I read some other threads about masks, but wanted to add my opinion after having bought one.

I bike 3-4 days a week, 40km round trip in Tokyo. The air here is not terrible, but I have to ride on major roads and there are no nice trails or anything like that. I spend a lot of time behind buses and delivery trucks and after the first few weeks of commuting, I got tired of that taste in my mouth. By the time I got to work, I could feel a dry tickle in my throat and would cough. I hated to think I was breathing in some garbage into my lungs every day.

I read quite a few people who said they had a hard time breathing with these masks on when they really got going, so I was a bit nervous. However, I found a local supplier and decided to try it out. If it didn't work, I could use it when I clean my house or something. The mask arrived the next day (and I found that my local shop - Nukaya Ebisu also sells them) and I tried it on the way home.

It fits well. I have a small head, but ordered the large. I'm 6ft 1in guy, not sure what my hat size is. It fits well, and is comfortable. I had the nosepiece a little snug the first time, but after adjusting that it really is comfortable. It doesn't get in the way of my helmet, but does mess with my earphones. Also, depending on your sunglasses, the nosepiece maybe in the way and mess up the fit.

I have some pretty good sized hills on my ride and even going at full huff and puff up them, I have no problems taking deep breathes with this thing on. I noticed a huge difference in quality of the air I was breathing. I can sit behind a bus and feel the heat of the exhaust, but barely taste or smell anything weird coming through the mask. I'm very satisfied with it. I bought extra filters and wear it morning and night.

If you're worried about pollutants, I say try it. If you can't adjust or feel it's in the way, you can always use it for painting or cleaning.