Of course there are other carcinogens around like carbon monoxide - but -- and it's a big but - We don't shut ourselves into rooms full of running cars.
There are many studies out there on cancers in locations other than the respiratory system which suggest a link to cigarette smoke. Bladder and breast cancer are among them.

Over a singing career that spans 40 years I've heard all the arguments from smokers about why I have to put up with their habit. It's been 30 years since I was able to accept invitations to gatherings in restaurants, pubs, (and private homes where smoking is allowed). I have permanant breathing problems from working in smoke - as a singer and also as a nurse.
The ban didn't come soon enough for me. I am glad it came in time for my grandchildren.
Cheers, Joy