When I was young, I figgured what would be the big deal smoking a few cigarettes per day. By the time I was 25 I realized that two packs per day was getting out of control.

I don't believe there is any harmful information but the whole picture is that most folks cannot smoke a few per day. Eventually cigarettes will own most smokers.

You probably have to differentiate risk. I assume for some risks associated with smoking there is a linear correlation with exposure, where it's on/off for others, or e.g. following a logarithmic function.

Nicotine, CO and radioactive toxicity have a long half-life, where primary radical damage or reactive carcinogen burden is dose dependent stochastics.

Also people forget, you are much, much more likely to die from cardio-vascular damage or COPD as a consequence of smoking, than lung cancer.