Respiration and Respiratory Systems — страница 2
conducted, but neither screening method appears to have a major impact on mortality. Therefore, attention has been turned to prevention by every means possible. Foremost among them are efforts to inform the public of the risk and to limit the advertising of cigarettes. Steps have been taken to reduce asbestos exposure, both in the workplace and in public and private buildings, and to control air pollution. The contribution of air pollution to the incidence of lung cancer is not known with certainty, though there is clearly an "urban" factor involved. Persons exposed to radon daughters are at risk for lung cancer. The hazard from exposure was formerly thought to be confined to uranium miners, who, by virtue of their work underground, encounter high levels of these radioactive materials. However, significant levels of radon daughters have been detected in houses built over natural sources, and with increasingly efficient insulation of houses, radon daughters may reach concentrations high enough to place the occupants at risk for lung cancer. A recent survey of houses in the United States indicated that about 2 percent of all houses had a level of radon daughters that posed some risk to the occupants. Major regional variations in the natural distribution of radon occur, and it is not yet possible to quantify precisely the actual magnitude of the risk. In some regions of the world (such as the Salzburg region of Austria) levels are high enough that radon daughters are believed to account for the majority of cases of lung cancer in nonsmokers. Workers exposed to arsenic in metal smelting operations, and the community around the factories from which arsenic is emitted, have an increased risk for lung cancer. Arsenic is widely used in the electronics industry in the manufacture of microchips, and careful surveillance of this industry may be needed to prevent future disease. Some types of lung cancer are unrelated to cigarette smoking. Alveolar cell cancer is a slowly spreading condition that affects men and women in equal proportion and is not related to cigarette smoking. Pulmonary adenocarcinoma of the lung also has a more equal sex incidence than other types, and although its incidence is increased in smokers, it may also be caused by other factors. It is common to feel intuitively that one should be able to apportion cases of lung cancer among discrete causes, on a percentage basis. But in multifactorial disease, this is not possible. Although the incidence of lung cancer would probably be far lower without cigarette smoking, the contribution of neither this factor nor any of the other factors mentioned can be precisely quantified.