A biological effect occurs when a change can be measured in a biological system after the introduction of some type of stimuli. A biological effect becomes a safety hazard when it “causes detectable impairment of the health of the individual or of his or her offspring”.

RF Thermal effects

Biological effects that result from heating of tissue by RF energy are often referred to as “thermal” effects. It has been known for many years that exposure to high levels of RF radiation can be harmful due to the ability of RF energy to heat biological tissue rapidly.

This is the principle by which microwave ovens cook food, and exposure to very high RF power densities, i.e., on the order of 100 mW/cm2 or more, can clearly result in heating of biological tissue and an increase in body temperature.  Tissue damage in humans could occur during exposure to high RF levels because of the body’s inability to cope with or dissipate the excessive heat that could be generated.  Under certain conditions, exposure to RF energy at power density levels of 1-10 mW/cm2 and above can result in measurable heating of biological tissue (but not necessarily tissue damage).  The extent of this heating would depend on several factors including radiation frequency; size, shape, and orientation of the exposed object; duration of exposure; environmental conditions; and efficiency of heat dissipation.

Two areas of the body, the eyes and the testes, are known to be particularly vulnerable to heating by RF energy because of the relative lack of available blood flow to dissipate the excessive heat load (blood circulation is one of the body’s major mechanisms for coping with excessive heat).  Laboratory experiments have shown that short-term exposure (e.g., 30 minutes to one hour) to very high levels of RF radiation (100-200 mW/cm2) can cause cataracts in rabbits.  Temporary sterility, caused by such effects as changes in sperm count and in sperm motility, is possible after exposure of the testes to high- level RF radiation (or to other forms of energy that produce comparable increases in temperature).

RF “non-thermal” effects

Certain biological effects reported after exposure of animals (“in vivo”) and animal tissue (“in vitro”) to relatively low levels of RF radiation.  These reported effects have included certain changes in the immune system, neurological effects, behavioral effects, evidence for a link between microwave exposure and the action of certain drugs and compounds, a “calcium efflux” effect in brain tissue (exposed under very specific conditions), and effects on DNA.

Also some studies on mobile phone use and the risks of certain brain tumors, for at least 10 years since first exposure, found a more than twofold risk increase having glioma.