НАУЧНАЯ БИБЛИОТЕКА - РЕФЕРАТЫ - Respiration and Respiratory Systems
Respiration and Respiratory Systems
Respiration and Respiratory Systems
Up to the time of World War II, cancer of the lung was a relatively rare
condition. The increase in its incidence in Europe after World War II was
at first ascribed to better diagnostic methods, but by 1956 it had become
clear that the rate of increase was too great to be accounted for in this
way. At that time the first epidemiological studies began to indicate that
a long history of cigarette smoking was associated with a great increase in
risk of death from lung cancer. By 1965 cancer of the lung and bronchus
accounted for 43 percent of all cancers in the United States in men, an
incidence nearly three times greater than that of the second most common
cancer (of the prostate gland) in men, which accounted for 16.7 percent of
cancers. The 1964 Report of the Advisory Committee to the Surgeon General
of the Public Health Service (United States) concluded categorically that
cigarette smoking was causally related to lung cancer in men. Since then,
many further studies in diverse countries have confirmed this conclusion.
The incidence of lung cancer in women began to rise in 1960 and
continued rising through the mid-1980s. This is believed to be explained by
the later development of heavy cigarette smoking in women compared with
men, who greatly increased their cigarette consumption during World War II.
By 1988 there was evidence suggesting that the peak incidence of lung
cancer due to cigarette smoking in men may have been passed. The incidence
of lung cancer mortality in women, however, is increasing.
The reason for the carcinogenicity of tobacco smoke is not known.
Tobacco smoke contains many carcinogenic materials, and although it is
assumed that the "tars" in tobacco smoke probably contain a substantial
fraction of the cancer-causing condensate, it is not yet established which
of these is responsible. In addition to its single-agent effects, cigarette
smoking greatly potentiates the cancer-causing proclivity of asbestos
fibres, increases the risk of lung cancer due to inhalation of radon
daughters (products of the radioactive decay of radon gas), and possibly
also increases the risk of lung cancer due to arsenic exposure. Cigarette
smoke may be a promoter rather than an initiator of lung cancer, but this
question cannot be resolved until the process of cancer formation is better
understood. Recent data suggest that those who do not smoke but who live or
work with smokers and who therefore are exposed to environmental tobacco
smoke may be at increased risk for lung cancer, eloquent testimony to the
power of cigarettes to induce or promote the disease.
Because lung cancer is caused by different types of tumour, because it
may be located in different parts of the lung, and because it may spread
beyond the lungs at an early stage, the first symptoms noted by the patient
vary from blood staining of the sputum, to a pneumonia that does not
resolve fully with antibiotics, to shortness of breath due to a pleural
effusion; the physician may discover distant metastases to the skeleton, or
in the brain that cause symptoms unrelated to the lung. Lymph nodes may be
involved early, and enlargement of the lymph nodes in the neck may lead to
a chest examination and the discovery of a tumour. In some cases a small
tumour metastasis in the skin may be the first sign of the disease. Lung
cancer may develop in an individual who already has chronic bronchitis and
who therefore has had a cough for many years. The diagnosis depends on
securing tissue for histological examination, although in some cases this
entails removal of the entire neoplasm before a definitive diagnosis can be
Survival from lung cancer has improved very little in the past 40 years.
Early detection with routine chest radiographs has been attempted, and
large-scale trials of routine sputum examination for the detection of
malignant cells have been 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.