Not quite as harmless to humans as they suggest, apparently:

"Although not primarily an anti-personnel device, those who have been exposed to HPM report that its effect is agonising. The radiation penetrates below the skin, boiling nerve cells. It can blind. It induces uncontrollable panic (early research into HPM was as a crowd control agent)."

http://www.guardian.co.uk/g2/story/0,3604,896930,00.html