I have been an asthmatic all my life. Several years ago, my grandmother, now in her late 80s, told me that when she was a girl, she smoked some sort of marijuana for her asthma. Thinking her crazy, I looked around and found the following paper published in 1975
Effects of Smoked Marijuana in Experimentally Induced Asthma
One quick comment on Tashkin's paper: The cannabis they were using was "2 percent THC" - thats DIRT WEED in my book - I can't even find schwag thats that low quality!
My personal experience is that smoke is bad, so reduce the smoke either by using a vaporizer, smoking better bud, or both.
This is medical science - THC does help approx. 80% of asthmatics. I am one of them.
p.s. It turns out that, yes, grandma was smoking cannabis - Right now, I think it was little balls of hashish, sold prior to 1937. For the record, she HATED smoking it.