Research has generated scientific evidence that secondhand smoke (that is, in the case of cigarettes, a mixture of smoke released from the smoldering end of the cigarette and smoke exhaled by the smoker) causes the same problems as direct smoking, including heart disease,[5] cardiovascular disease, lung cancer, and lung ailments such as COPD, bronchitis and asthma.[6] Specifically, meta-analyses have shown lifelong non-smokers with partners who smoke in the home have a 20–30% greater risk of lung cancer, and those exposed to cigarette smoke in the workplace have an increased risk of 16–19%.[7]
http://en.wikipedia.org/wiki/Second_hand_smoke
Feel free to smoke in your own home if you live alone, but smoking around others or even where they live (it lingers in the air for hours after the act of smoking) is simply irresponsible. You're not only hurting yourself (which is your decision); you're hurting those around you (which is not your decision).
