When going on the OAT trip to Yunnan Province in China last September, I showed my doctor the face mask we were given as we exited the plane in Japan on a prior trip. (The start of the H1N1 epidemic). My doctor just laughed and said it wouldn't be very effective, and no, I didn't need it in Yunnan.
For the upcoming GCT trip, which includes a few days in Beijing, I bought N95 masks from Amazon with breathing valves. The breathing valves are apparently cooler, don't fog eye-glases, and are eaiser for people with asthma to breath in, than the masks without such valves. This is from various reviews, not from experience.
Read many reviewsand checked other sources before choosing. Purchased a set that is primarily for for dust , sand and similar pollution but some readers said they also fended off respiratory illinesses when near sick people on long airplane flights.. They do cost more than $5 for a box of 50. Hope the pollution will have cleared by then, but my husband and I will be prepared.