I've used masks plenty of times outdoors, both with hospital mask and with N95 molded mask. Often use it when riding my bike. I don't care about what the Chinese think about me wearing one. Since 2003 SARS, at least in Beijing there is a decent enough contingent that wears hospital-type masks when they have colds, so as not to spread. But not to the extent you see this in Japan. Hospital masks are a bit of a psychological item and don't really protect you from much (definitely don't help for pollution), but they can help keep whatever illness you have from passing to others.