Before the Mevissen et al. (1993) study, a group in Georgia examined mammary carcinogenesis in magnetic-field exposed animals that were initiated with N-nitroso-N-methylurea (NMU) (Beniashvili et al. 1991). In the groups of animals exposed to a 60-Hz magnetic field at 20 µT for 3 hr per day for the lifetime of the animals, the incidence of NMU-induced mammary tumors increased over that in sham-exposed animals or in animals exposed for only 0.5 hr per day.

An additional mammary carcinogenesis study was performed in which DMBA was used to initiate mammary tumors in rats. Löscher and co-workers (1993) reported a significant increase in mammary-tumor induction in the rats exposed to a magnetic field. All rats received four weekly doses of 5-mg DMBA beginning at 52 days of age. After DMBA administration, 99 rats were exposed to 50-Hz magnetic fields at a flux density of 0.1 mT for 24 hr per day for 3 months. Another 99 rats were sham exposed. After 3 months of exposure, mammary-tumor incidence was about 50% higher in the exposed group (51 tumors) than in the sham-exposed group (34 tumors). The difference was statistically significant ( p < 0.05). The tumors were also larger in the exposed group (p = 0.0134), but a difference was not found in the number of tumors per tumor-bearing rat. Note that this exposure is about 1,000 times that of the usual residential field strengths.

REPRODUCTIVE AND DEVELOPMENTAL EFFECTS

This section deals with in vitro and in vivo reproductive and developmental biologic effects of electric and magnetic fields at frequencies of 50 or 60 Hz in exposures that are relevant to those associated with power transmission and use. It is divided into considerations of effects of electric fields and magnetic fields. This division is somewhat artificial because all time-varying electric fields have an associated magnetic field; however, at these low frequencies, the fields can be considered independently to a high degree of accuracy. Nonmammalian and mammalian studies are also considered separately. The studies are summarized in Appendix A, Table A4-2.

Nonmammalian Studies of 50- or 60-Hz Electric Fields

Fish

Embryonic effects of concurrent exposure to power-frequency electric and magnetic fields have been studied in Medaka fish by Cameron et al. (1985). Two-to four-cell-stage embryos were exposed for 48 hr either to 60-Hz electric fields that produced a current density of 300 mA/m2, to a magnetic field of 100 µT (1.0 G) root mean square (rms), or to combined fields. No significant developmental delays were reported immediately after exposure. Delays averaging 18 hr were detected 36-73 hr after removal from the magnetic field and



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