Biological responses to weak 60-Hz electric and magnetic fields must vary as the square of the field strength.

Under quite general conditions, the biological response j(G) to a very weak continuous 60-Hz electric or magnetic field G is shown to be proportional to the square of the field strength. This conclusion follows from the continuity of the function j(G) and the first three derivatives of j(G) with respect to G over the amplitude of G. That continuity is ensured in nominally discontinuous systems by the presence of thermal noise. I argue the validity of the conjecture that all plausible biological responses to weak 60-Hz fields vary with the square of the field strength. A specific model is used to illustrate characteristic dependencies of biological responses to exposure times.