Chronic toxicity results and ongoing studies of 1,3-butadiene by the National Toxicology Program

A follow-up study of mortality among women in the North American synthetic rubber industry

OBJECTIVE

To evaluate mortality from cancer and other diseases among 4863 women employed at eight North American styrene-butadiene rubber plants. Cancers of the lymphohematopoietic tissues, breast, and ovary were of strongest a priori interest.

METHODS

Cause-specific standardized mortality ratios (SMRs) and 95% confidence intervals (CIs) were estimated.

RESULTS

The observed number of deaths was approximately equal to that expected for leukemia (10 observed/13 expected), Hodgkin lymphoma (1/1.6), multiple myeloma (7/7.9), non-Hodgkin lymphoma (15/14), and cancers of the breast (72/74) and ovary (21/22). Ever-hourly women had more than expected deaths from lung (47/30, SMR = 159, CI = 117 to 211) and bladder (6/1.8, SMR = 332, CI = 122 to 723) cancers. Exposure-response analysis, done only for lung cancer, indicated no trend for butadiene or styrene.

CONCLUSIONS

The observed excesses of lung and bladder cancers may be attributable to nonoccupational factors rather than to workplace exposures.

Chemical Exposures in the Synthetic Rubber Industry and Lymphohematopoietic Cancer Mortality

OBJECTIVE

This study evaluated the association between exposure to several chemicals and mortality from lymphohematopoietic cancer (LHC) among 16,579 synthetic rubber industry workers who were followed up from 1943 to 1998.

METHODS

Poisson regression analyses examined LHC rates in relation to butadiene, styrene, and DMDTC exposure. Models provided maximum likelihood estimates of the relative rate for the contrast between categories of one agent, adjusting for other agents and for additional potential confounders.

RESULTS

Cumulative exposure to 1,3-butadiene was associated positively with all leukemia (relative rates of 1.0, 1.4, 1.2, 2.9, and 3.7, respectively, for exposures of 0, >0 to <33.7, 33.7 to <184.7, 184.7 to <425.0, and 425.0+ ppm-years), chronic myelogenous leukemia and to a lesser extent with chronic lymphocytic leukemia. Adjusting for styrene and DMDTC attenuated these associations. After controlling for butadiene, neither styrene nor DMDTC displayed a consistent exposure-response trend with all leukemia, chronic myelogenous leukemia, or chronic lymphocytic leukemia.

CONCLUSIONS

This study found a positive association between butadiene and leukemia that was not explained by exposure to other agents examined.

Leukemia and exposure to 1,3-butadiene, styrene and dimethyldithiocarbamate among workers in the synthetic rubber industry

This study evaluated relations between exposure to 1,3-butadiene (BD), styrene (STY) and dimethyldithiocarbamate (DMDTC) and mortality from leukemia among synthetic rubber industry workers. Subjects were 13130 men employed for at least 1 year during 1943-1991 at any of six plants that manufactured synthetic rubber. Death certificates and medical records identified workers with leukemia. Cumulative exposure estimates were based on plant- and time period-specific process and task characteristics, linked to subjects' work histories. Poisson regression estimated relative rates (RRs) for workers exposed to each agent compared to unexposed workers. Leukemia (N=59) was positively associated with BD ppm-years (RRs of 1.0, 1.2, 2.0 and 3.8, for exposures of 0, >0-<86.3, 86.3-<362.2 and 362.2+ ppm-years; only the RR for the highest exposure category was statistically significant), STY ppm-years (RRs of 1.0, 1.2, 2.3 and 3.2, for exposures of 0, >0-<20.6, 20.6-<60.4 and 60.4+ ppm-years; only the RR for the highest exposure category was statistically significant) and DMDTC mg-years/cm (RRs of 1.0, 2.3, 4.9 and 2.9, for 0, >0-<566.6, 566.6-<1395.1 and 1395.1+ mg-years/cm; the RR for each non-zero exposure category was statistically significant) after adjusting for age and years since hire. After further adjusting each agent-specific set of RRs for the other two agents, a positive but imprecise relation remained for BD and DMDTC but not for STY. The association with BD was stronger for ppm-years due to exposure intensities >100 ppm than for ppm-years due to lower concentrations. BD and DMDTC, but not STY, were positively associated with leukemia in multivariable analyses. The independent effect of each agent was difficult to evaluate because of correlations with other agents and imprecision.

Toxicokinetics of inhaled 1,3-butadiene in monkeys: comparison to toxicokinetics in rats and mice

1,3-Butadiene is a potent carcinogen in mice and a weaker carcinogen in rats. People are exposed to butadiene through its industrial use--largely in rubber production (over 3 billion pounds of butadiene were produced in 1989)--and because it is common in the environment, occurring in cigarette smoke, gasoline vapor and in the effluents from fossil fuel incineration. Epidemiological studies have provided some evidence for butadiene carcinogenicity in people. Differences in the uptake and metabolism of inhaled butadiene between rodents and primates, including people, might be reflected in differences in its toxicity. In order to compare uptake and metabolism in primates to that in rodents--for which data were already available--we exposed cynomolgus monkeys (Macaca fascicularis) to 14C-labeled butadiene at concentrations of 10.1, 310 or 7760 ppm for 2 hr. Exhaled air and excreta were collected during exposure and for 96 hr after exposure. The uptake of butadiene as a result of metabolism was much lower in monkeys than in rodents. For equivalent inhalation exposures, the concentrations of total butadiene metabolites in the blood were 5-50 times lower in monkey than in the mouse, the more sensitive rodent species, and 4-14 times lower than in the rat. If the toxicokinetics of butadiene in people is more like that of the monkey than that of rodents, then our data suggest that people will receive lower doses of butadiene and its metabolites than rodents following equivalent inhalation exposures to butadiene. This has important implications for assessing the risk to humans of butadiene exposure based on animal studies.

Neoplastic lesions induced by 1,3-butadiene in B6C3F1 mice

1,3-butadiene (CAS No. 106-99-0) was evaluated for carcinogenicity and chronic toxicity by inhalation exposure in B6C3F1 mice because of its high production volume, widespread exposure of workers, and the lack of carcinogenicity and toxicity data (NTP Report #228, 1984). Butadiene (BD) had long been considered to have low toxicity. The long-term studies established that BD is a potent mouse carcinogen with multiple organ carcinogenicity (Huff et al. 1985; Melnick et al. 1988). This paper presents morphological descriptions and illustrations of the neoplastic lesions induced by 1,3-butadiene in B6C3F1 mice.