Carcinogens are agents that cause cancer, and include chemicals, radiation, and some viruses. While avoiding contact with carcinogenic agents is wise, it is virtually impossible to steer clear of them completely. Ultra-violet radiation from the sun, substances in food, and even oxygen can induce malignancies. In spite of the pervasive nature of carcinogens, however, not all individuals develop cancer, which suggests that mere contact with a carcinogenic agent is insufficient to produce this lethal disease. That is because organisms have evolved protective mechanisms to prevent cancer, and some of these defenses work by thwarting the potentially harmful effects of carcinogens.

Breathing second hand smoke increases a person's heart rate and blood pressure, and delivers dangerous amounts of carbon monoxide to the blood. It is estimated that 2 percent of lung cancer deaths are caused by passive smoking.

Cancer-Causing Chemicals

References to cancer have been found in the annals of...