TESTS of a controversial weapon that鈥檚 designed to heat people鈥檚 skin with a
microwave beam have shown that it can disperse crowds. But critics are not
convinced the system is safe.
Last week, the Air Force Research Laboratory (AFRL) in New Mexico finished
testing the system on human volunteers. The Air Force now wants to use this
Active Denial Technology (ADT), which it says is non-lethal, for peacekeeping or
riot control at 鈥渞elatively long range鈥濃�攑ossibly from low-flying
aircraft.
ADT uses a 2-metre dish to create a narrow beam of microwaves that can be
scanned across a crowd or even aimed at individuals. AFRL is using infrared
photography to analyse the heating effect on the volunteers鈥� bodies.
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AFRL says that the 3-millimetre wavelength radiation penetrates only 0.3
millimetres into the skin, rapidly heating the surface above the 45 掳C pain
threshold. At 50 掳C, they say the pain reflex makes people pull away
automatically in less than a second鈥攊t鈥檚 said to feel like fleetingly
touching a hot light bulb. Someone would have to stay in the beam for 250
seconds before it burnt the skin, the lab says, giving 鈥渁mple margin between
intolerable pain and causing a burn鈥�.
But critics question the AFRL鈥檚 claims that the weapon鈥檚 undisclosed exposure
levels are safe. John Pike of think tank Globalsecurity.org fears that the beam
power needed to scare people may be too close to the level that would injure
them. Air Force scientists helped set the present skin safety threshold of 10
milliwatts per square centimetre in the early 1990s, when little data was
available, says Louis Slesin, editor of Microwave News.
That limit covers exposure to steady fields for several minutes to an
hour鈥攂ut heating a layer of skin 0.3 mm thick to 50 掳C in just one
second requires much higher power and may pose risks to the cornea, which is
more sensitive than skin. A study published last year in the journal Health
Physics showed that exposure to 2 watts per square centimetre for three
seconds could damage the corneas of rhesus monkeys.