Introduction to RF Radiation Safety / 9 / 9.1 / 9.1.1

9.1.1 Early Discoveries And Medical Applications

The origins of RF safety science can be traced to investigations of electricity during the late nineteenth century. Researchers observed that biological tissues responded differently as the frequency of an applied current increased. Low-frequency currents readily stimulated nerves and muscles, whereas stimulation became less pronounced at higher frequencies and heating became increasingly important.

The French physiologist Jacques-Arsène d'Arsonval demonstrated that relatively large high-frequency currents could pass through the body without producing the intense neuromuscular stimulation associated with lower-frequency electricity. The accompanying heating effect contributed to the development of diathermy, in which high-frequency energy was deliberately used to heat tissue for therapeutic purposes.

During the first half of the twentieth century, long-wave and short-wave diathermy became established medical practices. These controlled applications demonstrated that RF energy could be absorbed by biological tissue and converted into heat. They therefore helped establish one of the central principles of RF safety: sufficiently intense RF exposure can produce physiologically significant heating.

Medical exposure was controlled and intentional, however, and did not by itself create an immediate need for general exposure limits. That need became more apparent as powerful communications and radar systems brought workers into environments containing much stronger RF fields.

9.1.2 Wireless Communications, Radar, And Occupational Exposure

Wireless communications expanded rapidly during the early twentieth century. Radio transmitters became more powerful and were increasingly used for broadcasting, navigation, maritime communication, aviation, and military operations. Nevertheless, relatively few people routinely worked close to high-power transmitting antennas, and systematic investigation of occupational RF exposure remained limited.

The development of high-power radar during World War II changed this situation. Radar equipment combined high transmitter powers, pulsed operation, microwave frequencies, and the possibility of personnel working close to antennas and transmission systems. These conditions created exposure circumstances that differed substantially from those associated with most earlier radio equipment.

Postwar investigations demonstrated that sufficiently intense RF and microwave fields could raise tissue temperature and cause injury. Excessive heating consequently became the principal established adverse effect upon which early RF exposure limits were based. The growing use of radar and other high-power systems also made it necessary to translate laboratory findings into practical limits that could be applied in workplaces.