Could be second hand smoke, mold, long exposure to diesel fumes,asbestos in house, etc.
It’s not just tobacco...but we know ..without a doubt..the tobacco can cause it.
There's been a fair amount of news about Radon gas in recent years, a gas that's in the ground in many areas and can seep into homes and possibly cause lung cancer and other respiratory problems.
I've not heard about mold. I'm thinking there would have to be prolonged exposure?