Value chain · CLM-1339
In plain terms
A 1950s experiment found that 3 GHz microwaves have to be about four times stronger than infra-red heat before skin starts to hurt, because most of the microwave heat goes deeper than the nerves that sense pain. Pain began at about half a watt per square centimetre.
The claim as recorded
A 1952 paper in the Journal of Physiology measured the burning-pain threshold of human skin for 3 GHz (10 cm) microwaves delivered through contact applicators. The intensity that could just be tolerated in prolonged exposure was 0.13 cal/cm2/s over a 7.6 x 7.0 cm aperture and 0.19 cal/cm2/s over a 3.8 x 2.5 cm aperture, about 0.54 and 0.79 W/cm2 (our conversion). The paper puts the tolerated intensity of 10 cm microwaves at over four times that of short- and long-wave infra-red, which it gives as about 0.03 cal/cm2/s for large areas. It explains the gap by depth: only 20 per cent of the microwave energy entering the skin is absorbed in the first 1.5 mm, and the rest, absorbed deeper, does not influence pain sensation. Burning pain came at a skin temperature of about 46 deg C, with no systematic change with radiation power.
Evidence behind this: T1, a measured result in a peer-reviewed physiology journal with method and measurement error stated (power +-5 per cent, skin temperature +-0.1 deg C). Small sample: six subjects per condition.
Scope and limits
Six male subjects; Table 1: forearm, thigh, calf; Table 2 adds fingertip and thenar eminence; partial-body contact exposure, not far-field whole-body exposure; 3 GHz only. The paper says the microwave power was non-uniform across the aperture and that detailed comparison is difficult (p.7). Table 1 power over the large aperture gives about 0.34 W/cm2 (18.3 W over 53.2 cm2, our arithmetic), below the 0.54 W/cm2 threshold figure; the derivation is on p.6, whose scan and text layer are garbled, and was not read. Conversion uses 1 cal = 4.184 J (ours). A pain threshold is a sensation threshold, not an injury threshold or an exposure limit. The statement that deeper heating is not felt is the paper's, for 3 GHz; what it means for pain as a warning at other frequencies is not stated.
What it assumes
Contact exposure through a cork-filled flare or Perspex-filled guide at a room temperature of 20 deg C with minimal air movement. The 0.13 and 0.19 figures are the paper's threshold intensities, not aperture averages.
Where it comes from
PDF p.2 / printed 2: "10 cm free-space wave-length (frequency = 3 x 10^9 c/s)" (normalised); probable error in power measurement "+-5%". PDF p.3 / printed 3, Table 1: tolerated power 18.3, 19.0 and 18.0 W, mean final skin temperature 45.8 deg C; small applicator "radiation power = 4.8 W"; "no systematic change of burning pain temperature with change in radiation power". PDF p.7 / printed 7: "for the 7.6 x 7.0 cm aperture the threshold intensity is 0.13 cal. cm^-2 sec^-1, while for the smaller aperture (3.8 x 2.5 cm) it rises to 0.19" (normalised); infra-red on large areas "approximately 0.03 cal. cm^-2 sec^-1". PDF p.9 / printed 9: "is over four times that of both short- and long-wave infra-red radiation"; "20% of the energy entering the skin"; the rest "does not influence pain sensation"; pain "at a skin temperature of approximately 46" deg C.
Basis: cal/cm2/s incident intensity as printed (W/cm2 by our conversion); W through the applicator; deg C skin temperature
Related records
- Answers question
- QST-001 not published in this release
Cite this
energyaperture.com/c/clm-1339
CLM-1339, Energy Aperture, as of 1952.