That’s not the only thing they do. They also reduce your capacity for work… and life. I have a reason to believe that people dying from “overwork” are actually because:

1. People work (and play) more when they are young and in school etc.
2. Get used to their ability to work as much and subconsciously set a mental bar.
3. Get into office space full of secondary smoke / start smoking
4. Smoke reduces their ability
5. They don’t realise their reduced ability and keep on working as much as previously set bar.
6. dedz

Just a hypothesis. No scientific backing.

… other than first hand exp with reducing ability to work after long term exposure in a heavily contaminated environment.