Overview
Lung damage is a side effect of some cancer treatments. This damage may include inflammation, which reduces the amount of oxygen you can absorb, and/or scarring, which reduces the amount of air you can breathe. Both of these result in uncomfortable symptoms, including shortness of breath and fatigue. Treatment for lung damage is primarily aimed at relieving symptoms.

• What are lung toxicities?
• What causes lung toxicity?
• What are the symptoms of lung toxicity?
• How are lung toxicities managed?
• What else can I do?

When you read through this (and other) information you will find that there is in fact very little you can do - it is basically untreatable. It seems to appear a few months after cancer treatment, but I haven't been able to find out whether there is still a risk after a year or more, although logically you would think the passage of time and exposure to other things like dust, smoke etc. would make it more likely. Not one medical specialist has ever mentioned this to me, even though I do have a low lung capacity as evidenced by my response to anaesthetic: quite often I cause a panic because my level of oxygen drops so much due to being unable to breathe properly.  After one occasion they had me on high-level oxygen and breathing into a spirometer six times a day. I continue to have shallow breathing breathing and try to remind myself to deep-breathe every time I am in a good place where the air is clean. Of course deep breathing isn't going to help where the air itself is polluted/dusty or full of pollens and other allergens.
 
So unfortunately this is just another example of the way in which the bad effects of cancer treatments are not fully explained or maybe not explained at all. Also I can't understand why they don't take proper case histories from patients before the therapies start: for instance, how many among us was even asked whether we have lung problems, or bladder issues, or whatever? At least we might be forewarned about what we might expect and not be blindsided by it.

Poor @Blossom1961, so very sorry to hear about the awfulness you are going through. Everyone of course is wishing you well, but apart from cyber-hugs there's so little we can do ... awful to feel so helpless. Hope you are feeling better today, let us know how you are doing. 
A.