It does sound like a tough situation.
Just from my basic experience in quitting a job, or being fired/laid-off, for that matter; it is common practice to confront your supervisor, first. Second, if that doesn't work, go to the next up the ladder, until your last resort is to quit. However, upon quitting, the general rule to unemployment is not having a certain period without benefits.
If you have a copy of the letter, and if you happened to have any other documents indicating your health situation in regards to the environment, ie., the bad air is conflicting with your health, then you may have a strong case. Perhaps even getting a doctor's note that may suggest the environment was part of the cause for they hemoptysis.
Another point to make is that we, as CF'rs, when asked if we can perform our duties without any handicap, the answer is often "No". However, in your case, the environment (which if it was a recent problem) was causing you to be unable to perform your work, but that is the fault of the location you were at...assuming you didn't have this problem in the first 5 years of working there, that might be an angle worth asking a lawyer or legal representative.
I'm just a CF'r who's had my days of being in the ringer, and thought this might be usefull information...Good Luck and I hope you win (your winning might provoke more places to enforce having cleaner air!)