms per cubic metre Target Value None
Cadmium Annual mean 5 nanograms per cubic metre Target Value None
Lead Annual mean 0.25 micrograms per cubic metre Objective None
Nickel Annual mean 20 nanograms per cubic metre Target Value None
Ozone Running 8 hour mean 120 micrograms per cubic metre Target Value Up to 25 8-hour periods
Ozone Running 8 hour mean 100 micrograms per cubic metre Objective Up to 10 8-hour periods
Polyaromatic hydrocarbons (benzo(a)pyrene) Annual mean 1 nanogram per cubic metre Target Value None
Polyaromatic hydrocarbons (benzo(a)pyrene) Annual mean 0.25 nanogram per cubic metre Objective None
Sulphur dioxide 15 minute mean 266 micrograms per cubic metre Objective Up to 35 15-minute periods

Environmental Assessment Levels

If you exceed these assessment levels, you might need to take further action to reduce your impact on the environment. The Environment Agency will tell you what you need to do.

‘Further action’ might include doing a cost benefit analysis of alternative waste recovery and disposal methods, or installing new equipment, like an abatement plant.

Where an environmental standard or environmental assessment level (EAL) is not listed for a substance you are assessing you can propose a new EAL.

To derive a new EAL, you should use the Environment Agency hazard characterisation method for determining tolerable concentrations in air (TCAs) within section 7 and annex 5 of our 2012 consultation document Derivation of new environmental assessment levels to air. You need to select the option appropriate for the substance and whether the critical effect has a threshold or has no threshold.

We may need to do a further review and consult on your proposals. Therefore, you need to submit your proposal with a sufficiently detailed explanation to explain how you have derived it. We have examples available in Appendix C: summary of toxicological evidence for MEA and NDMA of our 2021 Consultation response document: new EALs for emissions to air.

We are in the process of periodically updating EALs using our hazard characterisation method for determining TCAs.

In the first phase consultation, published in 2021 we reviewed EALs for 12 substances:

  • arsenic
  • benzene
  • chloroform
  • chromium VI
  • ethylene dichloride
  • methyl chloroform
  • mono-ethanolamine (MEA)
  • naphthalene
  • N-nitrosodimethylamine (NDMA)
  • tetrachloroethylene
  • trichloroethylene
  • vinyl chloride

In the second phase consultation, published in 2023 we reviewed EALs for 13 substances following our consultation:

  • acrylamide
  • butadiene
  • cadmium
  • chromium III
  • copper
  • ethylene oxide
  • hydrogen chloride
  • hydrogen cyanide
  • mercury
  • methyl chloride (chloromethane)
  • methylene chloride (dichloromethane)
  • nickel and selenium
Substance Averaging time Concentration in micrograms per cubic metre Derivation method or information source
Acetaldehyde 1 hour mean 9,200 Old EAL derivation method from EH40/2001 OEL
Acetaldehyde Annual mean 370 Old EAL derivation method from EH40/2001 OEL
Acetic acid 1 hour mean 3,700 Old EAL derivation method from EH40/2001 OEL
Acetic acid Annual mean 250 Old EAL derivation method from EH40/2001 OEL
Acetic anhydride 1 hour mean 40 Old EAL derivation method from EH40/2001 OEL
Acetic anhydride Annual mean 1 Old EAL derivation method from EH40/2001 OEL
Acetone 1 hour mean 362,000 Old EAL derivation method from EH40/2001 OEL
Acetone Annual mean 18,100 Old EAL derivation method from EH40/2001 OEL
Acetonitrile 1 hour mean 10,200 Old EAL derivation method from EH40/2001 OEL
Acetonitrile Annual mean 680 Old EAL derivation method from EH40/2001 OEL
Acrylamide Annual mean 0.05 Hazard characterisation method for determining TCA (2023)
Acrylic acid 1 hour mean 6,000 Old EAL derivation method from EH40/2001 OEL
Acrylic acid Annual mean 300 Old EAL derivation method from EH40/2001 OEL
Acrylonitrile 1 hour mean 264 Old EAL derivation method from EH40/2001 OEL
Acrylonitrile Annual mean 8.8 Old EAL derivation method from EH40/2001 OEL
Allyl alcohol 1 hour mean 970 Old EAL derivation method from EH40/2001 OEL
Allyl alcohol Annual mean 48 Old EAL derivation method from EH40/2001 OEL
Ammonia 1 hour mean 2,500 Old EAL derivation method from EH40/2001 OEL
Ammonia Annual mean 180 Old EAL derivation method from EH40/2001 OEL
Aniline 1 hour mean 240 Old EAL derivation method from EH40/2001 OEL
Aniline Annual mean 8 Old EAL derivation method from EH40/2001 OEL
Antimony and compounds (as antimony) except antimony trisulphide and antimony trioxide 1 hour mean 150 Old EAL derivation method from EH40/2001 OEL
Antimony and compounds (as antimony) except antimony trisulphide and antimony trioxide Annual mean 5 Old EAL derivation method from EH40/2001 OEL
Arsine 1 hour mean 48 Old EAL derivation method from EH40/2001 OEL
Arsine Annual mean 1.6 Old EAL derivation method from EH40/2001 OEL
Benzene 24 hour mean (short term) 30 Hazard characterisation method for determining TCA (2021)
Benzylchloride 1 hour mean 158 Old EAL derivation method from EH40/2001 OEL
Benzylchloride Annual mean 5.2 Old EAL derivation method from EH40/2001 OEL
Beryllium (total in the PM10 fraction) Annual mean 0.0002 EPAQS Metals and Metalloids (2009)
Boron trifluoride 1 hour mean 280 Old EAL derivation method from EH40/2001 OEL
Bromine 1 hour mean 70 EPAQS Halogen and Hydrogen Halides (2006)
Bromomethane 1 hour mean 5,900 Old EAL derivation method from EH40/2001 OEL
Bromomethane Annual mean 200 Old EAL derivation method from EH40/2001 OEL
1, 3 butadiene 24 hour mean (short term) 2.25 Hazard characterisation method for determining TCA  (2023)
Butane 1 hour mean 181,000 Old EAL derivation method from EH40/2001 OEL
Butane Annual mean 14,500 Old EAL derivation method from EH40/2001 OEL
Cadmium and its compounds (as cadmium) 24 hour mean (short term) 0.03 Hazard characterisation method for determining TCA  (2023)
Carbon disulphide 24 hour mean (short term) 100 WHO Air Quality Guidelines for Europe (2000)
Carbon disulphide Annual mean 64 Old EAL derivation method from EH40/2001 OEL
Carbon monoxide 1 hour mean 30,000 WHO Air Quality Guidelines for Europe (2000)
Carbon tetrachloride 1 hour mean 3,900 Old EAL derivation method from EH40/2001 OEL
Carbon tetrachloride Annual mean 130 Old EAL derivation method from EH40/2001 OEL
Chlorine 1 hour mean 290 EPAQS Halogen and Hydrogen Halides (2006)
Chloroform 24 hour mean (long term) 100 Hazard characterisation method for determining TCA (2021)
Chromium (III) compounds (as chromium) 24 hour mean (long term) 2.0 Hazard characterisation method for determining Tolerable Concentrations in Air  (2023)
Chromium VI compounds (as chromium) Annual mean 0.00025 Hazard characterisation method for determining TCA (2021)
Copper and its compounds (as copper) 24 hour mean (long term) 0.05 Hazard characterisation method for determining TCA  (2023)
Dibutyl phthalate 1 hour mean 1,000 Old EAL derivation method from EH40/2001 OEL
Dibutyl phthalate Annual mean 50 Old EAL derivation method from EH40/2001 OEL
Diethyl ether 1 hour mean 154,000 Old EAL derivation method from EH40/2001 OEL
Diethyl ether Annual mean 12,300 Old EAL derivation method from EH40/2001 OEL
Diethyl ketone 1 hour mean 89,500 Old EAL derivation method from EH40/2001 OEL
Diethyl ketone Annual mean 7,160 Old EAL derivation method from EH40/2001 OEL
Diisobutyl phthalate 1 hour mean 1,500 Old EAL derivation method from EH40/2001 OEL
Diisobutyl phthalate Annual mean 50 Old EAL derivation method from EH40/2001 OEL
Diisopropyl ether 1 hour mean 131,000 Old EAL derivation method from EH40/2001 OEL
Diisopropyl ether Annual mean 10,600 Old EAL derivation method from EH40/2001 OEL
Dimethyl sulphate 1 hour mean 15.6 Old EAL derivation method from EH40/2001 OEL
Dimethyl sulphate Annual mean 0.52 Old EAL derivation method from EH40/2001 OEL
Dimethylformamide 1 hour mean 6,100 Old EAL derivation method from EH40/2001 OEL
Dimethylformamide Annual mean 300 Old EAL derivation method from EH40/2001 OEL
Dioxane 1 hour mean 36,600 Old EAL derivation method from EH40/2001 OEL
Dioxane Annual mean 910 Old EAL derivation method from EH40/2001 OEL
Ethyl acrylate 1 hour mean 6,200 Old EAL derivation method from EH40/2001 OEL
Ethyl acrylate Annual mean 210 Old EAL derivation method from EH40/2001 OEL
Ethylbenzene 1 hour mean 55,200 Old EAL derivation method from EH40/2001 OEL
Ethylbenzene Annual mean 4,410 Old EAL derivation method from EH40/2001 OEL
Ethylene dibromide 1 hour mean 234 Old EAL derivation method from EH40/2001 OEL
Ethylene dibromide Annual mean 7.8 Old EAL derivation method from EH40/2001 OEL
Ethylene dichloride Annual mean 3 Hazard characterisation method for determining TCA (2021)
Ethylene oxide Annual mean 0.002 Hazard characterisation method for determining TCA  (2023)
Formaldehyde 30 minute mean 100 WHO Air Quality Guidelines for Europe (2000)
Formaldehyde Annual mean 5 Old EAL derivation method from EH40/2001 OEL
Hydrazine 1 hour mean 2.6 Old EAL derivation method from EH40/2001 OEL
Hydrazine Annual mean 0.06 Old EAL derivation method from EH40/2001 OEL
Hydrogen bromide 1 hour mean 700 EPAQS Halogen and Hydrogen Halides (2006)
Hydrogen chloride 1 hour mean 750 EPAQS Halogen and Hydrogen Halides (2006)
Hydrogen cyanide 24 hour mean (long term) 2.0 Hazard characterisation method for determining TCA  (2023)
Hydrogen fluoride 1 hour mean 160 EPAQS Halogen and Hydrogen Halides (2006)
Hydrogen fluoride Monthly mean 16 EPAQS Addendum to Halogens and Hydrogen Halides Report (2009)
Hydrogen iodide 1 hour mean 520 EPAQS Halogen and Hydrogen Halides (2006)
Hydrogen iodide Monthly mean 5 EPAQS Addendum to Halogens and Hydrogen Halides Report (2009)
Hydrogen sulphide 24 hour mean (short term) 150 WHO Air Quality Guidelines for Europe (2000)
Hydrogen sulphide Annual mean 140 Old EAL derivation method from EH40/2001 OEL
Manganese and compounds (as manganese) 1 hour mean 1,500 Old EAL derivation method from EH40/2001 OEL
Manganese and compounds (as manganese) Annual mean 0.15 WHO Air Quality Guidelines for Europe (2000)
Mercury and its inorganic compounds (as mercury) 1 hour mean 0.6 Hazard characterisation method for determining TCA  (2023)
Mercury and its inorganic compounds (as mercury) 24 hour mean (long term) 0.06 Hazard characterisation method for determining TCA  (2023)
Methanol 1 hour mean 33,300 Old EAL derivation method from EH40/2001 OEL
Methanol Annual mean 2,660 Old EAL derivation method from EH40/2001 OEL
Methyl chloride (chloromethane) 24 hour mean (long term) 18 Hazard characterisation method for determining TCA  (2023)
Methyl chloroform 24 hour mean (long term) 5,000 Hazard characterisation method for determining TCA (2021)
Methyl ethyl ketone 1 hour mean 89,900 Old EAL derivation method from EH40/2001 OEL
Methyl ethyl ketone Annual mean 6,000 Old EAL derivation method from EH40/2001 OEL
Methyl propyl ketone 1 hour mean 89,500 Old EAL derivation method from EH40/2001 OEL
Methyl propyl ketone Annual mean 7,160 Old EAL derivation method from EH40/2001 OEL
Methylene chloride (dichloromethane) 24 hour mean (short term) 2,100 Hazard characterisation method for determining TCA  (2023)
Methylene chloride (dichloromethane) Annual mean 770 Hazard characterisation method for determining TCA  (2023)
Mono-ethanolamine (MEA) 1 hour mean 400 Hazard characterisation method for determining TCA (2021)
Mono-ethanolamine (MEA) 24 hour mean (long term) 100 Hazard characterisation method for determining TCA (2021)
Naphthalene 24 hour mean (long term) 3 Hazard characterisation method for determining TCA (2021)
N-hexane 1 hour mean 21,600 Old EAL derivation method from EH40/2001 OEL
N-hexane Annual mean 720 Old EAL derivation method from EH40/2001 OEL
Nickel and its compounds, except nickel carbonyl (as nickel) 1 hour mean 0.7 Hazard characterisation method for determining TCA  (2023)
Nitric acid 1 hour mean 1,000 Old EAL derivation method from EH40/2001 OEL
Nitric acid Annual mean 52 Old EAL derivation method from EH40/2001 OEL
Nitrogen monoxide 1 hour mean 4,400 Old EAL derivation method from EH40/2001 OEL
Nitrogen monoxide Annual mean 310 Old EAL derivation method from EH40/2001 OEL
N-nitrosodimethylamine (NDMA) Annual mean 0.0002 Hazard characterisation method for determining TCA (2021)
Orthophosphoric acid 1 hour mean 200 Old EAL derivation method from EH40/2001 OEL
Para-dichlorobenzene 1 hour mean 30,600 Old EAL derivation method from EH40/2001 OEL
Para-dichlorobenzene Annual mean 1,530 Old EAL derivation method from EH40/2001 OEL
Phenol 1 hour mean 3,900 Old EAL derivation method from EH40/2001 OEL
Phenol Annual mean 200 Old EAL derivation method from EH40/2001 OEL
Phosgene 1 hour mean 25 Old EAL derivation method from EH40/2001 OEL
Phosgene Annual mean 0.8 Old EAL derivation method from EH40/2001 OEL
Phosphine 1 hour mean 42 Old EAL derivation method from EH40/2001 OEL
Polychlorinated biphenyls (PCBs) 1 hour mean 6 Old EAL derivation method from EH40/2001 OEL
Polychlorinated biphenyls (PCBs) Annual mean 0.2 Old EAL derivation method from EH40/2001 OEL
1-propanol 1 hour mean 62,500 Old EAL derivation method from EH40/2001 OEL
1-propanol Annual mean 5,000 Old EAL derivation method from EH40/2001 OEL
2-propanol 1 hour mean 125,000 Old EAL derivation method from EH40/2001 OEL
2-propanol Annual mean 9,990 Old EAL derivation method from EH40/2001 OEL
Propylene oxide 1 hour mean 720 Old EAL derivation method from EH40/2001 OEL
Propylene oxide Annual mean 24 Old EAL derivation method from EH40/2001 OEL
Selenium and compounds, except hydrogen selenide (as selenium) 24 hour mean (long term) 2.0 Hazard characterisation method for determining TCA  (2023)
Sodium hydroxide 1 hour mean 200 Old EAL derivation method from EH40/2001 OEL
Styrene 1 hour mean 800 Old EAL derivation method from EH40/2001 OEL
Styrene 1 week (long term) 260 World Health Organisation Air Quality Guidelines for Europe (WHO 2000)
Sulphur hexafluoride 1 hour mean 759,000 Old EAL derivation method from EH40/2001 OEL
Sulphur hexafluoride Annual mean 60,700 Old EAL derivation method from EH40/2001 OEL
Sulphuric acid 1 hour mean 300 Old EAL derivation method from EH40/2001 OEL
Sulphuric acid Annual mean 10 Old EAL derivation method from EH40/2001 OEL
Tetrachloroethylene 24 hour mean (long term) 40 Hazard characterisation method for determining TCA (2021)
Tetrahydrofuran 1 hour mean 59,900 Old EAL derivation method from EH40/2001 OEL
Tetrahydrofuran Annual mean 3,000 Old EAL derivation method from EH40/2001 OEL
Toluene 1 hour mean 8,000 Old EAL derivation method from EH40/2001 OEL
Toluene 1 week (long term) 260 World Health Organisation Air Quality Guidelines for Europe (WHO 2000)
1,2,4-trichlorobenzene 1 hour mean 2,280 Old EAL derivation method from EH40/2001 OEL
1,2,4-trichlorobenzene Annual mean 76 Old EAL derivation method from EH40/2001 OEL
Trichloroethylene Annual mean 2 Hazard characterisation method for determining TCA (2021)
Trimethylbenzenes, all isomers or mixture 1 hour mean 37,500 Old EAL derivation method from EH40/2001 OEL
Trimethylbenzenes, all isomers or mixture Annual mean 1,250 Old EAL derivation method from EH40/2001 OEL
Vanadium 24 hour mean (short term) 1 WHO Air Quality Guidelines for Europe (2000)
Vinyl acetate 1 hour mean 7,200 Old EAL derivation method from EH40/2001 OEL
Vinyl acetate Annual mean 360 Old EAL derivation method from EH40/2001 OEL
Vinyl chloride 24 hour mean (short term) 1,300 Hazard characterisation method for determining TCA (2021)
Vinyl chloride Annual mean 10 Hazard characterisation method for determining TCA (2021)
Xylene (o-, m-, p- or mixed isomers) 1 hour mean 66,200 Old EAL derivation method from EH40/2001 OEL
Xylene (o-, m-, p- or mixed isomers) Annual mean 4,410 Old EAL derivation method from EH40/2001 OEL
Zinc oxide 1 hour mean 1,000 Old EAL derivation method from EH40/2001 OEL
Zinc oxide Annual mean 50 Old EAL derivation method from EH40/2001 OEL

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Updates to this page

Published 1 February 2016
Last updated 21 May 2024 + show all updates

  1. The main changes are to the ‘Calculating averaging periods’ section. Explained the new long term 24 hour mean EALs and how they can be screened against using the existing methods. Clarified which averaging times are long term (process contribution screen against 1%) and short term (process contribution screen against 10%). Clarified the two phases (2021 and 2023) of the EAL update project.

  2. Updated the section of the guidance on 'Grouping air emissions' to explain what to do if you release volatile organic compounds into the air.

  3. We have updated EALs for acrylamide, butadiene, cadmium, chromium III, copper, ethylene oxide, hydrogen chloride, hydrogen cyanide, mercury, methyl chloride (chloromethane), methylene chloride (dichloromethane), nickel and selenium, following our consultation, Review of Environmental Assessment Levels (EALs) for emissions to air: second phase.

  4. Updated the value for PM2.5 in the table 'Air Quality Standards Regulations 2010 Limit Values' from 25 micrograms per cubic metre to 20 micrograms per cubic metre.

  5. Added 'Central European Time' to the substance ozone under Screening for protected conservation areas.

  6. Updated the screening for protected conservation areas table - included a daily oxides of nitrogen critical level of 200 micrograms per cubic meter for detailed assessment where the ozone and sulphur dioxide concentrations are proven to be low. The 75 microgram per cubic meter daily oxides of nitrogen critical level remains in effect for all screening assessments. Added the ozone critical levels to be used in conjunction with daily oxides of nitrogen critical level of 200 micrograms per cubic meter. Updated the format of the environmental assessment levels (EALs) table. Corrected EAL averaging times to the appropriate time period. Gave carbon disulphide, hydrogen sulphide, methylene chloride and vanadium 24 hour EALs rather than the default 1 hour. This makes these EALs consistent with the originally published EAL list. Corrected the formaldehyde short-term EAL averaging time to 30 minutes to be consistent with the original published EAL. Added a 1 hour to 30 minute conversion factor to use with this EAL. Removed the vanadium long-term EAL because the short-term EAL is lower and more protective. Moved UK Air Quality Strategy objective for benzo(a)pyrene out of the EAL table to the Air Quality Standards Regulations 2010 Target Values and UK Air Quality Strategy objectives table.

  7. We have updated environmental assessment levels (EALs) for arsenic, benzene, chloroform, chromium VI, ethylene dichloride, methyl chloroform, naphthalene, tetrachloroethylene, trichloroethylene and vinyl chloride and added 2 new ones – mono-ethanolamine and N-nitrosodimethylamine (NDMA) following our consultation 'New air environmental assessment levels'.

  8. The following EALs have reverted back to their original levels as of 13/05/21 as consultation responses are still being processed for 'New air environmental assessment levels'. (arsenic, benzene, chloroform, chromium VI, ethylene dichloride, methyl chloroform, naphthalene, tetrachloroethylene, trichloroethylene and vinyl chloride and removed 2 new ones (mono-ethanolamine and N-nitrosodimethylamine (NDMA).

  9. Updated the screening distances for natural gas (or fuels with a similarly low sulphur content) fired combustion plants, with more than 500MW thermal input and some larger combustion plants using more sulphurous fuels with more than 50MW thermal input to within 15km of protected conservation areas. Also updated the EALs for arsenic, benzene, chloroform, chromium VI, ethylene dichloride, methyl chloroform, naphthalene, tetrachloroethylene, trichloroethylene and vinyl chloride and added 2 new ones (mono-ethanolamine and N-nitrosodimethylamine (NDMA)) following our consultation 'New air environmental assessment levels'

  10. New guidance added on deriving a new Environmental Assessment Level.

  11. Amendments to sections: 'Screen out insignificant PECs' now called 'Screen out PECs from detailed modelling' regarding the second stage of screening. And ‘Screening for protected conservation areas’ - a change was made which incorrectly pre-empted work currently being undertaken around thermal size and screening distances. The text has been changed back to the original text; 10km for an installation or 15km for a coal/oil fired power station. Where thermal size is large (greater than 50 megawatt) a larger screening distance may be more appropriate and it is recommended that further advice is sought from National Permitting Service.

  12. Minor changes to wording to clarify scientific and legal interpretation of definitions.

  13. First published.

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