Welcome to my ghetto. MEC sells them in Toronto, and some days, they are needed. (funny today is another smog alert day, now that you mention it).

Some riders I know who are mildly asthmatic have found they can go faster breathing through one of those some days, as it keeps the gunk out of the lungs. I briefly tried one myself and found that any benefits of less particulate were offset by reduced air intake through the hepa mesh, although it was better than I thought it would be.

I personally haven't noticed much problem with smog alert days....but they are increasing in frequency and severity here, so I may end up coughing up black tar at the end of the ride one of these days. Then maybe I'll wear one. Under the full-face.