run4fun says:I've run with a mask during the winter months for several years due to an annoying case of adult onset asthma. The first couple of times I felt like it was difficult to breath, but I eventually got used to it, and no longer have any problem. I recommend you buy a mask that does not lie flat against the mouth. Find one with a bit of a hollowed out space in front. I run at very high altitudes, where the air is quite thin. I still get plenty of oxygen. Just give yourself some adjustment time.