Secondhand smoke is a kind of mixed smoke that is burned or exhaled by cigarettes, pipes or cigars when smokers smoke. Secondhand smoke is the most common pollutant in many smoking places. Smoke emitted during smoking can release more than 4,000 kinds of gases and particles, most of which are very strong irritants, and at least 40 of them can cause cancer in humans or animals. After smokers quit smoking, these particles can still stay in the air for several hours, can be inhaled by other non-smokers, and may be mixed with the decay products of radon gas, causing great harm to human health.