What sort of mask do you wear in HK that can protect against PM2.5 or gasses, that you can still exercise in without passing out?
Personally, the only ones I've found that can do that are the full-on respirators, but I have never seen anyone using those for exercise.
In London, I found the Respro masks were useful for cycle commuting, but they were only good for large soot/dust particles, not the problem that we are facing in HK. They claim nowdays to filter PM2.5, but in my experience, they are way too hot for HK. Also their rather vague writeups of filter efficacy lead me to be wary of any of their claims, but they were certainly great for UK winters.