compelling findings in animals or human beings, the committee finds no justification for any conclusion other than that magnetic fields with field strengths from 0.01 to 1.0 µT have no significant effects in cultured cell systems.

  • Magnetic-field exposures of approximately 100 µT (1 G) have been observed in independently replicated studies to produce effects on ornithine decarboxylase (ODC) activity, one of the membrane-mediated signal-transduction pathways, and numerous peer-reviewed reports have been done on the effects of magnetic fields on other components of the signal-transduction pathways. However, a mechanism through which these magnetic fields produce such biologic effects is unknown.

    Positive results in the field-strength range of 50 to 500 µT (0.5 to 5.0 G) have been observed and reproduced for only the signal-transduction effect on ODC activity. For other effects—genotoxicity, intracellular calcium concentrations, and general patterns of gene expression—no convincing and reproducible results have been observed.

  • Magnetic-field strengths greater than 500 µT (5 G) have induced changes in intracellular calcium concentrations and general patterns of gene expression as well as in several components of signal transduction. However, no reproducible genotoxicity is observed at any field strength.

Again, effects of the sort seen here are typical of many experimental manipulations and do not, per se, indicate a health hazard. For the positive results that have been observed, as in the bone-healing studies, the effects cannot be extrapolated to lower field strengths; therefore, it is not known whether the effects observed at higher field strengths are induced by mechanisms distinctly different from those that might cause effects at residential and occupational field strengths.

The committee's overall conclusion based on analysis of in vitro experimentation is that magnetic-field exposures at 50-60 Hz have been shown to induce changes in cultured cells only at field strengths that exceed residential exposure levels by factors of 1,000 to 100,000.

UTILITY AND LIMITATIONS OF IN VITRO STUDIES

In vitro studies are useful for documenting responses of selected cell systems to chemical and physical agents. Interpreting probable responses of cells in culture in terms of potential or putative target-cell response in the body is problematic, however, and requires similar exposures and appropriate surrogates for target-cell populations in vivo. For example, a number of extracellular signals induce a common set of early-response genes. The early response is not, however, sufficient to determine the biologic outcome. The initial response of cells to the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) is activation of protein kinase C (PKC) and induction of the early-response genes fos, myc, and jun. Phorbol esters were originally identified as promoters of papilloma development



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