From the American Cancer Society: Secondhand smoke causes cancer Secondhand smoke (SHS) is classified as a “known human carcinogen” (cancer-causing agent) by the US Environmental Protection Agency (EPA), the US National Toxicology Program, and the International Agency for Research on Cancer (IARC – a branch of the World Health Organization). Tobacco smoke is a mixture of gases and particles. It contains more than 7,000 chemical compounds. More than 250 of these chemicals are known to be harmful, and at least 69 are known to cause cancer. SHS has been linked to lung cancer. There is also some evidence suggesting it might be linked to lymphoma, leukemia, and brain tumors in children, and cancers of the larynx (voice box), pharynx (throat), nasal sinuses, brain, bladder, rectum, stomach, and breast in adults.
Non-smokers who breathe in secondhand smoke take in nicotine and other toxic chemicals just like smokers do. The more secondhand smoke you are exposed to, the higher the level of these harmful chemicals in your body.