Long-term exposure to a magnetic field (5 mT at 60 Hz) increases X-ray-induced mutations

Exposure to extremely low frequency magnetic field (ELFMF) at 400 mT has be en shown to induce mutations (Mutat. Res., 349: 109-114, 1996; Int. J. Radi at. Biol., 71 : 75-79, 1997; and Biochem. Biophys. Res. Commun., 243: 579-5 84, 1998). However, whether ELFMF at low fur densities (under 1 mT) induces mutations is debatable. We investigated the effect of long-term exposure t o 5 mT ELFMF at 60 Hz on mutant frequency. Chinese hamster ovary K1 (CHO-K1 ) cells were exposed or sham-exposed to 5 mT ELFMF for up to 6 weeks with o r without X-irradiation (3 Gy), and the mutant frequency of the hypoxanthin e-guanine phosphoribosyl transferase (HPRT) gene was analyzed. Long-term ex posure to 5 mT ELFMF did not increase mutations, suggesting a threshold for mutation induction greater than 115 mA/m(2) or a magnetic density of 5 mT. However, enhancement of the X-ray-induced mutation rate was observed after treatment with X-irradiation followed by long-term exposure to 5 mT ELFMF. At little as a I-week exposure to ELFMF after X-irradiation enhanced the m utation rate. We also found that 400 mT exposure enhanced the mutation rate induced by X-irradiation (Mutat. Res., 349: 109-114, 1996). These results suggest that exposure to more than 5 mT ELFMF may promote X-ray-induced mut ations.