Anti-pollution masks (Beijing)

Do you wear an anti-pollution mask?

Has your opinion about masks and air pollution changed whilst you have been in China?

What's a recommended type or brand to wear?

Wherd can you buy genuine masks?

In London there have been warnings occasionally about bad air days but you're unlikely to see people rushing to purchase masks. No doubt it's an attitude thing, especially for men I imagine the idea of wearing one would seem unmanly, regardless of the health benefits.

I've read previous threads on this subject but excuse the repition, I just wanted some up to date advice and opinion.