I remember from the SARS scare of 2002-3 that many Chinese at the time donned masks as a form of protection. American TV reproduced many images of these masks which, as I remember them, all resembled hospital-issue masks. I figured that, with SARS and avian flu off the front pages, masks were no longer part of Chinese daily wear. And for the most part that is true; most Chinese do not wear masks on the street. But I have been surprised to see a fair number of Chinese wearing them.
Of course, the air in Nanjing is often fairly polluted, as the haze that informs so many of my photographs confirms, so a mask might really prove a helpful protection against some air-born pollutants. The masks fit over the ears, and stretch around to cover the mouth and usually the nose as well, so in this respect they certainly help filter city air. In addition, although daytime temperatures here have been warm, overnight temperatures usually cool down quite a bit, so it might be that in the early morning or late afternoon a warm mask over one's mouth is necessary in a way it would not be in June or July.
But the masks are fascinating in their variety and design. Below I have posted examples of a few different masks I managed to photograph, but this selection cannot do justice to the wide array of designs one encounters on the streets. Nor should you infer from the fact that all these photos come from people on bikes or scooters that masks are useful only at higher speeds; quite a few pedestrians also sport masks, but I haven't been able to figure out how to photograph them without causing offense.
Here are a couple of snaps I took of two masks I couldn't resist buying.