There is some evidence that marijuana relaxes the bronchial passageways.
I was a terrible asthmatic as a child. I had to bring a pulmoaide machine to school and use it two times per day (and twice at home in the morning and evening). I tried everything, but I suffered from asthma to the point that I had to limit my physical activity. They also put me on allergy shots 2x per week (4 total shots each time) which as a kid was the worst.
As an adult I tried a few treatments, but I as pretty much dependent on my quick acting albuterol inhaler. I used to carry one on my person at ALL times because I never knew when I would tighten up and as I am sure you know that feeling is close to unbearable and without meds doesn't go away on its own in a timely manner.
Well now, at 31, I keep an inhaler in my gym back, but I don't always use it proactively. I rarely have to use my inhaler except when exercising strenuously. This is so much different than before or any other period in my life.
Now I am not going to draw direct causation, i do know that sometimes childhood asthmatics get far better in adulthood, BUT I do smoke more now than I ever did. I try to use a vaporizer whenever possible as to not inhale tar, but I have had times where my lungs will tighten up and instead of going for the inhaler, I go for the bong, and the effect is very similar, a near instantaneous relief of the tightness, I feel like I can get more air.
Now if its the particule matter that causes your asthma, then yeah, smoking will only aggravate it, but you could try a non smoked form of marijuana or possible the vaporizor.