Secondhand smoke is smoke from burning tobacco products, such as cigarettes, cigars, or pipes. It is smoke that has been exhaled, or breathed out, by the person smoking. Tobacco smoke contains more than 7,000 chemicals, including hundreds that are toxic and about 70 that can cause cancer. Indoors or outdoors, there is no safe level of exposure to smoke from tobacco products. Smoke can travel between rooms though stairwells, air ducts, plumbing, electrical wiring routes, and even through tiny cracks in floors, doors, and walls. Women who are pregnant, babies, children, people with asthma, and other chronic health conditions are particularly at-risk of developing health problems from secondhand smoke. more