I've read that pollution in Beijing often makes exercise unwise. Do the masks actually make any difference?
I'm more interested in bicycling in other places, especially Foshan & Guangzhou.
Like @Tom said in the comment :
Masks block only a portion of the pollution
The problem with the Chinese haze is due to 2 types of particles : pm10 and pm2.5 and those last ones (pm2.5) are really small and you can't protect yourself even when wearing a mask. Event if the mask specially mentioned "works against pm2.5" because they are lying.
I used to lived there during one year and most of the embassy recommendations during high pollution days were : stay home, close windows, and no physical activity at all.
You can check real time pollution level of each particles with a lot of mobile apps like this one China Air Quality.
Did a quick web search. The first hit, not only shows hourly reports for Guangzhou, but it also has a chart of recommendations for different levels, and says that there are IOS and Android apps to get the U.S. Embassy's version. Whether China's "great firewall" prevents it from working, I cannot say. They imply it can be found in Apple's U.S. store, but not so.
Today's levels in GuangZhou are "Unhealthy for Sensitive Groups."
There is a link to stateair.net which has more info and allows one to download the historical values for the entire year.