Lung cancer is a global problem and it is the most frequent cancer in the world. Each year the incidences of lung cancer are unfortunately increasing. Although the use of tobacco has declined in many high income countries, it has become rapidly worse in developing and former socialist countries, where smoking rate is still high. Everyone knows that smoking causes lung cancer but how smoking causes lung cancer is a question that nobody really knows. In this article I will explain how smoking causes lung cancer.
A cigarette smoke is a very complicated mixture of different compounds. More than 4000 different compounds have been indentified in mainstream cigarette smoke. From these 4000 and a bit more compounds 60-70 are carciongens: heterocyclic hydrocarbons, N -heterocyclic amines, polycyclic aromatic hydrocarbons (PAH), N -nitrosamines, aromatic amines, organic compounds, aldehydes, inorganic compounds like hydrazine and some metals and free radicals.
Tobacco contains a high amount of N-nitrosamines and PAH-s, which are which are strong carciogens. Other compounds seem to have lower carciogenic potential. Various studies have shown that PAH-s can induce tumors in laboratory animals. PAH- are formed by incomplete combustion of tobacco. Studies have also shown that PAH-s can interact with the DNA, forming mutagenic DNA adducts. And the formation of DNA adducts may be the primary initiating event of carciogenesis.
The concentration of N-nitrosamines in tobacco is relatively high and heavy smokers have the highest exposure to these compounds. The tobacco-specific N -nitrosamines (TSNA), principally the nicotine-derived NNK, are the strongest respiratory carcinogens identified in tobacco products. Both malignant and benign tumors are formed in mice, hamsters and rats have been induced by NNK. The concentration of NNK in mainstream smoke has increased while those of benzo( a ) pyrene have decreased since the 1960s.
The smoke of cigarettes also contains a large amount of free radicals and free radicals cause oxidative damage. Both the particulate and gas phase are highly oxidative and therefore damage the lung. Cigarette smoke also induces proinflammatory cytokines and recruits activated macrophages and neutrophils to lung tissue.
Cigarettes contain a very high levels of metals. Somewhere around 30 metals have been found. Some evidence has shown that metals are effective initiators of the carcinogenic process, but can also be potential promoters during carcinogenesis. Cigarette smoke contains high levels of cadmium, chromium and also nickel. Chromium accumulates in the lung and tobacco smoking is the main source of cadmium exposure in humans. Unfortunately the complete contribution of these metals to lung cancer is poorly understood.
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