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The critical local effects following chronic inhalation exposures to chloroprene are considered to be non-neoplastic lesions of the nose and the lungs. In 2-year chronic inhalation bioassays conducted by NTP (1998), the incidence of non-neoplastic lesions of the lungs (bronchiolar hyperplasia or alveolar epithelia hyperplasia) and of the nose (chronic inflammation; atrophy or necrosis) were statistically increased in mice and rats treated at the lowest exposure concentration in the study (12.8 ppm). A No-Observed-Adverse-Effect-Concentration (NOAEC) was not established in these studies. Concerning neoplastic observations see chapter carcinogenicity.
Additionally repeated dose inhalation toxicity of chloroprene has been investigated in rats, mice and hamsters in different subacute and subchronic studies. In repeated dose toxicity studies conducted by NTP (1998) male and female F344/N rats and B6C3F1 mice were exposed to chloroprene (>96% pure) by inhalation at concentrations between 12 and 200 ppm for 16 days or 13 weeks. The LOAEL for the sub-acute inhalation toxicity of chloroprene in rats was 32 ppm (115.8 mg/m3) based on degeneration of the olfactory epithelium. The NOAEL for sub-acute inhalation toxicity of chloroprene in mice was 12 ppm (43.4 mg/m) based on observations of decreased bodyweight gains in the consecutively higher dose group. A NOAEL of 12 ppm (43.4 mg/m3) was proposed for
the sub-chronic inhalation toxicity of chloroprene in rats based on observation of olfactory epithelial degeneration at 32 ppm. NOAELs of 32 (115.8 mg/m3) and 12 ppm (43.4 mg/m3) were proposed for the sub-chronic inhalation toxicity of chloroprene in male and female mice respectively based on observations of squamous epithelial hyperplasia and anaemia at higher doses respespectively.

Key value for chemical safety assessment

Additional information

The repeated dose inhalation toxicity of chloroprene in rats and hamsters has been investigated in several non-standard studies. Although these studies were conducted prior to the introduction of GLP and formal testing guidelines, they have been conducted in general accordance with current test guidelines and can be regarded as reliable. The repeated dose toxicity of chloroprene in rats and mice was investigated in studies conducted as part of the US Department of Health and Human Services National Toxicology Program (NTP). The repeated oral dose toxicity of chloroprene was investigated in a single published study. There are no animal studies on the repeated dose toxicity of chloroprene following dermal exposures.

In a sub-acute (four-week) inhalation study, male and female SPF rats were exposed to chloroprene vapours at 0, 39, 161 or 625 ppm (6 hours/day) 5 days/week for 4 consecutive weeks. Mortality was observed in the highest and mid dose groups with severe growth retardation. Diminished food consumption and behavioural disturbances were observed in all the dose groups tested. Eye and nose irritation, nasal discharge and orange-coloured urine was found in the highest dose group. Loss of hair occurred in females at 161 and 625 ppm. The relative weights of liver, kidneys and lungs were increased in rats exposed at 625 ppm. Relative kidney weight was also increased in females of the mid dose group. The lungs of rats that died during the experiment invariably showed haemorrhages and perivascular odema. Liver damage consisting of centrilobular degeneration and necrosis occurred at the two highest dose levels. Minor compound-related histopathological changes were found in the kidneys, nasal cavity, spleen and adrenals of top dose animals. A No-Effect-Level was not observed in the study and was proposed to be lower than 39 ppm in rats.

In a sub-chronic (26 week) inhalation study, male and female Wistar rats were exposed to chloroprene vapours at 0, 10, 33 or 100 ppm (6 hours/day) 5 days/week. No treatment-related effects were observed at the lowest dose tested. Relative kidney weights were increased in female rats treated at 33 ppm. Effects observed at 100 ppm included clinical signs (restlessness, pilo-erection and a hunched posture in the initial weeks of exposure); slight retardation of growth for males only; slight kidney enlargement and liver enlargement proposed to be an adaptive response induced by increased metabolic workload. A sub-chronic inhalation No-Effect-Effect-Level of 33 ppm was proposed for rats based on observations of slight growth retardation and liver enlargement in the high dose group.

In a sub-acute (4 week) inhalation study, male and female Syrian gold hamsters were exposed to chloroprene vapours at 0, 39, 162 or 627 pm (6 hours/day) 5 days/week for 4 consecutive weeks. All animals died within 24 hours after a single 6 hour exposure to chloroprene at 627 ppm. Abnormal behaviour, nose and eye irritation, dyspnea and urinary and bowel incontinence were observed prior to death and extensive lung oedema was observed in post-mortem examinations. Mortality, lethargy, growth retardation, slight eye and nose irritation, increased red blood cell count, increased organs weights (heart, adrenal glands, liver and kidneys) and mid-zonal liver degeneration and necrosis were observed in the mid-dose group. Only transient restlessness, eye irritation, and slight irritation of the mucous membranes of the nasal cavity were observed at exposures to the lowest dose of 39 ppm. A No-Effect-Level was not observed in the study and was proposed to be lower than 39 ppm in hamsters.

Trochimowicz et al (1997) investigated the chronic inhalation toxicity of chloroprene in rats and hamsters. In the study, Syrian hamsters and Wister rats (both sexes) were exposed to chloroprene vapours at 0, 10 or 50 ppm (6 hours/day) 5 days/week for up to 24 and 18 months respectively. All treated rats exhibited slight restlessness during the first few weeks of exposure. At 50 ppm, rats also showed an increased incidence of alopecia, slight growth retardation, and an increased incidence of foci of altered liver cells (seen frequently in aged rats). Minimal increases in relative liver weight in female rats in both dose groups were reported. Hamsters showed only slight growth retardation and a reduction in amyloidosis at 50 ppm. No adverse effects were seen in either species following chronic inhalation exposures to chloroprene at 10 ppm.

Repeated dose NTP studies

In a series of repeated dose toxicity studies, male and female F344/N rats and B6C3F1 mice were exposed to chloroprene (> 96% pure) by inhalation (whole body exposures) for 16 days, 13 weeks, or 2 years.

In a 16-day inhalation study, groups of 10 male and 10 female F344/N rats were exposed to chloroprene vapour at 0, 32, 80, 200, or 500 ppm, 6 hours per day, 5 days per week, for 16 days. At the onset of treatment, rats receiving higher doses (200 or 500 ppm) became hypoactive, unsteady and had rapid shallow breathing. These effects worsened on day 2 of treatment with animals haemorrhaging from the nose. Three males receiving the highest dose died on day 2 or 3 of the study. Both sexes treated at 200 ppm and females treated at 500 ppm had reduced body weight gains. A normocytic, normochromic, responsive anemia; thrombocytopenia; and increases in serum activities of alanine aminotransferase, glutamate dehydrogenase, and sorbitol dehydrogenase occurred on day 4 in 200 ppm females and 500 ppm males. Increased relative kidney and liver weight occurred in female rats at 80 and 500 ppm and at 200 and 500 ppm respectively. Increased incidences of minimal to mild olfactory epithelial degeneration were reported in all exposed groups of males and females. Increased incidences of squamous metaplasia of the respiratory epithelium were observed in male rats at 500 ppm males whereas increased incidence of centrilobular to random hepatocellular necrosis were observed in 500 ppm males and 200 ppm females A NOAEL was not identified in the study as effects were observed at the lowest dose. The LOAEL for sub-acute inhalation toxicity was 32 ppm in both male and female rats based on degeneration of the olfactory epithelium. 

In a 16-day inhalation study, groups of 10 male and 10 female B6C3F1 mice were exposed to chloroprene vapours at 0, 12, 32, 80, or 200 ppm, 6 hours per day, 5 days per week, for 16 days. All mice treated at 200 ppm died on day 2 or day 3 of the study. Mice exposed at 200 ppm exhibited signs of necrosis, hepatocellular and thymic necrosis and hypertrophy of the myocardium. Male mice exposed at 32 or 80 ppm had significantly reduced bodyweight gains. Hematology and clinical chemistry in exposed mice were similar to those of controls. Mice treated at 80 ppm had decreased thymus weights, with female mice also showing increased liver weights. Increased incidences of multifocal random hepatocellular necrosis occurred in both sexes exposed at 200 ppm. Squamous epithelial hyperplasia of the forestomach was observed in mice exposed to 80 ppm.

NOAELs of 32 and 12 ppm were proposed for the NTP sub-acute inhalation toxicity of chloroprene in male and female mice respectively based on observations of decreased bodyweight gains in the consecutively higher dose group.

In a 13 week range-finding inhalation study groups of 10 male and 10 female F344/N rats were exposed to chloroprene vapours at 0, 5, 12, 32, 80, or 200 ppm, 6 hours per day, 5 days per week. One male rat exposed at 200 ppm died during the study. No reduction in bodyweight gains were observed in exposed rats. Male rats in the highest dose group had red or clear discharge around the nose and eye region. In this dose group, thrombocytopenia and transient increases in the activities of serum alanine aminotransferase, glutamate dehydrogenase, and sorbitol dehydrogenase was observed in mice on day 22, with normal platelet counts and other parameters returning to normal by week 13. Minimal to mild increases in activated partial thromboplastin and prothrombin times were observed. An alkaline phosphatase enzymeuria occurred on day 22. A normocytic, normochromic, and non-responsive anemia and a proteinuria were observed in week 13. Reduced liver nonprotein sulfhydryl concentrations were reported. Slightly reduced sperm motility was observed in the highest exposure group. Increased kidney weight occurred in male rats. The incidence of hepatocellular necrosis was increased. Scattered chronic inflammation also occurred in the liver. The incidence of hemosiderin pigmentation was increased. Increased incidences of minimal to mild olfactory epithelial degeneration and respiratory metaplasia occurred.

In rats exposed at 80 ppm thrombocytopenia and alkaline phosphatase enzymeuria occurred. Increased kidney weight occurred in female rats. Increased incidences of minimal to mild olfactory epithelial degeneration and respiratory metaplasia occurred. Rats exposed at 32 ppm showed slight neurobehavioral effects and olfactory epithelial degeneration. A NOAEL of 12 ppm was proposed for the sub-chronic inhalation toxicity of chloroprene in rats based on observation of olfactory epithelial degeneration at 32ppm.

In a 13 week range-finding inhalation study, groups of 10 male and 10 female B6C3F1 mice were exposed to chloroprene vapour at 0, 5, 12, 32, or 80 ppm, 6 hours per day, 5 days per week, for 13 weeks. All mice survived to the end of the study. Mice exposed at 80 ppm had reduced bodyweight gains. Female mice exposed at 32 or 80 ppm had reduced hematocrit concentrations and erythrocyte counts and increased platelet counts. Increased incidences of squamous epithelial hyperplasia of the forestomach occurred in mice exposed at 80 ppm. NOAELs of 32 and 12 ppm were proposed for the sub-chronic inhalation toxicity of chloroprene in male and female mice respectively based on observations of squamous epithelial hyperplasia and anaemia at higher doses respespectively.

Chronic studies at NTP:

The critical local effects following chronic inhalation exposures to chloroprene are considered to be non-neoplastic lesions of the nose and the lungs. In 2-year chronic inhalation bioassays conducted by NTP (1998), the incidence of non-neoplastic lesions of the lungs (bronchiolar hyperplasia or alveolar epithelia hyperplasia) and of the nose (chronic inflammation; atrophy or necrosis) were statistically increased in mice and rats treated at the lowest exposure concentration in the study (12.8 ppm). A No-Observed-Adverse-Effect-Concentration (NOAEC) was not established.

The only available animal study of chronic oral exposures to chloroprene was conducted by Ponomarkov and Tomatis (1980) as part of a developmental/reproductive study. Chloroprene dissolved in olive oil was administered by stomach tube to 17 female rats (BD IV strain) at a single dose of 100 mg/kg bw on gestational day 17 and to progeny from treated rats weekly at 50 mg/kg bw from weaning until 120 weeks. Litter sizes, pre-weaning mortality, survival rates and bodyweights did not differ between treated and untreated rats. Rats that died within 35 weeks of treatment showed severe congestion of the liver and the lungs, whereas rats autopsied after 80-90 weeks showed multiple liver necrosis. Rats surviving more than 90 weeks showed degenerative lesions of the liver parenchymal cells.

Justification for classification or non-classification

According to Annex VI to Regulation (EC) No 1272/2008, chloroprene is classified for repeated dose toxicity hazard as: STOT RE 2 H373 – May cause damage to organs through prolonged or repeated exposures (affected organs: lung, liver and kidney; route of exposure: inhalation)