Tests of a controversial weapon that is 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� 鈥� possibly 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� it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�.
Little data
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� but 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.