Second-hand smoke, also known as environmental tobacco smoke (ETS), is one of the most harmful indoor pollutants. Exposure to ETS is a worldwide silent cause of mortality and morbidity. Although ETS had been decreased for 20 years, a lot of people who do not smoke still exposed to ETS at home, work, public places, and in vehicles. ETS is a risk factor for many important diseases such as lung cancer, chronic obstructive lung disease, asthma, cardiovascular diseases, upper and lower respiratory tract infections, and sudden infant death. In this article, the harmful effects of ETS and the effects of smoke-free environment regulation on ETS exposure were reviewed.