Appendix C: summary of toxicological evidence for MEA and NDMA
Updated 3 September 2021
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This appendix contains additional information on the derivation of new environmental assessment levels (EALs) for mono-ethanolamine (MEA) and N-nitrosodimethylamine (NDMA).
Mono-ethanolamine (CAS Number 141-43-5)
MEA – also known as 2-aminoethanol, or ethanolamine – is a colourless, viscous liquid with an ammoniacal odour (HSE 2016), whose vapour is denser than air. It is widely used in industry in the production of detergents and soaps, dyestuffs, rubber vulcanisation, and as a scrubber for acidic gases in enclosed atmospheres such as submarines. MEA is used in a range of consumer products including cosmetics and personal care products, washing and cleaning products, coating products, biocides, inks and toners, and adhesives and sealants. It is rapidly biodegradable in the environment and has an estimated phototransformation half-life in air of about 11 hours (HSE 2016).
Regulatory standards
- none
Recommended environmental assessment level in air
-
long-term EAL – 0.1 mg/m3 as a 24-hour mean
-
short-term EAL – 0.4 mg/m3 as a 1-hour mean
Overview
There are few authoritative reviews on the adverse effects from exposure to MEA (CNESST 2019, HSE 2016, SCOEL 1996). It is a strong respiratory, ocular and skin irritant. CNESST (2019) concluded that MEA is a skin and respiratory sensitiser, but this opinion has been disputed (HSE 2001 and 2016).
Toxicokinetics
Although MEA is a normal component of the human diet, part of the membrane-constituting class of glycerophospholipids and a degradation product of the amino acid serine, there is no quantitative information on its systemic uptake through oral or inhalation routes (HSE 2016). SCOEL (1996) concluded that MEA is absorbed through the skin, lungs and gastrointestinal tract.
MEA is rapidly metabolised in the liver and incorporated into phospholipids in cellular membranes through the formation of phosphoryl ethanolamine and cytidinediphosphate ethanolamine (HSE 2016, SCOEL 1996). Excess MEA is converted through acetaldehyde to carbon dioxide and exhaled. In a study of dermal uptake in mice (Klain et al. 1985), MEA was widely distributed and extensively metabolised. Following inter-peritoneal injection in rats, the highest concentrations were reported in the fatty tissues of the spleen, kidneys and small intestine (Taylor and Richardson 1967).
Urea, glycine, serine, choline, and uric acid were the major urinary metabolites in mice (Klain et al. 1985).
Short- and long-term exposures
The acute toxicity of MEA is low (SCOEL 1996), but it is a respiratory, ocular, and skin irritant (HSE 2016, TCEQ 2015). No deaths or abnormal clinical signs after 14-days were reported in Sprague-Dawley rats exposed to 1,300 mg/m3 for 6 hours (a proprietary study conducted in 1988).[footnote 1] Necropsy findings were also reported to be ‘unremarkable’. In a sub-acute study, Wistar rats exposed to a respirable MEA aerosol at more than 50 mg/m3 for 6 hours a day, 5 days a week showed signs of respiratory irritation including submucosal inflammation and squamous metaplasia in the larynx and trachea.[footnote 2]
Repeated continuous exposure at levels above 66 ppm (168 mg/m3) caused pathological lesions to the lung, liver, kidneys, spleen and testes in dogs, guinea pigs, and rats exposed for up to 90 days (Weeks et al., 1960).[footnote 3] Dogs exposed at 66 mg/m3 showed immediate signs of restlessness and discomfort, indicated by nose-pawing, muzzle-licking, and shallow-rapid respiration. Exposure of rats, dogs and guinea pigs to MEA vapour was also reported to produce skin irritation at levels as low as 5 ppm (13 mg/m3), although SCOEL (1996) suggested that this may have been potentiated by direct skin contact with liquid that had condensed on the surface of the inhalation chamber. Localised respiratory inflammation was also observed at concentrations more than 50 mg/m3 in Wistar rats exposed for 6 hours a day, 5 days a week, for 4 weeks (proprietary study, see Pivotal Studies).
Respiratory sensitisation resulting in occupational asthma has been identified as a concern by CNESST (2019), but this opinion has been disputed (HSE 2001 and 2016, SCOEL 1996). Although several case reports (Gelfand 1963, Makela et al. 2011, Sallie et al. 1994, Savonius et al. 1994) have identified symptoms of respiratory sensitisation, there are difficulties in interpretation due to concomitant exposures, uncertainties in the concentration and duration of exposure, and other factors such as the sensitivity of those studied to multiple allergies. HSE (2016) noted that symptoms of occupational asthma had been identified in only a limited number of case reports despite the widespread use of MEA. In a mechanistic study by Kamijo et al. (2009) significant bronchoconstriction was observed in guinea pigs, but there was no evidence of respiratory sensitisation (HSE 2016). The authors suggested that the mechanism for bronchoconstriction possibly involved agnostic effects at the histamine H1 and muscarinic receptors.
Evidence for neurobehavioral effects have been reported. Repeated inhalation exposure at levels above 66 ppm (168 mg/m3) caused neurobehavioral changes in dogs, guinea pigs, and rats exposed continuously to MEA vapour for up to 90 days (Weeks et al., 1960). Rats exposed for 2 to 3 weeks to 5 ppm (13 mg/m3) exhibited lethargy (Weeks et al. 1960).
Evidence for reproductive toxicity has also been reported (Weeks et al. 1960, Mankes 1986), but SCOEL (1996) concluded that this occurred at exposure levels much higher than those that induced either irritation or neurobehavioral effects.
Genotoxicity and carcinogenicity
‘In vitro’ genotoxicity was investigated in 3 bacterial reverse mutation assays, a chromosome aberration assay in rat hepatocytes and 2 mammalian cell gene mutation assays (mouse lymphoma [L5178Y] and Chinese hamster lung fibroblasts [V79]). Negative results were reported in all studies (HSE 2016). Negative results were also obtained from an ‘in vivo’ mouse micronucleus test where clear signs of substance related toxicity were observed at the highest dose. HSE (2016) concluded that based on the tests performed, the results for MEA were consistently negative and that they gave no cause for additional concerns. SCOEL (1996) also reported that MEA was not mutagenic in bacteria and did not induce cell transformation.
No specific carcinogenicity studies have been reported (HSE 2016, SCOEL 1996). HSE (2016) noted that hyperplasia and metaplasia were observed in the respiratory tract in the 28-day proprietary study with Wistar rats (see Pivotal studies). However, they concluded that MEA is a corrosive substance and that the relevance of these respiratory tract lesions to human carcinogenicity was questionable.
Pivotal studies
In a sub-acute study submitted as evidence in support of an application under REACH (HSE 2016), Wistar rats were exposed to a respirable MEA aerosol at 10 mg/m3, 50 mg/m3, or 150 mg/m3 for 6 hours a day, 5 days a week, for 4 weeks.[footnote 4] Each concentration group consisted of 10 rats (5 of each sex). Animals were monitored for mortality, clinical signs of toxicity, bodyweight, food consumption, ophthalmological effects, haematological and clinical chemical effects, and were subject to necropsy at the end of the study including gross pathology and histological investigation. No systemic effects were observed at any concentration level. No histopathological effects were seen in any other organ outside the respiratory tract. Exposure at 150 mg/m3 resulted in:
- submucosal inflammation (levels I, II) in males and females
- degeneration of submucosal glands (level I) in males and females
- focal epithelial necrosis (level I) in males and females
- focal squamous metaplasia, (level I) in males and females; (level II) in 1 male and 2 females
- focal epithelial hyperplasia (level II) in males and females observed in the larynx
- focal squamous metaplasia (carina) accompanied by inflammation in males observed in the trachea
At 50 mg/m3, the following was reported in the larynx:
- submucosal inflammation (level I and II) in males and females
- squamous metaplasia (level I and II) in few males and females
No treatment-related weight changes, gross lesions or microscopic findings at the low concentration (10 mg/m3). HSE (2016) concluded that the NOAEC for localised and systemic effects were 10 mg/m3 and 150 mg/m3, respectively.
In a sub-chronic study by Weeks et al. (1960), male adult beagles (n=3 per exposure group), 6-week old male guinea pigs (n=22 or 30 per group), and 8-week old female rats (n=45 per group) were continuously exposed to MEA at concentrations of 12 to 26 ppm (30.5 – 66.0 mg/m3), or 66 to 102 ppm (167.6 to 259.1 mg/m3) for between 24 and 90 days. Additionally, 4 to 6 week old male and female rats (n=20) and male beagles (n=3) were exposed to 5 to 6 ppm (12.7 – 15.2 mg/m3) for 40 and 60 days, respectively.
Dogs tolerated a much higher concentration of MEA than rodents, with 2 dogs surviving 30-days exposure at the highest doses (167.6 to 259.1 mg/m3). At their respective highest dose, 83% of the rats and 75% of the guinea pigs died after 28- and 24-days exposure, respectively. The 2 surviving dogs developed lung irritation (that is, moist rales) by the middle of the second week, which was associated with a low grade fever that ran a course of about 2 weeks. Depressed, lethargic, and apathetic states were noted in all animals that survived the high dose. Other common effects like skin lesions on ground contact points (feet, nose, lips, and chin) and skin points of tension (around extensor surface of larger joints) showed dark eschars (that is, dry scabs that form on burned skin) which covered ulcerated skin beneath. These skin-related effects may be associated with the animals constantly in contact with MEA condensate as it accumulated on the walls and floors of the exposure chambers throughout the experiments.
All animals survived their respective intermediate concentrations (30.5 to 66.0 mg/m3) of MEA vapour for 90 days. Signs and symptoms were similar to those seen at the higher concentrations, but not so severe. Dogs exposed to 66.0 mg/m3 showed ‘immediate’ signs of restlessness and discomfort, indicated by nose-pawing, muzzle-licking, and shallow-rapid respiration. Whilst the specific duration is not stated, as irritation is primarily concentration-dependent, it was assumed the ‘immediate’ irritant effects reported would also occur at a 1-hour duration). Throughout the experiment these dogs were more irritable than controls, and after a few days of exposure were less alert and bordered variably on lethargy. Slight tremors of rear leg muscles were also noted. Also, skin at floor contact points on the chest and scrotum of the dogs became irritated, which was relieved by ointment.
Dogs exposed to 30.5 mg/m3 for 90 days did not show immediate behavioural changes. No significant weight changes occurred nor did physical examinations reveal any changes. However, after several days their skin became irritated and soothing ointment was applied, which relieved the condition and the skin showed no further signs of irritation. Concurrently, lethargy or depression appeared and lasted about 3 weeks before their behaviour returned to normal. Rodents exposed to 30.5 to 38.1 mg/m3 became less active than the controls after about 3 days, and showed definite lethargy after about 10 days, which lasted throughout the balance of the exposure. In addition to hair loss, rodents showed an approximate 10% reduction in weight gain and approximately a 40% increase in water consumption.
For the low exposure group, young (4 to 5 weeks old) male and female rats and mature beagles were exposed to 12.7 to 15.2 mg/m3 for 40 days and 60 days, respectively. All animals survived exposure to these low concentrations. Neurobehavioral changes in animals were noted after 2 to 3 weeks of continuous exposure at these concentrations. In dogs, a slight decrease in alertness and activity was noted. Two of the 3 exposed dogs also showed slight weight loss concurrently. No changes from normal were observed in pulse, temperature, and heart and lung sounds. Skin irritation and hair loss occurred on chest-floor contact areas, and the scrotum became bare and spotted with small scattered black eschars. All rats exposed to 12.7 mg/m3 showed pelt discoloration after 12 days and transitory hair loss on the head and back after 3 weeks, which was more pronounced in the females. Additionally, some slowness in movement developed in rats after 3 weeks, which lasted throughout the 40-day exposure duration.
Based on the results of this study, 30.5 mg/m3 was selected by TCEQ (2015) as the acute LOAEC for nasal irritation symptoms (for example, nose-pawing, shallow breathing) in dogs. Neurobehavioral changes seen in rats and dogs at 12.7 mg/m3 were selected by TCEQ (2015) and HSE (2016) for the chronic LOAEC.
Short-term exposure
HSE (2016) and TCEQ (2015) have proposed HBGVs for MEA.
Health and Safety Executive
HSE (2016) proposed a DNEL of 3.8 mg/m3 to protect the general public from local and systemic effects from acute exposure. It was based on the 15-minute STEL of 7.6 mg/m3, which had been recommended by SCOEL (1996) to prevent worker exposure to irritating levels. In the study by Weeks et al. (1960), exposure of rats, guinea pigs, and dogs to MEA vapour had produced skin irritation at levels as low as 12.7 mg/m3. HSE (2016) applied a further UF of 2 to account for wider sensitivity in the general population.
Texas Commission on Environmental Quality
TCEQ (2015) proposed an acute air monitoring comparison value (AMCV) of 0.32 mg/m3. It was based on a NOAEC of 30.5 mg/m3 for behavioural signs of nasal irritation (nose-pawing, shallow breathing) in dogs subject to continuous exposure for up to 90 days (Weeks et al. 1960). A UF of 90 was applied (a factor of 3 for interspecies variation, a factor of 10 for intra-species variation, and a further factor of 3 for deficiencies in the database for acute toxicity). No adjustment was applied to the use of a sub-chronic study because irritation was considered a concentration-based effect.
Long-term exposure
HSE (2016), TCEQ (2015), and the industry REACH dossier have proposed HBGVs.
Health and Safety Executive
HSE (2016) derived a DNEL of 0.5 mg/m3 to protect the general public from systemic effects from long-term exposure. It was based on the OEL of 2.5 mg/m3 as an 8-hour TWA derived by SCOEL (1996), which itself used the LOAEC of 12.7 mg/m3 for signs of lethargy and sluggish movement seen in rodents (Weeks et al. 1960) and a UF of 5 for interspecies variation. HSE (2016) corrected the OEL to continuous exposure (8/24) and divided it by an additional UF of 2 for wider sensitivity in the general population.
REACH chemical dossier
The industry REACH dossier for MEA on the ECHA dissemination portal derived a DNEL of 2 mg/m3 to protect the general public from localised effects from long-term exposure.[footnote 5]. It was based on a NOAEL of 10 mg/m3 from a proprietary sub-acute study in Wistar rats for inflammation and lesions in the respiratory tract (see Pivotal studies). Concentration was considered more important than exposure duration for an irritant effect and therefore no correction was applied for intermittent exposure. A UF of 5 was applied (a factor of 1 for interspecies variation and a factor of 5 for intra-species variation).
Texas Commission on Environmental Quality (TCEQ)
TCEQ (2015) proposed a chronic AMCV of 0.023 mg/m3, which was based on a LOAEC of 12.7 mg/m3 for signs of lethargy and sluggish movement observed in rodents (rats and guinea pigs) exposed to MEA vapour for up to 90 days (Weeks et al. 1960). A UF of 540 was applied, a factor of:
- 3 for extrapolation from a LOAEC to NOAEC
- 3 for interspecies variation
- 10 for intra-species variation
- 2 for the sub-chronic to chronic exposure duration
- 3 for deficiencies in the database for chronic toxicity
Summary
Several authoritative organisations have proposed HBGVs for MEA, although the overall toxicological database is small.
Short-term exposure guidelines have been proposed by HSE (2016) and TCEQ (2015) based on localised irritation observed in rodents (rats and guinea pigs) and dogs in a repeat dose inhalation study (Weeks et al. 1960). Although TCEQ (2015) used a higher LOAEC of 30.5 mg/m3 as the POD, they also applied a larger UF, which lead to a lower guideline.
Table 1: Summary of health-based guidance values for short-term exposures
| Guideline | Value (mg/m3) | Duration | Critical effect | Pivotal reference(s) |
|---|---|---|---|---|
| Current EAL | None | - | - | - |
| AMCV | 0.32 | 0.5 – 1-hour | Irritation | Weeks et al. 1960 |
| DNEL (HSE 2016) | 3.2 | 15 mins | Irritation | Weeks et al. 1960 |
Long-term chronic exposure guidelines were proposed by HSE (2016), TCEQ (2015), and the industry REACH dossier. HSE (2016) and TCEQ (2015) used the same endpoint of neurobehavioral effects seen in rodents and dogs (Weeks et al. 1960), whilst the industry REACH dossier used respiratory irritation observed in a sub-acute rodent study (see Pivotal Studies). The POD were similar (10 or 12.7 mg/m3) and the wide difference in health-based guidance values is explained by the choice of UF (5, 10, and 540).
Table 2: Summary of health-based guidance values for long-term exposures
| Guideline | Value (mg/m 3) | Duration | Critical effect | Pivotal reference(s) |
|---|---|---|---|---|
| Current EAL | None | - | - | - |
| AMCV | 0.0.23 | Life-time | Neurobehavioral | Weeks et al. 1960 |
| DNEL (HSE 2016) | 0.5 | Life-time | Neurobehavioral | Weeks et al. 1960 |
| DNEL (REACH dossier) | 2 | Life-time | Irritation | Propriety Study 2010 |
Recommendations
Short-term EAL
The critical health effect from short-term inhalation exposure to MEA vapour is considered to be localised respiratory irritation. The pivotal study for the derivation of a short-term EAL is the sub-acute duration rodent study submitted as evidence in support of an application under REACH (HSE 2016). In this study, Wistar rats were exposed to a respirable aerosol at 10 mg/m3, 50 mg/m3, or 150 mg/m3 for 6 hours a day, 5 days a week, for 4 weeks. The POD is considered to be the NOAEC of 10 mg/m3, which was identified for localised irritation of the respiratory tract (HSE 2016). No correction for continuous exposure is applied to the POD because irritation is considered a concentration-dependent effect. The short-term EAL of 0.4 mg/m3 is obtained by dividing the POD from a relevant sub-acute animal study by a UF of 25 (a factor of 2.5 for interspecies variation and a factor of 10 for intra-species variation). This is consistent with the approach to the derivation of an acute DNEL for respiratory irritation under REACH.
Long-term EAL
The critical health effects from long-term inhalation exposure are considered to be respiratory irritation and neurobehavioral toxicity. In accordance with the guidance for the derivation of EALs, the DNEL proposed by HSE (2016) would be recommended as the long-term EAL because it was proposed by a UK authoritative body. However, it is derived from the OEL proposed by SCOEL (1996). Most notably, HSE (2016) corrected for continuous exposure from the OEL (8 hours to 24 hours) despite the observation that the underlying study by Weeks et al. (1960), on which the OEL was based, used a regime of continuous and not intermittent exposure. The high UF adopted by TCEQ (2015) in the derivation of the chronic AMCV is considered a little over-cautious. Therefore the pivotal study for the derivation of a long-term EAL is considered to be the same sub-acute rodent study used for the short-term EAL. The POD is considered to be the NOAEC of 10 mg/m3, which was identified for localised irritation of the respiratory tract (HSE 2016). The long-term EAL of 0.1 mg/m3 is obtained by dividing the POD by a UF of 100 (a factor of 10 for interspecies variation and a factor of 10 for intra-species variation). No factor for sub-acute to chronic duration is required because irritation is considered a concentration-based effect.
Abbreviations and definitions
AMCV
Air monitoring comparison values are established in the state of Texas to evaluate the potential effects as a result of exposure to chemicals in air. They are not considered ambient air standards. Exceedances do not necessarily indicate a problem, but triggers a more in-depth review.
DNEL
Derived no-effect level is defined as the level of exposure to a substance above which humans should not be exposed. DNEL apply to health effects with a threshold.
EAL
Environmental assessment level
ECHA
European Chemicals Agency
HBGV
Health-based guidance value
HSE
Health and Safety Executive
LOAEC
Lowest observable adverse effect concentration
NOAEC
No observed adverse effect concentration
OEL
Occupational exposure limit are broadly defined as a measurable concentration of a substance in air that represents a point of reference for the development of workplace strategies to protect workers from health risks associated with inhalation of chemical substances.
POD
Point of departure
REACH
Registration, Evaluation, Authorisation and Restriction of Chemicals is a European Union regulation that addresses the production and use of chemical substances and their potential impact on human health and the environment.
STEL
Short-term exposure limit (usually 15 minutes) for use in the workplace
UF
Uncertainty factor
References
HSE, 2016. Substance Evaluation Report for 2-aminoethanol, Version 2. Health and Safety Executive.
SCOEL, 1996. Recommendation from the Scientific Committee on Occupational Exposure Limits for Ethanolamine. SCOEL/SUM/24.
N-nitrosodimethylamine (NDMA)
N-nitrosamines are a group of hydrocarbons with the generic chemical formula of (R1R2)-N-N=O, where R1 and R2 are alky groups (SEPA 2014). They are formed primarily from the reaction of amines with oxidising agents including chlorine disinfectants, nitrites, and atmospheric nitrogen oxides and are detected in drinking water, food stuffs, personal care products, and tobacco smoke. Secondary amines are the most susceptible to nitrosating agents (SCCS 2011). N-nitrosamines have also been measured in flue gases from CC systems (see Table 1 for examples from various studies), which use a number of amines and amine-based solvents as reagents (SEPA 2014). Atmospheric N-nitrosamine concentrations are a steady-state balance of the rate of formation from ongoing emissions of amines and the rapid rate of removal including photolysis and wash out (many N-nitrosamines are water soluble).
Table 1: Specific N-nitrosamines
| N-nitrosamine | CAS reference |
|---|---|
| N-nitrosodibutylamine (NDBA) | CAS 924-16-3 |
| N-nitrosodiisobutylamine (NDIBA) | CAS 997-95-5 |
| N-nitrosodimethylamine (NDMA) | CAS 62-75-9 |
| N-nitrosodiethylamine (NDEA) | CAS 55-18-5 |
| N-nitrosodiethanolamine (NDELA) | CAS 1116-54-7 |
| N-nitrosodiisononylamine (NDINA) | CAS 643014-99-7 |
| N-nitrosomorpholine (NMOR) | CAS 59-89-2 |
| N-nitrosopiperidine (NPIP) | CAS 100-75-4 |
NDMA, (CH3)2N3O, is the simplest structural N-nitrosamine and is one of the most widely studied in terms of its human and environmental toxicity (SEPA 2014). It is a volatile, combustible, yellow oily liquid at room temperature, miscible with water, slightly lipophilic with a log Kow of 0.57, and moderately volatile with a Henry’s Law Constant of 3.34 Pa/m3/mol (IPCS 2002). NDMA has been widely detected in cosmetics, food, medicines, and drinking water (IPCS 2002, SEPA 2014, and US FDA 2019). In response to the public consultation on the derivation of new environmental assessment levels, Environment Agency (2015) concluded that “… for the class of amine degradation products known as nitrosamines, we will establish the EAL for N-nitrosodimethylamine (NDMA) (CAS No. 62-75-9).” Therefore this review focuses on the derivation of an EAL for NDMA.
Regulatory standards
- none
Recommended environmental assessment level in air
-
long-term EAL – 0.2 ng/m3 as an annual average
-
short-term EAL – none
Overview
There are several authoritative reviews on the adverse effects from exposure to NDMA (ATSDR 1989, IARC 1987, NIPH 2011, SCCS 2011, US EPA 1987, and IPCS 2002).[footnote 6] N-nitrosamines are amongst the most potent carcinogens (SCCS 2011). However, most available evidence concerns its oral toxicity. NDMA is likely to be highly carcinogenic to humans and information on other adverse health effects is limited (IPCS 2002).
Toxicokinetics
IPCS (2002) reported that there was limited quantitative data on human absorption of NDMA by any route of exposure. NDMA has been detected in the urine of rats and dogs exposed through inhalation, which suggests that it is absorbed by the pulmonary system (ATSDR 1989). However, reliable quantitative data for the inhalation route is unavailable. In oral studies, it is rapidly and extensively absorbed (up to 90%) through the lower intestinal tract (IPCS 2002). IPCS (2002) also inferred that NDMA is absorbed through the skin (around 0.03%) because it was found in rat urine after dermal administration.
NDMA was widely distributed through systemic circulation in several animal studies (IPCS 2002). It was found in mother’s milk and in the foetus of rats after injection into the maternal bloodstream. Pharmacokinetic studies in several animal models has revealed that NDMA is rapidly metabolised in the liver and excreted through the urine. Metabolism involves either hydroxylation, which leads to the formation of formaldehyde and monomethylnitrosamine, or denitrosation, which leads to the formation of formaldehyde and methylamine, through the cytochrome P450 (CYPE2E1).
Monomethylnitrosamine is unstable and readily rearranges to the strongly reactive methyldiazonium ion, which is known to alkylate biological macromolecules including DNA, RNA and various proteins (IPCS 2002). The methyldiazonium ion is an important agent in the observed hepatotoxicity of NDMA (Lee et al. 1996). Whilst quantitative information in human studies is limited, IPCS (2002) and SCCS (2011) concluded that investigations using human liver preparations showed little qualitative differences with animal metabolism.
NDMA and its metabolites were excreted through urine in animal studies (IPCS 2002). Radio-labelled carbon dioxide was detected in exhaled air 1-hour after intraperitoneal administration to rats (ATSDR 1989). In an inhalation experiment in beagles, 23% of the administered dose was exhaled in 30-mins after a 3-hour exposure duration (Raabe 1986).
Short- and long-term exposures
In human case reports, effects from exposure to unspecified amounts of NDMA in air for relatively short periods have included enlarged liver and spleen, hepatic cirrhosis, jaundice, ascites, and death (ATSDR 1989, IPCS 2002). Liver failure, brain haemorrhage, and death has been attributed to acute ingestion events of unspecified amounts of NDMA. One further case involved ingestion of at least 4 doses of about 250 to 300 mg over a 2-year period (IPCS 2002).
NDMA was acutely toxic to animals through inhalation with 4-hour LC 50 of 78 ppm (240 mg/m3) in rats and 57 ppm (176 mg/m3) in mice, and 2 of 3 dogs had died a day after exposure to 16 ppm (49 mg/m3) for 4-hours (ATSDR 1989).[footnote 7] Hepatotoxicity was the prominent effect observed in animals through inhalation (ATSDR 1989). In all 3 species, exposure produced haemorrhagic necrosis in the liver, whilst an increased blood clotting time was also observed in the dogs. ATSDR (1989) concluded that high concentrations of NDMA vapours are likely to be irritating, but there was limited information available on dermal and ocular effects. Doolittle et al. (1984) reported reddened eyes and piloerection in rats exposed to 500 ppm (1,540 mg/m3) or 1000 ppm (3,080 mg/m3 for 4-hours. No evidence for sensitisation was identified in the available literature (IPCS 2002).
IPCS (2002) found no repeat dose studies in humans or animals identified for the inhalation route. Hepatic effects including hepatocyte vacuolization, portal venopathy, and tissue necrosis or haemorrhage were observed in a number of mammalian species in sub-acute studies (up to 34-days) at ingested doses greater than 0.2 mg/kg bodyweight/day (ATSDR 1989). In addition, ‘congestion’ was reported in other organs including the kidneys, lung, spleen, and myocardium (IPCS 2002) and gastrointestinal haemorrhaging was also observed in rats administered 10 mg/kg bodyweight/day (Barnes and Magee 1954) and in mink administered up to 0.6 mg/kg bodyweight/day for up to 37-days (Carter et al. 1969). Renal effects including glomerulus dilation and slight thickening of the Bowman’s capsule were observed in mink receiving 0.2 mg/kg bodyweight/day for an unspecified period (Martino et al. 1980).
No inhalation studies in humans or animals of the reproductive toxicity, neurotoxicity, and immunotoxicity of NDMA were identified and data from oral studies was very limited (IPCS 2002). Interpretation of the results is complicated by the high doses administered, which would probably have resulted in acute or repeat dose organ toxicity (or both). Embryo toxicity and lethality in single dose developmental studies has been reported at doses from 20 to 30 mg/kg bodyweight/day and a range of reversible immunological effects have been observed in animals at levels as low as 5 mg/l in drinking water (IPCS 2002). A two-fold increase in stillbirths and neonatal deaths (combined) was observed in mice administered an estimated daily intake of 0.02 mg/kg bodyweight/day through drinking water for 75-days before mating and throughout pregnancy and lactation (Anderson et al. 1978). However, exposure had no effect on maternal fluid consumption, litter size, or average bodyweight of the weanlings, and there were no consistent gross or histopathological abnormalities observed in the stillborn foetuses or dead neonates to account for increased mortality (IPCS 2002). Moreover, increased mortality was not observed in another study in which mice were administered a single intra-peritoneal injection of 7.4 mg/kg bodyweight on GD16 or GD19 (Anderson et al. 1989).
Genotoxicity and carcinogenicity
In numerous ‘in vitro’ studies conducted in bacterial and mammalian cells, there is strong evidence that NDMA is mutagenic and clastogenic (ATSDR 1989, IARC 1978, IPCS 2002, and US EPA 1987). Increased frequencies of gene mutations, chromosomal damage, sister chromatid exchange, and unscheduled DNA synthesis were observed in a wide variety of cell types, in assays conducted in the presence or absence of metabolic activation. Positive results were observed in human as well as rodent cells (IPCS 2002).
Similarly, clear evidence of genetic effects has been observed in, mainly oral, ‘in vivo’ studies (IPCS 2002). Clastogenic effects including micronuclei, sister chromatid exchange, and chromosomal aberrations were reported in hepatocytes, bone marrow, spleen cells, peripheral blood lymphocytes, oesophageal cells, and kidney cells in rodents after dietary exposure or intraperitoneal injection. An increased frequency of micronucleated bone marrow cells was observed in female mice after inhalation exposure to 1,030 mg/m3 (Odagiri et al. 1986). In oral and injection studies, evidence of DNA damage has been observed in the liver, kidneys, and lungs of rats, mice, and hamsters (IPCS 2002).
Relevant epidemiological studies have suggested an association between dietary exposure to NDMA and cancers of the stomach, upper digestive tract, bowel and lung (IPCS 2002). Dietary exposure was estimated from reported levels in foodstuffs combined with patient recollection of food consumption either in the preceding year before illness (González et al. 1994, Goodman et al. 1992, and Pobel et al. 1995) or over the preceding 5 to 10 year period (De Stefani et al. 1996). In a population-based cohort study, Knekt et al. (1999) reported an increased relative risk of bowel cancer in the group with the highest dietary intake. IPCS (2002) concluded that the human database was still rather limited and that the studies all had various methodological issues. For example, use of dietary recall and only partial matching or control of confounders such as alcohol consumption.
Most of the older animal studies were also considered limited by current standards. However, IPCS (2002) concluded that the weight of evidence of the carcinogenicity of NDMA in mammalian species was consistent and convincing, and the pattern of tumour development was characteristic of direct interaction with genetic material. IARC (1978, 1987) concluded that there was sufficient and convincing evidence from animal studies that NDMA was probably carcinogenic to humans (Group 2A).
NDMA was shown to induce tumours in hamsters, mice, and rats, at relatively low doses, irrespective of the route of exposure (oral or inhalation), in a wide range of tissues including the liver, Leydig cells, lungs, kidney, and nasal cavity (IPCS 2002, NIPH 2011). Tumours were induced in the lung, liver and kidney in mice and rats exposed to 0.2 mg/m3 for 17 or 25-months, respectively (IARC 1978). Marked increases in tumours of the nasal cavity were observed in female rats exposed intermittently (4-times a week, 4 to 5 hours a day) to NDMA concentrations of 0.12, 0.6 and 3 mg/m3 for 30-weeks (Klein et al. 1991).
In the most recent chronic oral study, an increased incidence of hepatic tumours was observed at doses as low as 0.1 mg/kg body weight/day in Colworth-Wistar rats administered NDMA in drinking-water (Brantom 1983, Peto et al. 1984, 1991a and 1991b). Although it was only investigated in a limited number of studies, tumour formation occurred in the absence of significant non-neoplastic effects and the time-scale to first tumour was relatively short (IPCS 2002). Tumour incidence was increased following administration of a single dose or repeated doses for short periods (around 2 or 3 weeks) and they were also observed in the offspring of exposed pregnant rats and mice.
The mode of action of the observed carcinogenicity of NDMA has been attributed to its metabolism and the generation of the methyldiazonium ion (IPCS 2002). Lee et al. (1996) concluded that formation of the ion, a powerful alkylating agent, through the hydroxylation metabolic pathway, was critical to NDMA hepatotoxicity. DNA adducts formed included N 7-methylguanine (65% of all adducts initially formed) and O 6-methylguanine (about 7% of adducts). In particular, the formation and persistence of O 6-methylguanine (specifically the activity of O 6-methylguanine DNA-methyltransferase) correlated with observed variations in tumour incidence among different animal species and genetic strains (IPCS 2002). O 6-methylguanine has been detected in human tissues exposed to NDMA (Herron and Shank 1980) and it is considered that the pathways for NDMA metabolism are similar in humans and rodents (IPCS 2002). SCCS (2012) noted that activity of the cytochrome P450 (CYPE2E1) was only 20% of adult activity in neonates, but children between 1 and 10 years old had only slightly lower activities compared to adults. They concluded that no adjustment factor for toxicokinetic age differences was required because the extent of bioactivation in children was unlikely to be higher compared to adults.
Pivotal studies
In a long-term study by Klein et al. (1991), 4 groups of 36 female rats were exposed to 0.04, 0.2, or 1 ppm of NDMA vapour (equivalent to 0.12, 0.6, and 3 mg/m3), 4 times a week, 4 to 5 hours a day, for up to 30-weeks. Length of exposure varied between animals within dose groups from a minimum of 57 days to a maximum of 207 days. A summary of the main findings is shown in Table 2.
Table 2: Histopathological findings of nasal tumours in rats after long-term exposure (Klein et al. 1991)
| Findings | NDMA dose group 0 mg/ m3 | NDMA dose group 0.12 mg/ m3 | NDMA dose group 0.6 mg/ m3 | NDMA dose group 3.0 mg/ m3 |
|---|---|---|---|---|
| Esthesioneuroblastomas | - | 2 | 2 | 9 |
| Mucoepidermoid tumours (inc. carcinomas) | - | 11 | 30 | 7 |
| Squamous-cell carcinomas | - | - | 2 | 1 |
| Neurogenic sarcomas | - | - | - | 1 |
| Osteogenic sarcomas | - | - | - | 2 |
| Total number of tumour-bearing animals | 0 | 13 | 31 | 19 |
A significant reduction in median survival time (9 months) was observed in the rodents treated in the highest dose group (3 mg/m3). Tumours occurred mainly in the nasal cavity and were observed in all dose groups: 0.12 mg/m3 (13/36), 0.6 mg/m3 (31/36), and 3 mg/m3 (19/36). At the highest exposure, 47% of nasal tumours were esthesioneuroblastomas, with a median manifestation time of 320 days, whereas only 6% and 15% of this tumour type were observed at 0.6 mg/m3 and 0.12 mg/m3, respectively. These tumours were found in the olfactory part of the nasal mucosa. Mucoepidermoid tumours were the most prevalent in the lower dose groups and were located mainly in the respiratory part of the nasal mucosa. Squamous-cell carcinomas were found in the anterior nasal cavity and were localised in the bones of the nasal or basal skull. No tumours were seen in respiratory or olfactory regions of the septal mucosa. Klein et al. (1991) suggested that their observations indicated that both tumour manifestation and tumour type was dependent on the exposure concentration of NDMA and, or the total amount inhaled during the study by each dose group.
In a long-term drinking-water study designed to provide detailed information on dose-response (Brantom 1983, Peto et al. 1984, 1991a and 1991b), 15 dose groups of 60 male and 60 female Colworth-Wistar rats were exposed to NDMA at a wide range of drinking-water concentrations from 0.033 to 16.8 mg/l (equivalent to estimated daily intakes of between 0.001 to 0.697 mg/kg bodyweight/day in males and 0.002 to 1.224 mg/kg bodyweight/day in females). A control group consisted of 120 male and 120 female rats. Groups of animals were taken for interim sacrifice after 12 and 18-months.
Survival of the animals reduced with increasing dose and animals in the highest dose group did not last longer than 12-months. There were no significant differences in body weight between the animals in the control and dose groups. A dose related increase in the incidence of liver tumours (especially hepatocellular carcinomas and biliary cystadenomas) incidence was observed in male and female rats (see Table 3). Non-neoplastic hepatic effects included formation of hyperplastic nodules and shrinkage of hepatocytes.
Table 3: Incidence of hepatic tumours observed in male and female Colworth-Wistar rats after life-time exposure (up to 2-years) to NDMA in drinking water (Brantom 1983, Peto et al. 1991a and 1991b)
Tumour types included carcinomas (C), haemangiosarcomas (H), and biliary cystadenomas (BC). % animals of each dose group with evidence of tumour at death or sacrifice. Group 1 consisted of 192 animals (the control group) and Groups 2 to 16 consisted of 48 animals (the dose groups).
Male
| Group | Dose mg/kg/day | Animals with C (%) | Animals with H (%) | Animals with BC (%) |
|---|---|---|---|---|
| 1 | 0 | 1 | 1 | 1 |
| 2 | 0.001 | 2 | 0 | 4 |
| 3 | 0.003 | 2 | 0 | 4 |
| 4 | 0.005 | 4 | 2 | 4 |
| 5 | 0.011 | 2 | 4 | 4 |
| 6 | 0.022 | 6 | 0 | 2 |
| 7 | 0.044 | 10 | 2 | 2 |
| 8 | 0.065 | 13 | 2 | 8 |
| 9 | 0.087 | 10 | 13 | 13 |
| 10 | 0.109 | 25 | 13 | 23 |
| 11 | 0.131 | 29 | 29 | 27 |
| 12 | 0.174 | 33 | 21 | 25 |
| 13 | 0.218 | 58 | 6 | 29 |
| 14 | 0.261 | 60 | 15 | 40 |
| 15 | 0.348 | 77 | 6 | 29 |
| 16 | 0.697 | 88 | 6 | 4 |
Female
| Group | Dose mg/kg/day | Animals with C (%) | Animals with H (%) | Animals with BC (%) |
|---|---|---|---|---|
| 1 | 0 | 1 | 1 | 2 |
| 2 | 0.002 | 0 | 2 | 2 |
| 3 | 0.005 | 0 | 0 | 8 |
| 4 | 0.010 | 4 | 2 | 0 |
| 5 | 0.019 | 4 | 0 | 6 |
| 6 | 0.038 | 10 | 2 | 10 |
| 7 | 0.076 | 6 | 4 | 15 |
| 8 | 0.115 | 10 | 2 | 71 |
| 9 | 0.153 | 10 | 6 | 69 |
| 10 | 0.191 | 8 | 2 | 83 |
| 11 | 0.229 | 13 | 6 | 92 |
| 12 | 0.306 | 15 | 4 | 90 |
| 13 | 0.382 | 25 | 0 | 85 |
| 14 | 0.459 | 38 | 0 | 69 |
| 15 | 0.612 | 69 | 6 | 33 |
| 16 | 1.224 | 73 | 10 | 8 |
HBGV for short-term exposure
No authoritative HBGV for NDMA were identified.
HBGV for long-term exposure
Only NIPH (2011) and US EPA (1987) have proposed HBGV for NDMA. IPCS (2011) and SCCS (2011) have estimated relevant PODs from the available animal data, which has been used to characterise risks to the general public using a margin of exposure approach. Discussion of these evaluations has been included because they provide additional characterisation of the carcinogenicity of NDMA and evaluation of one of the two pivotal studies (Brantom 1983, Peto et al. 1984, 1991a and 1991b).
Norwegian Institute of Public Health
NIPH (2011) proposed a DMEL of 0.3 ng/m3 for NDMA which was based on a TDL05 of 0.018 mg/kg bodyweight/day for BC in female rats (IPCS 2002). The POD was derived from the 2-year drinking-water study in Colworth-Wistar rats (Brantom 1983, Peto et al. 1991a and 1991b). Linear extrapolation was used to go from the experimental TDL05 to an ELCR of 1 in 1,000,000 in the general population using a unit risk of 2.8 x 10-6 per ng/kg bodyweight (0.05/18,000). The dose of 0.35 ng/kg bodyweight (1 / 2.8) was converted to an equivalent air concentration of 0.31 ng/m3 using the allometric and exposure scaling factor from rats to humans (divide by 1.15 m3/kg bodyweight) recommended in the REACH guidance (ECHA 2012).[footnote 8] NIPH (2011) also calculated a T25 value of 0.15 mg/kg bodyweight/day for the incidence of BC in female rats (Peto et al. 1991a and 1991b), but the details of its derivation from the original data were not reported.
For comparative assessment, NIPH (2011) also derived a HBGV of 0.3 ng/3, at an ELCR of 1 in 100,000 or lower, from the incidence of tumours in the nasal cavity of female rats (Klein et al. 1991). The method used was reported to be consistent with the ‘large assessment factor’ approach for the derivation of a DMEL using the REACH guidance (ECHA 2012). However, no further information was provided. NIPH (2011) acknowledged problems with the Klein et al. (1991) study including the short and intermittent exposure pattern and uncertainties related to the actual exposure regimen. However, they concluded that this and similar studies suggested that NDMA is a more potent carcinogen through inhalation.
Scientific Committee on Consumer Safety
The European Commission’s Scientific Committee on Consumer Safety (SCCS) reviewed the origin and toxicity of nitrosamines and secondary amines in cosmetic products and derived dose descriptors for the carcinogenic potency of a number of compounds including NDMA (SCCS 2011 and 2012). They proposed a T25 of 0.058 mg/kg bodyweight/day and a BMDL10 of 0.027 mg/kg bodyweight/day for NDMA.
The individual T25 values were calculated (as described in Dybing et al. 1997) for a number of relevant studies in male and female rats and in most cases was based on malignant liver tumours (see Table 4). The T25 value was the mean of the combined response in males and females across four studies (n = 6).
Table 4: Calculation of T25 for NDMA from carcinogenicity studies in rats (SCCS 2011)
| T25mg/kg bw/day | Species and sex | Target | Exposure regime | Lowest tumour frequency % | Reference |
|---|---|---|---|---|---|
| 0.11 | F344 male | Lung | 30w / gavage | 32 | Lijinsky et al 1987 |
| 0.041 | F344 male | Liver | 30w / drinking water | 50 | Lijinsky and Reuber 1984 |
| 0.032 | F344 female | Liver | 30w / drinking water | 45 | Lijinsky and Reuber 1984 |
| 0.059 | Wistar male | Liver | 104w / drinking water | 22 | Peto et al 1991a |
| 0.044 | Wistar female | Liver | 104w / drinking water | 45 | Peto et al 1991a |
| 0.061 | Wistar female | Liver | 93w / feed | 18 | Arai et al. 1979 |
SCCS (2011) applied the BMD approach to tumour incidence data from the 2-year drinking water study in Colworth-Wistar rats (Peto et al. 1991a) using the US EPA Software BMDS 2.1.1. The incidence of all fatal liver tumours for the low dose range (control and the six lowest dose groups) were modelled for the following endpoints: liver cells, bile ducts, mesenchyme, and Kupffer cells. Larger dose ranges were included for some models. Data for males and females were modelled separately. The BMDL10 of 0.64 ppm for male rats was selected as the lowest reliable POD and the full modelled dataset, as reported, is shown in Table 5.[footnote 9] It was converted to a BMDL10 of 0.027 mg/kg bodyweight/day using the dosing conversions of O’Brien et al. (2006) from the drinking-water concentrations reported in the original study (Peto et al. 1991a and b).[footnote 10]
US EPA IRIS database
US EPA (1987) recommended a unit risk of 1.4 x 10-2 per μg/m3 for life-time exposure, which was based on a simple route-to-route extrapolation of the modelled hepatic cancer risk (all tumour types) from the 2-year carcinogenicity study of NDMA administered in drinking water to Colworth-Wistar rats (Peto et al. 1984).[footnote 11] According to US EPA (1987), the Peto et al. (1984) study design had been optimised specifically for using the Weibull model and so it was used to conduct the linear extrapolation from the experimental data. After correction for background response, the oral slope factor of 7.8 (mg/kg)/day was adjusted for the difference in bodyweight between rat and human to 51 (mg/kg)/day.[footnote 12] The unit risk per μg/m3 was calculated from the slope factor by assuming a human bodyweight of 70 kg and an inhalation rate of 20 m3/day. An ELCR of 1 in 100,000 corresponded to an air concentration of 0.7 ng/m3.
Table 5: Results of benchmark dose modelling and calculation of BMD10 and BMDL10 values for the incidence of liver tumours in male rats exposed to drinking water in a 2-year study (Peto et al. 1991a)
Dose conversion from ppm to mg/kg bw/day after O’Brien et al. (2006). The ‘bold’ row indicates the value selected for the risk assessment.
| Model | Dose groups (max 16) | Model acceptance p-value | BMD10 ppm | BMDL10 ppm | BMDL10 Mg/kgbw/day |
|---|---|---|---|---|---|
| Full | 7 | ||||
| Reduced | 1 | ||||
| Probit | 2 | 0.15 | 1.2 | 0.84 | 0.035 |
| Probit | 16 | <0.00000001 | 1.50 | 1.37 | 0.057 |
| Log probit | 3 | 0.91 | 3.5 | 0.64 | 0.027 |
| Log probit | 16 | 0.054 | 1.46 | 1.24 | 0.052 |
| Logistic | 2 | 0.14 | 1.2 | 0.90 | 0.038 |
| Logistic | 16 | <0.000001 | 1.59 | 1.46 | 0.061 |
| Log logistic | 3 | 0.90 | 3.1 | 0.65 | 0.027 |
| Log logistic | 16 | 0.074 | 1.44 | 1.22 | 0.051 |
| Log logistic | 9 | 0.65 | 1.94 | 1.63 | 0.068 |
| Weibull | 3 | 0.90 | 3.0 | 0.65 | 0.027 |
| Weibull | 16 | 0.004 | 1.00 | 0.79 | 0.033 |
World Health Organization
IPCS (2002) concluded that carcinogenicity was the critical effect from exposure to NDMA. They used a margin of exposure approach to characterise the risks to the general public, using estimated intakes for the Canadian population from air, food and water. The POD chosen for the characterisation was the tumorigenic dose (TD05), the dose level that caused a 5% increase in hepatic tumour incidence over background in the selected 2-year oral study in Colworth-Wistar rats (Brantom 1983, Peto et al. 1991a and 1991b).
IPCS (2002) observed that the pivotal study contained an unusually large number of dose groups and controls, and they therefore excluded the upper dose groups from the dose-response modelling. The higher groups reported a downturn in tumour incidence, which may have been due to increased mortality from other causes. IPCS (2002) reasoned that these higher groups added little information to the shape of the curve in the dose range of interest (that is, the TD05) and their inclusion contributed to the lack of fit of the models. Quadratic models were used initially to fit the full dataset, less any dose groups contributing to the downturn in response at the upper end of the dose ranges.
In additional work, the multistage Global82 model (Howe and Crump 1982) was used with the dataset further reduced to 10 dose groups by not only removing the upper dose groups, but also by collapsing together adjacent similar dose groups by averaging the dose level and totalling the number of tumours within combined groups. Subsequently, the model fit for the critical range (that is, the TD05) was improved by the stepwise removal of further dose groups starting with the highest dose group (so the maximum number of dose groups in the final dataset for each tumour was 10). The datasets used for modelling the TD05 are shown in tables 6 and 7. Note that the oral intakes for each dose group no longer correspond with those reported in the pivotal study (and summarised earlier in table 3) because of the reduced number of combined groups.
Table 6: Modelled data of tumour incidence in male rats (IPCS 2002) from the 2-year study by Brantom (1983) and Peto et al. (1991a and 1991b)
Carcinoma
| Intake mg/kg bw/day | Incidence |
|---|---|
| 0 | 2/192 |
| 0.002 | 2/96 |
| 0.008 | 3/96 |
| 0.033 | 4/96 |
| 0.076 | 11/96 |
| 0.120 | 26/96 |
| 0.196 | 44/96 |
| 0.3045 | 66/96 |
Haemangiosarcoma
| Intake mg/kg bw/day | Incidence |
|---|---|
| 0 | 2/192 |
| 0.002 | 0/96 |
| 0.005 | 1/48 |
| 0.011 | 2/48 |
| 0.022 | 0/48 |
| 0.044 | 1/48 |
| 0.065 | 1/48 |
| 0.087 | 6/48 |
| 0.109 | 6/48 |
| 0.131 | 14/48 |
Biliary cystadenoma
| Intake mg/kg bw/day | Incidence |
|---|---|
| 0 | 2/192 |
| 0.002 | 4/96 |
| 0.008 | 4/96 |
| 0.033 | 2/96 |
| 0.076 | 10/96 |
| 0.120 | 24/96 |
| 0.196 | 26/96 |
| 0.3045 | 33/96 |
Table 7: Modelled data of tumour incidence in female rats (IPCS 2002) from the 2-year study by Brantom (1983) and Peto et al. (1991a and 1991b)
Carcinoma
| Intake mg/kg bw/day | Incidence |
|---|---|
| 0 | 2/192 |
| 0.0035 | 0/96 |
| 0.0145 | 4/96 |
| 0.057 | 8/96 |
| 0.134 | 10/96 |
| 0.210 | 10/96 |
| 0.344 | 19/96 |
| 0.459 | 18/48 |
| 0.612 | 33/48 |
Biliary cystadenoma
| Intake mg/kg bw/day | Incidence |
|---|---|
| 0 | 4/192 |
| 0.002 | 1/48 |
| 0.005 | 4/48 |
| 0.010 | 0/48 |
| 0.019 | 3/48 |
| 0.038 | 5/48 |
| 0.076 | 7/48 |
| 0.115 | 34/48 |
The final set of modelled TD05 values for each tumour are shown in table 8. A chi-square lack of fit test was performed on the datasets for the 3 tumour types. A P-value of less than 0.05 was used to indicate a significant lack of fit. Note that all P-values in table 6 are greater than 0.05. IPCS (2002) used the lowest modelled TD05 of 0.034 mg/kg bodyweight/day, which was for hepatic carcinomas in male rats, for their risk characterisation.
Table 8: Modelled TD05 values for hepatic tumours in male and female rats (IPCS 2002, Appendix 4)
| Male rats | TD 05 mg/kg bw/day | TDL 05 (95th LCL) mg/kg bw/day | Chi-square | df | P-value |
|---|---|---|---|---|---|
| Carcinoma | 0.038 | 0.024 | 2.17 | 5 | 0.82 |
| Haemangiosarcoma | 0.078 | 0.048 | 7.67 | 6 | 0.26 |
| Biliary cystadenoma | 0.035 | 0.029 | 10.25 | 6 | 0.11 |
| Female rats* | TD 05 mg/kg bw/day | TDL 05 (95th LCL)mg/kg bw/day | Chi-square | df | P-value |
|---|---|---|---|---|---|
| Carcinoma | 0.082 | 0.061 | 7.36 | 5 | 0.19 |
| Biliary cystadenoma | 0.034 | 0.018 | 7.04 | 5 | 0.22 |
*There was no evidence of a dose-response relationship for hepatic haemangiosarcomas in female rats and so this tumour type was not modelled (IPCS 2002).
Summary
Several authoritative organisations have characterised the carcinogenic risks from exposure to NDMA, although the overall toxicological database is small.
No short-term HBGV were found for NDMA (see table 9).
Table 9: Summary of health-based guidance values for short-term exposures
| Guideline | Value (mg/ m3 ) | Duration | Critical effect(s) | Pivotal reference(s) |
|---|---|---|---|---|
| Current EAL | None | - | - | - |
There is a consensus that NDMA is a genotoxic carcinogen in animals and it is probably a carcinogen in humans. Only NIPH (2011) and US EPA (1987) proposed long-term HBGV (see table 10). They based their opinion on the increased incidence of liver tumours observed in a 2-year drinking-water study in male and female Colworth-Wistar rats (Brantom 1983, Peto et al. 1984, 1991a and 1991b). NIPH (2011) calculated their guideline by linear extrapolation to an ELCR of 1 in 1,000,000 from the experimental TDL05 of 0.018 mg/kg bodyweight/day (IPCS 2002) for the incidence of bile duct cancers in female rats. It was converted to an equivalent air concentration using the allometric and exposure scaling factor from rats to humans recommended in the REACH guidance (ECHA 2012). On the other hand, US EPA (1987) appeared to use the incidence data for all hepatic cancers in deriving the oral slope factor, which was adjusted for the bodyweight difference between rats and humans, and converted to a unit risk in ambient air by assuming a bodyweight of 70kg and an inhalation rate of 20 m3/day. The outcome is about a four-fold difference in the estimates at an ELCR of 1 in 1,000,000.
Table 10: Summary of health-based guidance values for long-term exposures
| Guideline | Value (mg/m3) | Duration | Critical effect(s) | Pivotal reference(s) |
|---|---|---|---|---|
| Current EAL | None | - | - | - |
| DMEL ELCR (1 x 10-6) | 0.3 | Life-time | Hepatic carcinogenicity | IPCS 2002, NIPH 2011 Peto et al. 1991a and 1991b |
| US EPA ELCR (1 x 10-5) | 0.7 | Life-time | Hepatic carcinogenicity | Peto et al. 1984 |
| US EPA ELCR (1 x 10-6) | 0.07 | Life-time | Hepatic carcinogenicity | Peto et al. 1984 |
Recommendations
Short-term EAL
It is not proposed to establish a short-term EAL for NDMA because of its chronic toxicity.
Long-term EAL
Carcinogenicity is the critical health effect from long-term exposure to NDMA, although limited inhalation studies are available on which to base a long-term EAL.
Other authoritative bodies have used the 2-year drinking water study (Brantom 1983, Peto et al. 1984, 1991a and 1991b) to derive an inhalation HBGV using route-to-route extrapolation. However, when considering an extrapolation approach, it is important to take into account understanding of the toxicokinetics of the chemical concerned and the critical target organs for its toxicity. Limited data on NDMA toxicokinetics by the inhalation route is a clear deficiency in the available evidence, which makes extrapolation from the oral to inhalation route more uncertain. Whilst the inhalation study by Klein et al. (1991) reported nasal tumours (a site of contact effect), the drinking water studies found various hepatic tumours (a systemic effect).[footnote 13] There are clear differences in the target organs affected by the 2 exposure routes. It is therefore considered inappropriate to use route-to-route extrapolation as the basis for derivation of a long-term EAL for NDMA.
Klein et al. (1991) appears to be a suitable, if not ideal, pivotal study with dose related incidences of nasal tumours by the inhalation route. However, there are acknowledged challenges with interpreting this study including its short duration (shorter than a typical carcinogenicity study of 2-years) and ambiguities in the description of the complicated exposure regime.[footnote 14] New BMD modelling was undertaken nonetheless using the incidence of nasal tumours observed in female Sprague-Dawley rats. The US EPA benchmark dose software (BMDS) version 3.1.2 was used to fit dichotomous models to the incidence data for combined nasal tumours (esthesioneuroblastomas, mucoepidermoid tumours including carcinomas, and squamous-cell carcinomas) in the 0, 0.12, and 0.6 mg/m3 dose groups (see Table 2). Data from the highest dose group (3 mg/m3) was excluded from the modelling as a significant number of animals did not survive until the end of the experiment, which may have contributed to a downturn in tumour incidence. Inclusion of the highest dose group would have resulted in a lack of adequate fit of the models.
The final set of modelled BMD values are shown in Table 11. All models were considered to have a satisfactory significant fit of the data (all P-values are > 0.1). The AIC and the BMDL10 are the same for both Gamma and Weibull models.
Table 11: Results of the benchmark dose modelling (expressed in mg/m3) for incidence of nasal tumours in female rats exposed to NDMA through inhalation (Klein et al. 1991)
| Model | BMD10 | BMDL10 | BMDU10 | P-value | AIC |
|---|---|---|---|---|---|
| Gamma | 0.030637 | 0.023347 | 0.060377 | 0.94 | 78.2 |
| Weibull | 0.030637 | 0.023347 | 0.057005 | 0.94 | 78.2 |
The BMDL10 of 0.023 mg/m3, modelled from the Klein et al. (1991) data, was adjusted for continuous exposure by multiplying by (4/7) days and (4/24) hours to give a final POD of 0.002 mg/m3. The long-term EAL of 0.2 ng/m3 was obtained by dividing the POD by 10,000, which is a suitable margin of safety for minimal risk for a genotoxic carcinogen (COC 2018). The proposed long-term EAL is comparable to that derived by NIPH (2011) from the same pivotal study using the ‘large assessment factor’ approach for the derivation of a DMEL using the REACH guidance (ECHA 2012).
Abbreviations and definitions
AIC
Akaike information criterion
BMD
Benchmark dose
BMDL
Benchmark dose lower-confidence limit
DMEL
Derived minimal effect level
EAL
Environmental assessment level
ECHA
European Chemicals Agency
ELCR
Excess life-time cancer risk
GD
Gestation day
HBGV
Health-based guidance value
LC
Lethal concentration
POD
Point of departure
PPM
Parts per million
REACH
Registration, Evaluation, Authorisation and Restriction of Chemicals is a European Union regulation that addresses the production and use of chemical substances and their potential impact on human health and the environment.
UF
Uncertainty factor
US EPA
United States Environmental Protection Agency
References
ATSDR, 1989. Toxicological Profile for N-Nitrosodimethylamine, TP 141.
HOWE R.B., CRUMP K.S., 1982. Global82: A computer program to extrapolate quantal animal toxicity data to low doses. Ruston, Los Angeles: Science Research Systems.
LIJINSKY W., REUBER M.D., 1984. Carcinogenesis in rats by nitrosodimethylamine and other nitrosometylalkylamines at low doses. CANCER LETTERS, 22, 83 – 88.
US EPA, 1987. Chemical Assessment Summary for N-Nitrosodimethylamine CASRN 62-75-9.
Footnotes
-
European Chemicals Agency toxicology summary for 2-aminoethanol (CAS number 141-43-5) ↩
-
European Chemicals Agency toxicology summary for 2-aminoethanol (CAS number 141-43-5) ↩
-
1 ppm = 2.54 mg/m3 (SCOEL 1996). ↩
-
Human health toxicological assessment of contaminants in soil. Science Report – Final SC050021/SR2 ↩
-
European Chemicals Agency toxicology summary for 2-aminoethanol (CAS number 141-43-5) ↩
-
A review was also conducted by Health Canada in 2001, but it was used as the basis for the text in IPCS (2002) and has therefore not be reviewed or reported separately. ↩
-
1 ppm = 3.08 mg/m3 (ATSDR 1989, IPCS 2002). ↩
-
ECHA (2012) demonstrated that this single factor was the same as using an allometric scaling of the dose for differences in respiratory volume between rat and human of 4 plus the application of a bodyweight of 70 kg and an inhalation rate of 20 m3/day to convert the dose to an equivalent air concentration. ↩
-
Although the BMDL 10 of 0.56 ppm for female rats was lower than the value for male rats, the converted BMDL 10 of 0.041 mg/kg bodyweight/day was higher than its male derived counterpart. ↩
-
In O’Brien et al. (2006), the drinking water concentrations for each treatment group of male and female rats were reported as the equivalent doses for each treatment group. For example, for Group 4, the drinking-water concentration of 0.132 ppm from Peto et al. (1991a) was reported as 0.005 mg/kg bw/day for male rats and 0.010 mg/kg bw/day for female rats, respectively, giving a conversion factor of 1/26.4 and 1/13.2 from ppm to mg/kg bw/day. The median conversion factor across all treatment groups was 24.2 for male rats and 13.8 for female rats. SCCS (2011) used a conversion factor of 24 for male rats and 13.8 for female rats. If a conversion factor of 24.2 had been applied, the reported BMDL 10 would be 0.026 mg/kg bw/day. ↩
-
The estimated unit risk for the oral route was 1.4 x 10-3 per μg/l. ↩
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The cubic root of the ratio of assumed human bodyweight (70 kg) over rodent bodyweight (0.25 kg) is 6.5. ↩
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It is also noteworthy that Klein et al. (1991) found a lower incidence of other tumours compared to nasal tumours including, notably, just 2 incidences of hepatocellular carcinomas in the lower dose groups. This may have contributed to NIPH (2011) concluding that Klein et al. and similar studies suggested that NDMA may be a more potent carcinogen through inhalation. ↩
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Another important limitation in this study was the variable lifetime of the animals, ranging from 167 to 1,200 days, and the large number of fatalities in the highest dose group before the end of the experiment. ↩