[Xixi]

Just speculating here, but doesn't it have something to do with accumulation in the body? Your body obviously can't accumulate radiations. It also does process and eliminate the like of ethanol and many other substances. That's not the case, I believe, for heavy metals like mercury, lead or arsenic, that are eliminated much more slowly (if at all?).

That's why you can enjoy lychees by eating a few everyday if you fancy it, and will only poison yourself if you eat a lot at once. But you can poison someone by exposing them to a little bit of arsenic everyday (at least in the movies, not sure how true it is...)

I think tar from smoke accumulates, and there is a lot of it in second-hand smoke.

Obviously not a doctor, so I'm probably completely wrong...

[robocat]

The damage from smoking is accumulative. I have noticed it in the skin of young female smokers after a few years of heavy smoking. You can see it's affects on some other parts of the body over longer periods.

Drinking a lot of alcohol has a similar effects as tobacco on your health, and on the skin. If you know a heavy drinker then they quit for six months and then you see their face, there is noticeable rejuvenation. There are patterns of looks to older people that drink at pubs frequently.

[spider-mario]

You might enjoy this excerpt from E. T. Jaynes’ book “Probability Theory: The Logic of Science”:

https://bayes.wustl.edu/etj/prob/book.pdf#page=16 (the section that starts with “What is ‘safe’?”)