medium in the vicinity of the bone. A concentration gradient is set up, and new ions diffuse to the bone, becoming available for bone repair (Parkinson and Hanks 1989b).

More recently, McLeod and Rubin (1992) demonstrated that exposure to pulsed electric fields prevented osteopenia in turkey wings, stimulating an overall 10% increase in the bone area. They used 15-, 75-, and 150-Hz sinusoidal electric fields, with an estimated peak electric field in the tissue of no more than 0.01 mV/cm. The osteogenic influence was greatest at 15 Hz stimulation.

Summary An enormous body of work seeking a relationship between exposure to electric and magnetic fields and changes in calcium concentrations has been accumulated over the past two decades. Much of it shows some sort of positive association, albeit often requiring frequency windows, temperature windows, or power-density windows for an explanation. Some of the work, particularly the early work, is flawed by unstable preparations and uncontrolled thermal and exposure conditions. Many of the effects are difficult to observe or are only borderline significant and require pooling of data to obtain statistical significance. Many of the experiments have not been replicated adequately by others, perhaps because the exact experimental protocols have not been followed; in other cases, independent investigators were unable to replicate experiments.

Of the recent experiments summarized in Table A3-2, only three meet the exacting requirements of replication by independent laboratories, publication in peer-reviewed journals, and explicit identification of exposure strengths. Those experiments were on thymic lymphocytes in which Con-A stimulated cells showed an increase in calcium transport resulting from exposure to pulsed magnetic fields having flux densities about 10,000 times larger than those found in the average human environment (Liburdy 1992b; Walleczek and Budinger 1992; Yost and Liburdy 1993).