A scar on her arm reminds Kyoko Saito (not her real name) of an unpleasant experience she had a month ago. The Tokyo office worker was hurrying home one night after working three hours overtime, when she overtook three men chatting as they sauntered along the crowded sidewalk to the nearby station.

Just as she passed, a sharp pain shot up her arm.

"One of the men had flicked his cigarette, and the butt brushed against my left arm," Saito says. "He hadn't seen me just behind him. When he apologized to me I said I was all right, since I didn't see any sign of a wound. But it started to smart soon and became a blister the next day."