benzo(a)pyrene, a substance produced by the partial combustion of organic matter, such as that which occurs when smoking cigarettes, is not itself toxic, but it reacts with oxygen through the action of enzymes in the body to produce a species that can bind with DNA and cause cancer
smoking cigarettes + O2 bind with DNA = cancer
In discussing toxic responses of the respiratory system, it is important to make a distinction between the respiratory system as an entryway for toxic substances, referred to as inhalation toxicology, and the respiratory system adversely affected by toxicants, referred to as respiratory tract toxicology. The term pulmonary refers to lungs. Generally, toxicants that adversely affect the respiratory system are those that have been inhaled, including such well-known substances as asbestos, chromate, and silica. However, it is possible for systemic poisons transported from elsewhere in the body to act as respiratory system toxicants. One interesting possibility that has been suggested is lung cancer caused by the diol epoxide of benzo(a)pyrene (see Figure 7.3), which is formed by inhalation of benzo(a)pyrene, converted to the ultimate carcinogen in the liver, and transported back to the lung, where cancer develops
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