Cutaneous Perception of Microwaves*

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Abstract
The most obvious effect produced when microwaves are directed into an absorbing substance is the conversion of electromagnetic energy into heat. Awareness of microwave exposure is developed by several mechanisms among which are cutaneous thermal sensation or pain. Although the subjective awareness of warmth is only a rough indicator of microwave exposure, nevertheless, several investigators have established the thresholds for microwave induced thermal sensation and pain in man. The threshold for warmth perception is reached at a warming of the skin at a rate of about .001 °C per second. Threshold and intensity of temperature sensation depend to a large extent on the size of the skin area changing temperature. This paper is a review of the available information on cutaneous sensation of microwave energy which indicates that when a 40 cm2 area of the face is exposed to microwaves, thermal sensation can be elicited within 1 second at power densities of 21 mW/cm2for 10,000 MHz and 58.6 mW /cm2 for 3,000 MHz. Within 4 seconds the threshold is lowered by approximately 50%, i.e. 12.6 mW/cm2 (10,000 MHz) and 33.5 mW/cm2 (3000 MHz). On this basis, if the entire face were to be exposed, the threshold for thermal sensation to 10,000 MHz would be 4–6 mW/cm2 within 5 seconds or approximately 10 mW/cm2 for a 0.5 second exposure. Threshold for pain reaction to 3000 MHz exposure of a 9.5 cm2 area of the forearm ranges from 830 mW/cm2 for approximately 3 minutes to 3.1 W/cm2 for a 20 second exposure period. If a larger area (53 cm2) is exposed, the pain threshold for a 3 minute exposure is 560 mW/cm2. These data and other information on microwave sensation suggest that cutaneous perception of microwaves may provide a protection factor with sufficient margin of safety constituting a warning mechanism to prevent exposure to microwaves at levels that could be injurious.

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