Secondhand smoke, also known as environmental tobacco smoke, is harmful and contains over 4,000 chemicals, including 50 carcinogens. It can cause various health issues for nonsmokers like eye irritation, headaches, and increased risk of lung cancer and heart disease. Exposure to secondhand smoke is especially harmful for unborn children, infants, and young children, increasing risks like SIDS, respiratory infections, and ear infections. While laws aim to reduce exposure through smoking bans in public places, individuals can also take actions like establishing smoke-free homes and requesting smoke-free environments when visiting others.