#013: Microwave studies with human subjects, 1966.

Pick of the Week #13:  Microwave studies with human subjects.  Heating of Living Tissue. 1966.

Schwan, HP, A Anne, and L Sher.  1966.  Heating of Living Tissues, Aerospace Crew Equipment Laboratory, U.S. Naval Air Engineering Center, Philadelphia, Pennsylvania, U.S.A., NAEC-ACEL-534.  38 pp.

Abstract: A central forehead area of 4 subjects was exposed to free-field, 10 cm microwave irradiation and the reaction time to onset of warmth sensation measured.  Subjective awareness of warmth was found to be only a rough indication of personal hazard based on the currently accepted safety standards of 75 mW/cm2 for 2 min.

The U.S. Air Engineering Center did studies on 4 human subjects to determine how quickly they could perceive changes in temperature when exposed to high levels of microwave radiation.  They irradiated the forehead of these subjects using 10 cm wavelength (3 GHz frequency) at a power density of 74 and 54 mW/cm2.  Each of the 4 subjects was irradiated 30 times over a period of 5 days.

What is amazing about this study is not the time it took for these subjects to react (which was a matter of seconds) but the high levels to which they were exposed and the lack of follow-up studies to determine long-term effects or indeed to determine any changes other than heat perception.  The Americans were so certain that the only effect of microwave radiation was heating that they didn’t bother to determine any other responses to this radiation.  If you don’t look you can’t find!

The authors concluded that it might be better to choose some other part of the body than the forehead to determine heating effect of microwave radiation at such high power densities.  One reason for this is that the thickness of the skin affected heat perception.  Another reason provided is that this would “remove [the] fear of brain damage.”

Great effort went into designing a room and exposure of these subjects and one very interesting aside is the material used to absorb microwave radiation.  Three layers were used to protect parts of the body not exposed to microwave radiation.  This involved an absorbing material (Eccosorb), a second thinner absorbing layer (Teledeltos paper), and just in case some microwaves penetrated both layers, a sheet of copper to reflect the remaining radiation back into the absorbing layers.

Currently we have material (film, fabric, paint) that will reflect microwave radiation but what we need is material that will absorb this energy and, indeed, research on such material is actively being conducted.