1st of all, you are breathing in carcinogens all the time. The same with cigarette smoke. Pretty much everywhere you go, you are breathing it in. You don't have to be right next to a smoker, or in the same room as the smoker for their smoke to also damage you. If you where deodorants or perfumes, you are damaging your lungs. Breathing in fire smoke is damaging your lungs. The fact is our lungs are being damaged all the time - by pollution, by asbestos, by air fresheners, blah blah blah… believe me, you have inhaled more than just a single breath of smoke. You will drive yourself mental if you dwell on these things.
Sunlover, the fact that your mum worked with a chain smoker and is still healthy at 80, is not evidence that breathing in a single breathe of second hand smoke is not damaging; the same way that the fact there exists people who have smoked for majority of their lives and never got cancer is not evidence that smoking doesn't cause cancer. We know it does. Some people are less fortunate than others. Any breath of any carcinogenic is damaging. But the damage is insignificant if we are talking about a single breath. It takes a lifetime but like I said, some are less fortunate. This isn't just the media getting kicks - their is actual data - quantitative data and the risk can be calculated. But like anything, it is a risk - a probability based on statistics. As hypochondriacs, we dwell on these statistics but they are meaningless to us.
In the UK, 15% of those with lung cancer are non-smokers - meaning it requires a lifetime of second hand smoke to have any significance - but even then, you are unbelievably unlucky to get it. I think you are wasting your time worrying about second hand smoke - or worrying about damaging your lungs in anyway. It happens. It is pretty much inexorable. You are only going to make yourself worse (anxiety wise) thinking about these things.
Rather than worrying about it, you should just focus on trying to prevent damaging your lungs as much as possible but don't dwell on it because you can't expect to have perfect lungs. In the meantime, you need to focus on the real problem - your anxiety and thought patterns.