Value chain · CLM-1325
In plain terms
In 1979 American and British radiation officials worked out that the then-standard limit of 10 milliwatts per square centimetre was safe at 2.45 GHz (the frequency planned for power satellites; our context, not on the page), about nine times below the level that warms the body by 1 C (our arithmetic). They said it was too loose at much lower frequencies, where the body absorbs more.
The claim as recorded
A 1979 journal paper by staff of the US EPA and the UK NRPB proposes that RF limits follow heat-stress practice, taking a 1 C rise in core temperature as the occupational ceiling. Extrapolating animal threshold data to a 70 kg person, it estimates that about 92 mW/cm2 at 2.45 GHz would raise rectal temperature by 1 C in an hour, and that 10 mW/cm2 at 2.45 GHz deposits at most about 2.5 W/kg in the outer tissue layers. It concludes that 10 mW/cm2 gives an adequate margin for occupational and environmental exposure above about 1 GHz. Near whole-body resonance (about 70-80 MHz) it finds 10 mW/cm2 too permissive: modelling shows about a 1 C core rise in about 3 hours and thigh tissue near 44 C within 180 minutes, and above about 30 mW/cm2 a resting person's thermoregulation may be overloaded. It suggests about 1.5 mW/cm2, or perhaps 1 mW/cm2 for E-field polarisation, at those frequencies. It finds the 6-minute averaging time too short against the hour or more the body takes to reach thermal balance. It says the 20 to 30 minute weak-field effects reported in the literature do not imply harm, that no data show pulsed fields more hazardous than continuous fields of the same average power, and that chronic studies are needed.
Evidence behind this: T2, a peer-reviewed analysis; it permits "the paper estimates Y". The human figures are extrapolations from animal data and from other workers' thermal models, not human measurements.
Scope and limits
The 92 mW/cm2 is an extrapolation to man of a fit to animal data and, as the paper notes, ignores surface heating at 2.45 GHz. The resonance figures come from other authors' models (Guy et al. 1978) as used by the paper. The 1.5 mW/cm2 rests on an assumed linear link between tissue temperature and power density. The paper does not test non-thermal effects; it judges them not harmful on the short exposures then reported and asks for chronic studies. The ratio 92/10, about 9, is our arithmetic. The text layer renders the degree sign inconsistently ("1 degCrise"); figures were read in context. The views are stated not to be official EPA or NRPB positions.
What it assumes
A thermal mechanism of harm; adult thermoregulation scaled from rats, squirrel monkeys and rhesus monkeys; a comfortable environment (25 C, 50% RH, still air) for the resonance figures; a person at rest.
Where it comes from
PDF p.1 / printed 405 (inferred): 10 mW/cm2 "appears to provide an adequate margin of safety for both occupational and environmental exposure for frequencies above about 1 GHz"; "too high (perhaps by an order of magnitude) for some frequencies below 1 GHz"; "the present averaging period of 0.1 h seems unjustifiably short". PDF p.2 / printed 406: weak-field changes "are not implicative of adverse effects"; "chronic studies are indicated". PDF p.7 / printed 411: "There are no data which indicate that pulsed electromagnetic fields are any more hazardous than an equivalent average power CW field." PDF p.9 / printed 413: "about 92 mW/cm2"; "on the order of 2.5 W/kg would result from a 10 mW/cm2 exposure". PDF p.12 / printed 416: "a rise of rectal temperature near 1 C in a period of about 3 h". PDF p.13 / printed 417: "above approximately 30 mW/cm2 the human thermoregulatory system is potentially overloaded". PDF p.15 / printed 419: thigh "to increase to about 44 C within 180 min"; "a limiting exposure of about 1.5 mW/cm2 would be suggested". PDF p.17 / printed 421: "perhaps 1 mW/cm2, for E field polarization".
Basis: mW/cm2 incident; W/kg SAR; C rise in core and local tissue temperature; hours of exposure
Related records
- Answers question
- QST-001 not published in this release
Cite this
energyaperture.com/c/clm-1325
CLM-1325, Energy Aperture, as of 1979.