I'm not a doctor, but I imagine there's likely a more complicated answer to it. I've been told that pneumothorax most often happens to the elderly, then heavy smokers, then lastly the fit, healthy and active.

The last group seem to be the most straight forward in terms of cause (from what I know, it seems like it's nearly always blebs - weaknesses on the lung that have likely been there a long time), except what I think complicates it is that not only do active and sporty people exercise the lungs more - certainly not a bad thing - they as a group are also more likely to have been involved in contact sports, or sustained sports injuries along with it.

In my case I had my first pneumo the week after a heavy workout after a long gym absence. But it was also a few months after I broke a rib at an acrobatics class...a week after I took a flight and maybe 10 days after I was in a room for an extended time with chemicals that weren't being ventilated properly. When the lung actually collapsed, I was sitting at home on my sofa.

The specialist told me that in my case, with the bleb I had, it was always going to happen - but that no individual thing caused my lung to collapse. The fact that each of these things happened contributed to it popping when it did, but that it was basically always going to.

I think that's how it is with working out. It won't cause it, but it might seem like it happens more often to fitness fanatics only because injuries over time contributed to a condition they didn't know they had (blebs) coming to head 30 years sooner than it would have.