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Here's my thought process, correct me where I'm wrong. It seems the human body has thresholds.

Examples: You lift too much weight, and the muscles and tendons risk serious injury, but otherwise they tend to rebuild stronger to handle more weight. Too much sun can lead to sunburn or worse, but moderate UVB exposure can lead to melanogenesis and increase protection to UV damage.

Do the lungs behave in a similar manner where a small amount of smoking might actually strengthen your lungs before reaching the threshold where you're causing severe damage? My guess is the answer is "probably/somewhat" but that there haven't been any studies since it'd be difficult to analyze and quantify, but maybe somebody has an answer based on theory.

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From your examples only the first is really true. Exercise leads to better physical abilities (within some boundaries).

Sunlight on the other hand is always potentially dangerous, depending on the dose. But the damages accumulate over the years which finally lead to the so called senile lentigos (others call them aging spots) occuring in sun exposed skin from the age of 50.

And the same is true for cigarette smoke. Here you expose yourself to a nasty cocktail of potentially harmful to cancerogenous substances (tar, polycyclic aromatic hydrocarbons, benzene, nicotine and much more). These do potential harm to your lungs depending on the dose (meaning how many cigarettes you smoke a day), but there is nothing like a harmless amount. Your lungs will eventually recover from this (tar takes the longest to be removed), but it is definitely not good.

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Out of the womb, a person might be extremely prone to sunburn, but wouldn't 15 years of moderate exposure increase the overexposure threshold? Maybe my usage of the word "strengthen" is inappropriate, I mean more along the lines of increasing the threshold until damage. From my admittedly limited perspective, the human body seems to increase it's ability to withstand when exposed to harm, so long as the harmful factors are not prolonged and recovery time is allowed. –  coburne May 6 at 17:45
    
Sun exposure is difficult, since we need it to some degree to produce vitamin D. Nevertheless it can be pretty harmful, even in "small doses". The other problem are the harmful factors itself. There are a lot of things where the body can adapt and recover, like for drinking alcohol (which is also cytotoxic). For other substances which are cancerogenous this is not true. There every hit can be the fatal one which develops into a malignancy. Of course the dose plays a role here, too. But this is the reason why I can't consider smoking harmless. –  Chris May 6 at 18:17

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