Tests of a controversial weapon that is designed to heat people's 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 "relatively long range" - 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' bodies.
AFRL says that the 3-millimetre wavelength radiation penetrates only 0.3 millimetres into the skin, rapidly heating the surface above the 45 °C pain threshold. At 50 °C, they say the pain reflex makes people pull away automatically in less than a second - it's 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 "ample margin between intolerable pain and causing a burn".
But critics question the AFRL's claims that the weapon's 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 - but heating a layer of skin 0.3 mm thick to 50 °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.
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