RF Exposure

Human exposure to electromagnetic energy can have multiple effects on the body. Many studies over the years have shown heating , cell interaction, and behavioral changes to name a few. How the energy interacts with the body depends on a number of factors some of which include frequency, signal strength, exposure time, modulation, and a person natural immune system. The relationship between these and other factors makes it extremely difficult to pin point exact cause effect relationships. This complexity just adds to the controversy over high tension power lines causing cancer, computer monitors, cell phones, and other radio frequency emitting equipment in common use today. It should be pointed out that the field from high tension power lines fall in the frequency range of 50-60 Hz, computer monitors concerns are in the 50 to 300 Hz range. The lower frequency range is called ELF and VLF. These terms stand for Extremely Low Frequency and Very Low Frequency. Cell phone on the other hand operate in the 800 to 900 MHz range. (1 MHz equals 1 x 10⁶ Hz)

One must realize that all equipment that uses electricity emits radio frequency energy at some level. Many standards exist relating to how much energy a piece of electronic equipment can emit. Food and Drug Administration for medical, Federal Communications Commission for electronics and equipment with transmitters, Military standards for Military equipment, RTCA standards for Civil Avionics, all have a part in the effort to control unintended electromagnetic emissions. The standards that apply to human exposure relating to unintended and intended electromagnetic emissions are governed by the Occupational Safety & Health Administration. Figure 1 shows the limits for human exposure with respect to public and industrial situations. The public situation assumes no more than a 6 minute exposure in a given hour time frame. The industry situation assumes continuous exposure. The reason that the limit is lower between 30 MHz and 300 MHz is due to the fact that the human body absorbs more energy in this frequency range. Different parts of the human body absorb electromagnetic energy at different frequencies based on the organs size, fluid content, etc. Typical human bodies absorb the energy in this frequency range more efficiently and convert it to heat. So this graph is based upon the human Standard Absorption Rate (SAR). Above 300 MHz the body tends to react less to the fields, however there are specific frequency windows that the body can still absorb the fields efficiently. The frequency of a microwave oven for example, is a frequency that causes water molecule resonate or vibrate. This vibration absorbs the energy and converts it to heat due to the molecule movement when excited by the specific frequency. The limits set by FDA, FCC and other concerned with electronics emissions have standard levels far below the human exposure levels defined by OSHA. One typically sees energy levels at the intensity of the OSHA limits only from intended transmission sources (i.e. microwave tower, radio tower, radar tower, etc.). Since the field levels drop off rapidly as the distance from the antenna increases, keep out zones are established relative to the OSHA limits for public safety.

These intended and unintended emissions can be measured using sensitive equipment designed to characterize the electric and magnetic fields generated b such equipment. Depending on the situation, monitoring may require anywhere from a spot check to a week of data collection. Given the complex interaction of electric and magnetic fields with the human bodies electrical system, it is very difficult to predict the influence on a specific individual. The least complicated interaction to predict is the heating effects, therefore the limit profile shown above.