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Several studies suggest that exposure to 50 Hz magnetic fields may promote chemically induced breast cancer in rats. Groups of 100 female Sprague-Dawley rats were initiated with four weekly 5 mg gavage doses of 7,12-dimethylbenz[a]anthracene (DMBA) starting at 50 days of age. After the first weekly DMBA administration, exposure to ambient fields (sham exposed), 50 Hz magnetic fields at either 1 or 5 G field intensity or 60 Hz fields at 1 G for 18.5 h/day, 7 days/week was initiated. Exposure continued for 13 weeks. A vehicle control group without DMBA was included. In a second study, using lower doses of DMBA, groups of 100 female Sprague-Dawley rats were initiated with four weekly doses of 2 mg of DMBA starting at 50 days of age followed, after the first weekly DMBA administration, by exposure to ambient fields (sham exposed) or 50 Hz magnetic fields at either 1 or 5 G field intensity for 18.5 h/day, 7 days/week for 13 weeks. Rats were weighed and palpated weekly for the presence of tumors. There was no effect of magnetic field exposure on body weight gains or on the time of appearance of mammary tumors in either study. At the end of 13 weeks, the animals were killed and the mammary tumors counted and measured. Mammary gland masses found grossly were examined histologically. In the first 13 week study, the mammary gland carcinoma incidences were 92, 86, 96 and 96% for the DMBA controls, 1 G, 50 Hz, 5 G, 50 Hz and 1 G, 60 Hz groups, respectively. The total numbers of carcinomas were 691, 528 (P < 0. 05, decrease), 561 and 692 for the DMBA controls, 1 G, 50 Hz, 5 G, 50 Hz and 1 G, 60 Hz groups, respectively. In study 2, the mammary gland carcinoma incidences were 43, 48 and 38% for the DMBA controls, 1 G, 50 Hz and 5 G, 50 Hz groups, respectively. The total numbers of carcinomas were 102, 90 and 79 for the DMBA controls, 1 G, 50 Hz and 5 G, 50 Hz groups, respectively. There was no effect of magnetic field exposure on tumor size either by in-life palpation or by measurement at necropsy in either study. There was no evidence that 50 or 60 Hz magnetic fields promoted breast cancer in these studies in female rats. These studies do not support the hypothesis that magnetic field exposure promotes breast cancer in this DMBA rat model.

Chloroprene (2-chloro-1,3-butadiene) is a high production chemical used almost exclusively in the production of polychloroprene (neoprene) elastomer. Because of its structural similarity to 1,3-butadiene, a trans-species carcinogen, inhalation studies were performed with chloroprene to evaluate its carcinogenic potential in rats and mice. Groups of 50 male and female F344/N rats and 50 male and female B6C3F1 mice were exposed to 0, 12.8, 32 or 80 p.p.m. chloroprene (6 h/day, 5 days/week) for 2 years. Under these conditions, chloroprene was carcinogenic to the oral cavity, thyroid gland, lung, kidney and mammary gland of rats, and to the lung, circulatory system (hemangiomas and hemangiosarcomas), Harderian gland, kidney, forestomach, liver, mammary gland, skin, mesentery and Zymbal's gland of mice. Survival adjusted tumor rates in mice were fit to a Weibull model for estimation of the shape of the dose-response curves, estimation of ED10 values (the estimated exposure concentration associated with an increased cancer risk of 10%) and comparison of these parameters with those for 1,3-butadiene. Butadiene has been identified as a potent carcinogen in mice and has been associated with increased risk of lymphatic and hematopoietic cancer in exposed workers. Shape parameter values for most of the neoplastic effects of chloroprene and 1,3-butadiene were consistent with linear or supralinear responses in the area near the lowest tested exposures. The most potent carcinogenic effect of 1,3-butadiene was the induction of lung neoplasms in female mice, which had an ED10 value of 0.3 p.p.m. Since the ED10 value for that same response in chloroprene exposed mice was also 0.3 p.p.m., we conclude that the carcinogenic potency of chloroprene in mice is similar to that of 1,3-butadiene. Cancer potency of chloroprene is greater in the mouse lung than in the rat lung, but greater in the rat kidney than in the mouse kidney and nearly equivalent in the mammary gland of each species.

Several studies have suggested that exposure to 50 Hz magnetic fields promote chemically induced breast cancer in rats. Groups of 100 female Sprague-Dawley rats were initiated with a single 10 mg gavage dose of 7,12-dimethylbenz[a]anthracene (DMBA) at 50 days of age followed by exposure to ambient fields (sham exposed), 50 Hz magnetic fields at either 1 or 5 Gauss (G) field intensity or 60 Hz fields at 1 G for 18.5 h/day, 7 days/week for 26 weeks. A vehicle control group without DMBA was included. Rats were palpated weekly for the presence of tumors. There was no effect of magnetic field exposure on body weight gains or the time of appearance of mammary tumors. At the end of 26 weeks, the animals were killed and the mammary tumors counted and measured. Mammary gland masses found grossly were examined histologically. The mammary gland carcinoma incidence was 96, 90, 95 and 85% (P < 0.05, decrease) for the DMBA controls, 1 G 50 Hz, 5 G 50 Hz and 1 G 60 Hz groups, respectively. The total numbers of carcinomas were 649, 494 (P < 0.05, decrease), 547 and 433 (P < 0.05, decrease) for the DMBA controls, 1 G 50 Hz, 5 G 50 Hz and 1 G 60 Hz groups, respectively. The number of fibroadenomas varied from 276 to 319, with the lowest number in the 1 G 60 Hz exposure group. Measurement of the tumors revealed no difference in tumor size between groups. In this breast cancer initiation-promotion study in female Sprague-Dawley rats, there was no evidence that 50 or 60 Hz magnetic fields promoted breast cancer under the conditions of this assay. This study does not support the hypothesis that magnetic field exposure can promote breast cancer in this rat model.