My friend Marilyn sums it up here.
And she models the masks as well, which is a bonus for those in other countries who may have not even been aware that such things existed. Yes it's true, people do wear Darth-Vader-like masks in Tianjin when they ride their bikes in summer.
Another popular face covering are large scarves that can be wrapped all the way around a person's head to keep rather large hair styles in place. Supposedly these scarves keep the dust off your face too, but I have to admit I'm not convinced. I also have to admit that I've been absolutely petrified more than once upon witnessing a faceless bike rider hurtling toward me in the street. I don't have any pictures of this fashion phenomenon. You'll just have to take my word for it.