As a rider with respiratory issues in cold weather, I have to add that the face masks aren't always what they're cracked up to be when you're exerting. When you're trying to expand your lungs, the material over your nose and mouth limits how quickly you can breathe the air in and that can sometimes make the situation worse and put you at a defecit. Best rule of thumb for most people is to gauge their effort and have the courage to stay off the bike when the temps are iffy for their personal threshold, regardless of how or if they're treating their asthma.

My cut-off is actual temp or wind chill effect of 35 or lower. I rode for a few hours a week ago with a temp of 38 and did fine, but I'm acclimated to upper 30's feeling like "spring" - for someone used to warmer year-round weather, temps in the upper 40's might be a shock to the system.

I live in a region where I'd only get to ride for a month or two out of the year if I waited til temps were ideal. But having to layer something over the nose and mouth while you're working at an effort isn't really a great idea if you're still getting your conditioning in shape. People who could do their commute backwards in their sleep without breaking a sweat wouldn't probably have the mask concern.