Secondhand smoke (SHS) is a mixture of the side stream smoke released by a smoldering cigarette, pipe, hookah/waterpipe, or cigar, and the mainstream smoke exhaled by a smoker. SHS is a complex mixture containing thousands of chemicals, including formaldehyde, cyanide, carbon monoxide, ammonia, and nicotine. More than 250 of the chemicals in SHS are known to be harmful, and at least 69 are known to cause cancer.
Conclusive scientific evidence documents that SHS causes premature death and disease in children and adults who do not smoke. Exposure to SHS by adults has immediate adverse effects on the cardiovascular system, and long-term exposure to SHS causes coronary heart disease and lung cancer. Children exposed to SHS are at increased risk for sudden infant death syndrome, acute respiratory infections, middle ear disease, more severe asthma, respiratory symptoms, and slowed lung growth.
Secondhand aerosol is a mixture of chemicals in the aerosol exhaled by e-cigarette users. Some of the chemicals found in SHS are also present in secondhand aerosol. Although these levels are lower than in secondhand smoke, exposure is not risk-free. Besides nicotine, e-cigarette aerosol may contain heavy metals, volatile organic compounds, and fine and ultrafine particles that can be inhaled deeply into the lungs by both users and bystanders. Secondhand aerosol is often incorrectly referred to as "vapor".