Dimethyl ether

Endpoint summary

Description of key information

NOAEL = 47106 mg/m3 (2.5% or 25000 ppm in air), being the highest concentration tested in a 2-year chronic study in rat. 

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Quality of whole database:

Study is technically not feasible.

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed

Dose descriptor:

NOAEC

47 106 mg/m³

Study duration:

chronic

Species:

rat

Quality of whole database:

Extended database involving multiple studies of different duration up to 2 year chroic, and involving multiple species (rat and hamster) that consistly indicate low potential for toxicity by DME.

Repeated dose toxicity: inhalation - local effects

Link to relevant study records

Reference

Endpoint:

chronic toxicity: inhalation

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: Equivalent to Guideline study. This study was selected as the key study because the information provided for the hazard endpoint is sufficient for the purpose of classification and labelling and/or risk assessment.

Reason / purpose for cross-reference:

reference to same study

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 452 (Chronic Toxicity Studies)

GLP compliance:

yes

Species:

rat

Strain:

other: Crl:CD(R)(SD)BR

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Charles River Breeding Labs, Kingston, New York
- Age at study initiation: weanling rats
- Weight at study initiation: not reported
- Fasting period before study: none
- Housing: stainless steel, wire-mesh cages. Housed 3/cage upon arrival
- Diet (e.g. ad libitum): Purina Laboratory Chow Checkers #5001 (PLCC) available ad libitum except during exposures
- Water (e.g. ad libitum): ad libitum except during exposures
- Acclimation period: 12 days


ENVIRONMENTAL CONDITIONS
- Temperature (°C): 24±2°C
- Humidity (%): 50% ±10%
- Air changes (per hr): not reported
- Photoperiod (hrs dark / hrs light): not reported

Route of administration:

inhalation: gas

Type of inhalation exposure:

whole body

Vehicle:

air

Remarks on MMAD:

The test substance is a gas.

Details on inhalation exposure:

Four similarly constructed non test substance reactive inhalation chambers were dedicated to this study. They were operated in a single pass mode, slight negative pressure and cleaned at the end of each exposure day. Chamber concentrations of the test substance were regulated by controlling the flow rate of test substance into each chamber.

Test substance vapors were generated by warming cylinders containing liquefied test substance in a 21-27°C water bath. The vapors were metered into the intake manifold at the top of the chamber. Filtered, conditioned air also entered the top of the chamber, swept the test material into the respective exposure chambers and was exhausted out the bottom of the chambers. Chamber concentrations of the test substance were regulated by controlling the flow rate of test substance vapors into each exposure chamber. Filtered air, alone, was metered in a similar manner into the control chamber. Total flow of air in the control and test substance chambers was maintained at approximately 800 L/min.

Analytical verification of doses or concentrations:

yes

Details on analytical verification of doses or concentrations:

Chamber atmospheres were analyzed by a GC every half hour during the 6 hour daily exposure period. Target concentrations were 0, 0.2, 1.0 and 2.5%.

Duration of treatment / exposure:

2 years

Frequency of treatment:

6 hours a day/ five days a week (excluding holidays)

Dose / conc.:

0.21 other: %

Remarks:

analytical concentration

Dose / conc.:

1.02 other: %

Remarks:

analytical concentration

Dose / conc.:

2.5 other: %

Remarks:

analytical concentration

No. of animals per sex per dose:

100 animals/sex/concentration group

Control animals:

yes

Details on study design:

The study was designed to evaluate the potential chronic toxicity and oncogenicity of the test substance in male and female rats when exposed by inhalation. This route was chosen because it most appropriately simulates potential human exposure.

Observations and examinations performed and frequency:

DETAILED CLINICAL OBSERVATIONS: Moribund, dead rats, abnormal behavior and appearance / twice a day.

BODY WEIGHT: Rats weighed weekly- first 3 months/ twice a month for the remainder of the exposure.

FOOD CONSUMPTION: No

FOOD EFFICIENCY: No

WATER CONSUMPTION: No

OPHTHALMOSCOPIC EXAMINATION: No

HAEMATOLOGY: 3, 6, 9, 12, 18 months after the initiation of the study, evaluations were conducted. See Table 1 for parameters examined

CLINICAL CHEMISTRY: 3, 6, 9, 12, 18 months after the initiation of the study, evaluations were conducted. See Table 2 for parameters examined

URINALYSIS: 3, 6, 9, 12, 18 months after the initiation of the study, evaluations were conducted. See Table 3 for parameters examined

Sacrifice and pathology:

GROSS PATHOLOGY: evaluations on 10 rats/sex/exposure group after 6, 12, 18 months and all rats alive after 2 years. See Table 4
HISTOPATHOLOGY: evaluations on 10 rats/sex/exposure group after 6, 12, 18 months and all rats alive after 2 years. See Table 4

Other examinations:

Male and female rats were sampled between 606 and 653 days on test for the presence of antibodies to Kilham Rat Virus (KRV). This sampling was performed because KRV was detected in rats from a separate study that shared the same hallway. Antibodies to KRV were detected in 12 of 24 study rats that were sampled. The 12 antibody-positive rats were distributed evenly across all groups. Since the virus was present in all groups and since the adverse effects of the virus are exhibited primarily in reproductive efficiency in young rats, the interpretation of results from this study were not affected by the presence of KRV.

Statistics:

A one-way analysis of variance and the least significant difference test (LSD) were used to test statistical differences between groups in body weight and weight gain. Organ weight data were evaluated by using a one-way analysis of variance and Bartlett's test. Pair-wise comparisons were made by using the LSD and/or Dunnett's test. Clinical laboratory data were evaluated by a partially nested and crossed analysis of variance and by the LSD test.

The incidences of clinical observations and gross and histopathological lesions were compared to control group incidences by the Fisher's Exact test. A Bonferroni correction was applied to the comparison of clinical observations. Survival probability was generated by the Kaplan-Meier procedure and analyzed by using the Tarone test.

Clinical signs:

effects observed, treatment-related

Description (incidence and severity):

The incidences of stained wet inguinal/perineal area in male rats in the groups exposed to test substance vapors were greater than the control group. Since increases were observed in male rats in all exposure groups and since these increases were not concentration-related, the significance of this observation was not clear. Decreases in the incidences of swollen testes and in the incidences of swollen paws in male rats were not considered to be compound related. The incidences of torn ears for both male and female rats in the intermediate and high-exposure groups were greater than in the corresponding control groups. Torn ears may have been the consequence of tearing the ear punches used to identify the rats in this study. The double punching of one ear in high-exposure group rats, and the single punching of both ears in the intermediate exposure group may have lead to an increase in the incidence of torn ears.

Mortality:

mortality observed, treatment-related

Body weight and weight changes:

effects observed, treatment-related

Description (incidence and severity):

Body weights were greater and survival rates were less than the control group for male rats in the intermediate and high-exposure group.

Food consumption and compound intake (if feeding study):

not examined

Food efficiency:

not examined

Water consumption and compound intake (if drinking water study):

not examined

Ophthalmological findings:

not examined

Haematological findings:

effects observed, treatment-related

Description (incidence and severity):

Compound-related hematologic effects were not observed for male rats exposed to DME vapors for 2 years. The statistically significant decreases in erythrocyte count and hematocrit observed in male rats in the intermediate and high-exposure groups at the 6-month sampling time were interpreted to be evidence of in vitro hemolysis that was not compound-related. This conclusion was based on concurrent increases in MCH and MCHC. Other statistically significant changes in hematologic or clinical chemical parameters were within the range of expected biological variation and were interpreted not to be of biological significance.
At 3 months on test, there was evidence of a minimal compound-related hemolytic effect (decreases in erythrocyte and hematocrit) together with evidence of in vitro hemolysis (increased MCHC) for females in the high-exposure group. The decreases in erythrocyte count and hematocrit that were observed in the intermediate- and high-exposure groups at the 6 month sampling were interpreted not to be compound related but due to in vitro hemolysis. This interpretation was based on concomitant increases in MCH and MCHC values. Other statistically significant changes for female rats in hematologic or clinical chemical parameters that were measured during the study were within the range of expected biological variation and were interpreted not to be biologically significant or compound related.
The observed hemolytic effects seen in males and females were interpreted to be transient effects that were not representative of long-term effects of the test substance.

Clinical biochemistry findings:

no effects observed

Endocrine findings:

not examined

Urinalysis findings:

no effects observed

Behaviour (functional findings):

not examined

Immunological findings:

not examined

Organ weight findings including organ / body weight ratios:

effects observed, treatment-related

Description (incidence and severity):

The only consistent organ weight change was an increase in the absolute spleen weights of male rats in the high exposure group at the 6- and 12-month sacrifices. The changes in relative liver weights in female rats were not consistent; at the 12-month sacrifice, relative liver weights in the intermediate and high-exposure groups were less than the control group, and at the 18-month sacrifice, relative liver weight was greater than the control group and occurred only in the high-exposure group. All other changes in absolute or relative organ weights were considered transient or within the range of biological variation and therefore were not compound related.

Gross pathological findings:

no effects observed

Neuropathological findings:

not examined

Histopathological findings: non-neoplastic:

effects observed, treatment-related

Description (incidence and severity):

The only test substance-related histologic effect was slight splenic congestion in male rats in the high-exposure group observed at the 6-month sacrifice. This splenic observation was consistent with the increased spleen weight observed at 6 months.
No consistent histologic lesion was observed that would explain the decreased survival rate of male rats in the intermediate and high-exposure groups. The greater body weights in these groups could have been a contributing factor in the increased mortality. Although the test substance is known to have anesthetic properties that could lead to deceased physical activity and a consequent increase in body weight, clinical observations indicated normal physical activity of the rats in all groups. Therefore, increases in body weight along with the decrease in survival may be circumstantial and unrelated as to cause an effect.

Histopathological findings: neoplastic:

no effects observed

Description (incidence and severity):

Results are reported in Section 7.7.

Dose descriptor:

NOAEC

Effect level:

>= 2.5 other: %

Based on:

test mat.

Sex:

male/female

Basis for effect level:

other: No adverse findings noted

Critical effects observed:

not specified

Conclusions:

The NOAEC is at least 2.5% (25000 ppm)
The study and the conclusions which are drawn fulfill the quality criteria (validity, reliability, repeatability).

Executive summary:

Groups of 100 male and female rats were exposed for 6 hours/day, 5 days/week (excluding holidays) for up to two years to atmospheres containing either 0 (control), 0.2, 1.0, or 2.5% test substance vapors. Clinical chemical and hematological measurements were performed on 10 rats/sex/exposure group after approximately 3, 6, 9, 12, and 18 months of exposure. Gross and histopathological evaluations were conducted on 10 rats/sex/exposure group after 6, 12, or 18 months of exposure, on all rats alive after 24 months of exposure, and on all rats found dead or in a moribund condition, tissue integrity permitting.

Body weights were greater and survival rates were less than the control group for male rats in the intermediate- and high-exposure groups. No clear association could be made between body weight increases and decreased survival even though these changes were concurrent observations in the same exposure groups. No histological lesion was found that would explain the decrease in survival rate. A compound-related hemolytic effect was observed in male rats in the high-exposure group at six months on test. This effect was characterized by a decrease in erythrocyte count, increase in spleen weight, histological evidence of splenic congestion, along with normal bone marrow histology. A decrease in erythrocyte count was also observed in female rats in the high-exposure group at three months that was considered compound related. These changes were interpreted to be transient findings that were not representative for long term effects of the test substance.

The authors report a NOAEC for the study of 0.2% based on an increase in body weight and a decrease in survival in male rats exposed to 1.0 and 2.5% test substance and on hemolytic effects noted in male rats exposed to 2.5% test substance for six months. However, body weight gain (male only) was occasionally higher than controls in the early phases of the study, but this finding was not consistent throughout the study. Further, the observation of increased body weight was not seen in any other of the subchronic or repeated inhalation studies involving the test substance. While there was a statistically significant decrease in survival (males only) at the termination of the 2-yr study, no impact on survival was observed throughout the chronic toxicity part of the study (12 months). There were no pathological, clinical or clinical chemistry effects observed that would support a conclusion that there was a substance-related reduction in survival. The study authors did not consider the possibility that the statistical difference in survival at the 2-yr point is due to the high survival rate of the controls in this study, and that comparison with historical control values may have led to the dismissal of this finding. Finally, the observed hemolytic findings seen in males and females were interpreted to be transient findings that were not representative for long-term effects of the test substance.

Based on careful consideration of the chronic part of this study and the supporting information of several subchronic studies involving multiple species, the repeated inhalation NOAEC is determined to be at least 2.5% (47106 mg/m3). 

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed

Quality of whole database:

No respiratory irritation was noted during any of the inhalation exposures. None of the studies further pointed at effects on the respiratory system.

Repeated dose toxicity: dermal - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Quality of whole database:

Study is technically not feasible.

Repeated dose toxicity: dermal - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Quality of whole database:

Study is technically not feasible.

Mode of Action Analysis / Human Relevance Framework

DME is a stable molecule. Methyl groups are very stable and are poorly oxidizable. Auto-oxidation of DME hardly occurs. In contrast, auto-oxidation of DEE is 100 times higher than for DME. There is no evidence for formation of peroxides below 100°C.

DME is a gas, which does not show biological interactions, doesn't bind to receptors, and thus also doesn't undergo metabolic transformation. In these respects, it differs from DEE, and does more resemble the noble gases.

DME is well soluble in water. Presence in the body comes from completely passive diffusion from concentrations in the air, leading to an equilibrium of the partial DME pressure in the tissues and in the air. At very high concentrations the passive presence of DME in the membranes of cells, can lead to neuro-depression and at increasing concentrations even to narcosis. After bringing the organism back in fresh air without DME the DME will completely diffuse out of the body again, leading to a complete recovery. The same as is the case with noble gases.

Important to note for this is:

• The rapidity of the development of these symptoms almost ensures that the acute narcotic effects of DME is due to DME itself and not to metabolites. Repeated dose studies can be seen as consecutive daily (6-hour) acute toxicity studies. After ceasing the exposure, no DME is left in the body to exert any effect


• The concentrations needed for neuro-depressive effects, let alone narcosis, are very high. Neuro depressing effects of DME start from 4% in air. Information tests with human subjects show that from 5% in the air small effects in reaction time and typing skills start to become visible. At 10% no effects in first 10 minutes, incoordination developed after 21 minutes of exposure, and after one hour the subject was not able to perform simple tasks. At 14% in the air, narcosis occurred after 26 minutes, at with 20% after 17 minutes.


• The mechanism is a general one and also hydrocarbon gases, noble gases (and even nitrogen from the air under pressure) can show these effects. And these substances are exempted for registrations because of conceived lack of toxicity.


• There is no differences or specific sensitivity between organism for this effect (As the basic principle of function of  neuronal cells are the same).

 

As stated, DME shows no biological interactions at levels below those leading to neurodepression. This is supported by the overall NOAEL coming from a two-year chronic study in rat by inhalation, indicating a NOAEL of 2.5% at the exceptional highest concentration used in this study.

Additional information

Shorter duration inhalation studies, including 90-day exposures were conducted for this substance. In rats, no substance related adverse effects were observed at the highest concentration tested (20’000 ppm, 37685 mg/m3). In hamsters, clinical chemistry changes were not consistently concentration related or statistically significant, therefore, the rat was considered for use in assessment activities. Since a high quality, chronic inhalation exposure study was available in rats, it was the most appropriate study for further hazard assessments.

 

Male and female rats were exposed to 0, 0.2, 1 or 2.5% v/v via inhalation for 24 months. Body weight gain (male only) was occasionally higher than controls in the early phases of the study, but this finding was not consistent throughout the study. Further, the observation of increased body weight was not seen in any other of the subchronic or repeated inhalation studies with the test substance. While there was a statistically significant decrease in survival (males only) at the termination of the 2-yr study, no impact on survival was observed throughout the chronic toxicity part of the study (12 months). There were no pathological, clinical or clinical chemistry effects observed that would support a conclusion that there was a substance-related reduction in survival. The study authors did not consider the possibility that the statistical difference in survival at the 2-yr point is due to the high survival rate of the controls in this study, and that comparison with historical control values may have led to the dismissal of the effects. Based on careful consideration of the chronic part of this study and the supporting information of several subchronic studies involving multiple species, the repeated inhalation NOAEL is determined to be 2.5% (47106 mg/m3 or 25000 ppm).

 

 

Below the list of available inhalation studies in the DME dossier, ordered from shortest to longest duration. For completeness also two available acute studies involving single 4 hr inhalation exposure are added. All repeated dose studies involve 6 hr inhalation on each exposure day. The developmental toxicity studies (PNDT) are added last.

Study	Duration	species	Concentrations	Results	Comments
Haskell, 1980	Acute 4 hr	Rat	10 male /grp, Between 8.4 and 20.5%, 10/grp	4h-LC50 ca. 16%	At 8%: between delirium and light surgical anesthesia.
TNO, 1974	Acute 4 hr	Rat	5/sex, 2%	4h-NOAEC = 2%	Limit test
BASF, 2022	4-day, 6 h/d	Rat	5 /sex/grp; 0 and 2%	6h-NOEC = 2%	Extensive investigation for symptoms of neuro-depression/narcosis
Haskell, 1979	14-day, 6 h/d, 5d/w	Rat	10 males and 10 females; 0, 1, or 5%	not possible to verify adversity.	Narcotic effects (neuro depression) visible at 5%
TNO, 1976	28-day, 6 h/d, 5d/w	Rat	10/sex/grp; 0, 01, 0.1 and 1%	NOEC = 1%
TNO,1978	90-day, 6 h/d, 5d/w	Rat	10/sex/grp;0, 0.2, 1 and 2%	NOAEC = 2%	slight changes neutrophils (m+f); ALAT (m), slight decrease protein (f) – not statistically significant.
TNO, 1983	90-day, 6 h/d, 5d/w	Rat	10/sex/grp; 0, 0.1, 0.5, 1 and 2%	NOAEC = 2%	(GLP) slight changes neutrophils (decrease (m), increase (f)) - not toxicologically significant.
TNO, 1983	28-day, 6 h/d, 5d/w	Hamster	10/sex/grp; 0, 0.2, 1 and 2%	NOAEC = 2%	(GLP) WBC higher in males at d 28 (no dose-response)
TNO, 1983	90-day, 6 h/d, 5d/w	Hamster	30/sex/grp; 0, 0.1, 0.5, 1 and 2%	NOAEC = 2%	(GLP) tendency of lower WBC and lymphocytes
Collins, 1978	30-wk, 6 h/d, 5d/w	Rat	25/sex/grp; 0, 0.02, 0.2 and 2%	NOAEC = 2%	Indicated increased ALAT with decreased liver weight
Haskell, 1986	2 yr, 6 h/d, 5d/w	Rat	100/sex/grp; 0, 0.2, 1.0 and 2.5%	NOAEC = 2.5%	(GLP)
Haskell, 1981	10-day (GD 6-15); 6 h/d	Rat	27 females/grp; 0, 0.125%, 0.5%, and 2.0%	NOAEC = 2%	(error in reporting suggesting slightly decreased response to the sound)
TNO, 1981	11-day (GD 6-16); 6 h/d	Rat	25 females/grp;  0, 2.0%, and 2.8%	NOAEC = 2.8%	(GLP)

 

Some studies hint on a possible effect of DME such as on WBC (lymphocytes or neutrophils), or liver (ALAT or weight), but these effects are not statistical or biological significant or considered adverse within the study, and consistent and sometimes contradicting between studies.

The overall NOAEC of 2.5% can be drawn (on the basis of the highest NOAEC that is below the lowest LOAEC), derived from the 2-year chronic study, which includes all parameters relevant for evaluation. This is also below the NOAEC of 2.8% for developmental toxicity (where the possible reported narcotic effects at 2% from the PNDT from Haskell is overruled by all other studies available).

 

Additionally, there are two reports that express some concerns for possible narcotic effects observed at test concentrations:

- DME - 10-day inhalation rat, Haskell, 1979 (non-GLP):

This study involved exposures to 0, 1% and 5% of DME in air for 6h/day.
The report describes:During exposure, no unusual behavior was observed among rats in the control and low dose (1%) groups. Rats in the high dose (5%) group were unsteady and occasionally restless. They responded very slightly or not at all to sharp taps on the chamber, while those in the control and low dose groups responded normally.

Further under Clinical Signs:
5%: “very common, moderate sluggishness post-exposure”1%: “a moderate amount of sluggishness for a short time post-exposure”

‘Sluggishness’ is thus reported for both 1% and 5%, but tables on incidence, frequency, severity and duration are missing, making it difficult to assess.

- Developmental toxicity / teratogenicity rat, Haskell, HLR-459-81, 1981 (non-GLP)

For the RF part of the study, dams were exposed to 0, 0.5%, 2.0%, and 4.0% in air; 6 h/d, during GD 6-15.

Dams exposed to the test substance at the 4% (40’000 ppm) level showed no response to a sound stimulus during exposure and gained significantly less weight during the early exposure period than did the control dams. In the groups exposed to the lower levels, the only test substance-related effect demonstrated among the dams was a slight decrease in response to sound at 2% (20’000 ppm): the response of the 0.5% (5000 ppm) exposure group was described as ‘equivocal’.

The effects observed at 4% and 5% in the studies above are in agreement with the general available information. The reported ‘a slight decrease in response to sound at 2%’, the ‘moderate amount of sluggishness for a short time post-exposure’ at 1% and ‘equivocal response at 0.5%’ reported in the studies above, are not supported by the many other, higher quality, studies with DME. To this can also be remarked that the descriptions are not quantifiable, as actual individual tables are missing; it concerns subjective, overall judgments in the summary of the report.

 

Narcotic effects by DME are driven by peak exposures. The rapidity of the development of these symptoms almost ensures that the acute narcotic effects of DME is due to DME itself and not to metabolites, and after exposure ceases, there is rapid elimination of DME from the body. Repeated dose studies therefore can be seen as consecutive daily (6-hour) acute toxicity studies, and any possible narcotic/neuro-depressive effects would be possible during exposure at any exposure day, irrespective the total duration of days of the study.

In order to solve the discrepancy between observations between the two studies and all the later and more robust studies not mentioning any depressive effect, a new study was carefully designed to evaluate possible neuro-derepressing effects from 6 hr/day 2% DME in air exposure, both during and immediately after exposure, in the most complete and robust way possible (BASF, 2022).

This study indisputable shows that DME concentrations of 2% do not show any indication for narcotic effect.

Justification for classification or non-classification

Based on the results of repeated inhalation studies, the substance does not need to be classified for repeated dose toxicity according the EU Classification, Labelling and Packaging of Substances and Mixtures (CLP) Regulation (EC) No. 1272/2008.