There is evidence, both direct and indirect, suggesting that AC electric and magnetic fields increase the risk of certain cancers and other physiological and psychological abnormalities. Statistically significant clusters of primary brain tumors and childhood leukemia have been reported in close conjunction with high-tension lines and electrical substations. Electrical workers seem to have high rates of various cancers, and highway patrolmen and hairdressers alike claim to have developed cancers near where they habitually use their radar guns and hairdryers.
The currents induced by artificial fields may affect the nervous system directly, and the evidence of this is somewhat stronger than that of the cancer effect. Many people report difficulty in concentrating when exposed to high fields, and some studies suggest that fields suppress the production of the sleep-inducing hormone melatonin. Exposure to a fairly strong magnetic field of 300 milligauss (at 60 Hz) will consistently slow down subject's heart rate by about three beats per minute for three or four minutes after the field is activated, even though subjects cannot directly sense the field. 300 milligauss is the strongest magnetic field typically encountered in the home, but weaker fields may produce more subtle effects that take longer to manifest themselves.
Biological effects begin to occur at a continuous exposure of about one billionth of an amp of AC current per square centimeter. Preliminary results show that at five times that level an increase in protein production in cancer cells is seen; but when the field strength is increased 1000 times, the increase in protein production is only three times greater. These changes are seen for AC current at several different frequencies, including 60 Hz (60 oscillations per second).
Although how this happens is not fully understood, both magnetic and electric AC fields that surround the body can produce AC electric current inside the body. The best available theory is that this current interferes with the normal transport of ions across cell membranes. If this theory is correct, the body should be sensitive to current at any frequency up to about 1000 Hz; above that sensitivity will decrease (the exact frequency is not known and experimental measurement of it has not been attempted).
Based on the above evidence and some epidemiological studies, it would be prudent to avoid continuous exposure to any electromagnetic pollution that produces AC current inside the body higher than one billionth amp per square centimeter, at frequencies of 1000 Hz or below. No absolute hazard threshold has been established yet, but the lower limit for biological effects is probably within a factor of three from that level. Preliminary results also suggest that it's better to spend a short time well above this threshold than a long time just above it. At frequencies above 1000 Hz, the body is likely also to be sensitive, but not as sensitive as it is to lower-frequency current.
Magnetic and electric fields are vector quantities. This means they are specified as having a magnitude or field strength, measured in milligauss or kilovolts/meter respectively; as well as a direction. The effect on the body is more or less independent of the direction of the field, only the magnitude is important. Many of the studies of possible links between cancer and EM fields measured the field strength in one direction only. In these measurements, the meter's sensor had to be pointed in the same direction as the field was pointing, otherwise the reading would be less than the true magnitude of the field strength. (If the sensor were erroneously pointed perpendicular to the local field direction, the reading would be "zero", no matter how strong the field actually was.) To avoid this inaccuracy, all studies should be done with meters that read the true magnitude of the field, so a researcher can walk through a room with a meter and get an accurate, immediate reading of the field magnitude at every point along the path, regardless of which way the meter is held.
Any measurements of electromagnetic pollution should probably be frequency weighted, meaning that they read the product of magnetic field strength times frequency and/or electric field strengths times frequency, if the measurements are to gauge whether the current inside the body exceeds a threshold level. This frequency-weighting should extend up to about 1000 Hz and then sensitivity should decrease at higher frequencies.
Here is an example of frequency weighting. An external magnetic field of 3 milligauss or an electric field of 2.5 kilovolts/meter at 60 Hz will produce approximately one billionth amp per square centimeter. The current produced inside the body is proportional to field strength times frequency, so at 120 Hz (twice the frequency), only half as much field (1.5 milligauss and 1.25 kilovolts/meter respectively) is required to produce the same current inside the body.
Previous measurements that looked for a link between cancer and field strength have used several types of metering; some frequency-weighted, some not, and some measuring only 60 Hz fields. These different types of meters read different total field strengths thus making it more difficult to establish whether a link exists.
One more matter that complicates the interpretation of the strength of fields has to do with how magnetic fields induce current in the body. The current per area induced is proportional to field strength times frequency times the length of the body. For this reason, children exposed to magnetic fields experience less current per area than do adults, and lab rats experience about 1/10 as much. The multiplication by body length does not apply to electric fields, however, so both children and adults would experience the same current when exposed to them. Interestingly, a fairly strong magnetic field (500 milligauss) and electric field (about 2 kilovolts/meter) exist in nature, but these fields are static, and thus have a frequency of zero--they produce no current inside the body.