Those at greatest risk are smokers, and the risk increases with the number of cigarettes smoked. Among men who smoke more than two packs of cigarettes a day, the death rate from cancer of the lung is roughly 15 to 20 times higher than among men who do not smoke. Other factors influencing risk include the number of years of smoking, the age at which smoking commenced, and how deeply the smoker inhales. The best way to prevent lung cancer is to not smoke or be around people who do (passive smokers). People should not start smoking, and those who already smoke should quit. Everyone should avoid breathing in other people’s smoke. People should find out about cancer–causing chemicals they may be exposed to at work and take appropriate protective measures.