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Repeated dose toxicity: inhalation

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chronic toxicity: inhalation
Type of information:
experimental study
Adequacy of study:
key study
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Ancient study prior to GLP, non-guideline and non-specific analytical follow-up. 6-month exposure of rats should not be interpreted after 3rd month because controls were invalidated by heating equipment malfunction leading to control rat weight loss after 13th week. Reported data are reliable but a large amount of required data are missing or not reported.

Data source

Reference Type:
study report
Report Date:

Materials and methods

Test guideline
no guideline available
Principles of method if other than guideline:
A series of 6-month MMH chronic exposures to four animal species was undertaken to evaluate the safety factor and appropriateness of the current TLV for health of workmen.
GLP compliance:
prior to GLP
Limit test:

Test material


Test animals

other: see below
other: see below
Details on test animals and environmental conditions:
Each of the experimental animal groups, as well as their controls, consisted of 8 beagle dogs, 4 rhesus monkeys, 50 Wistar strain rats, and 40 ICR mice. All animals were female except for rats.

Administration / exposure

Route of administration:
inhalation: vapour
Type of inhalation exposure:
not specified
not specified
Remarks on MMAD:
MMAD / GSD: not applicable (vapour)
Analytical verification of doses or concentrations:
Details on analytical verification of doses or concentrations:
The chamber MMH concentrations were continuously monitored and controlled using a colorimetric method with an AutoAnalyzer. This method is not specific.
Duration of treatment / exposure:
6 months in all animals except half of the dogs
At the end of the first series of experiments, half of the dogs were held for 30 days postexposure observation to determine
+ 4 additional weeks (7 months exposure without reversibility period) in the other half of the dogs

In this summary, the rat data are ignored after week 13 due to invalidation of controls (heating problem - weight loss).
Frequency of treatment:
Exposures were conducted on a 6-hour/day 5-day/week basis at 0, 0.2, 1, 2 and 5 ppm MMH in four experiments.
Another experiment was conducted by continuous exposure at 0.2 ppm
No. of animals per sex per dose:
see above
Control animals:


Observations and examinations performed and frequency:
- weighed biweekly during the studies
- series of 15 clinical chemistry and 8 hematology tests (not conducted on rodents) biweekly during the studies
Sacrifice and pathology:
All exposed and control animals were sacrificed at the conclusion of the study and submitted for gross necropsy. Major organs from all dogs, monkeys, 10 rats, and 10 mice from each group were saved for histopathologic examination. Bone marrow studies on dogs were also performed at this time for their myeloid and erythroid elements (M/E ratio).

Results and discussion

Results of examinations

Clinical signs:
effects observed, treatment-related
mortality observed, treatment-related
Body weight and weight changes:
effects observed, treatment-related
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
Clinical biochemistry findings:
effects observed, treatment-related
Urinalysis findings:
not examined
Behaviour (functional findings):
not examined
Organ weight findings including organ / body weight ratios:
not examined
Gross pathological findings:
not specified
Histopathological findings: non-neoplastic:
effects observed, treatment-related
Histopathological findings: neoplastic:
not specified
Details on results:
Survival (no data on clinical signs):
- 15% and 27% deaths in mice at 2 and 5 ppm

Body weight:
- decreased in rats, except at 0.2 ppm intermittent exposure

Hematology: not investigated in rodents
- increase in methemoglobin, RBC fragility and/or Heinz bodies: at all dose-levels in dogs and monkeys
- decrease in RBC count, hematocrit and/or hemoglobin: at all dose-levels in dogs and monkeys
overall: clear dose-relationship and no apparent threshold for these effects; poorly time-dependent (maximal effect appears already during first month; the effect amplitude tends to decrease along prolonged exposure)

Clinical chemistry:
- mean bilirubin, alkaline phosphatase, and total inorganic phosphorus values for all exposed dog groups were increased: intrahepatic choleostasis

Pathology: done in dogs only:
- decrease in M/E ratio at all doses

Effect levels

open allclose all
Dose descriptor:
dog and monkey - hemolytic anemia
Effect level:
0.2 ppm
Based on:
test mat.
Basis for effect level:
other: intermittent exposure (6h/day 5day/week 6 months)
Dose descriptor:
rats - growth (no data on hemolysis)
Effect level:
0.2 ppm
Based on:
test mat.
Basis for effect level:
other: intermittent exposure (6h/day 5d/week, 13 weeks because of invalid controls thereafter) NOAEC may not cover hemolytic anemia (not investigated)

Target system / organ toxicity

Critical effects observed:
not specified

Applicant's summary and conclusion

Executive summary:

Chronic MMH inhalation leads to lethality in mice and decreased body weight gain in rats. In dogs and monkeys, it produces a dose-related hemolytic anemia (not investigated in rodents) with Heinz body formation for which there appears to be no threshold effect level. The anemia is reversible with removal from further exposure at least up to a level of 5 ppm intermittent exposure.

In dogs and monkeys, the 6-month intermittent exposure (6 h/day, 5 days/week) LOAEC was 0.2 ppm (nominal) due to hemolysis. In rats, the 3-month (longer exposure was not validated by appropriate controls) apparent NOEC was 0.2 ppm (nominal) but a large variety of required investigations were not carried out or reported, notably hematological investigations.

The maximal hematological effect appeared already during the first month, plateaued, and gradually the effect amplitude tended to decrease along prolonged exposure.

It may well be asked where man fits in the spectrum of species responses seen with MMH chronic exposure. In a study of the in vitro formation of methemoglobin by MMH (Leahy, 1970), blood samples from four species were compared to determine their equilibrium conversion rates for oxyhemoglobin. In this study, man was found to rank next to the dog in susceptibility with a higher conversion equilibrium than the rat and monkey.