Smoking and Lung Cancer
Cigarette smoking is the single greatest risk factor for lung cancer. There is a significant "dose-response" relationship between the number of pack-years smoked and lung cancer risk; that is, the more a person smokes and the longer he or she smokes, the greater the risk for lung cancer.
How does cigarette smoking cause lung cancer? This question has not yet been answered definitively, but the toxic mix of chemicals found in tobacco smoke is the likely factor. Major chemicals in cigarette smoke that have been studied with respect to lung cancer include:
- polycyclic aromatic hydrocarbons (PAHs)
- nicotine by-products
- tobacco-specific nitrosamines (TSNAs)
- metals (e.g., nickel, cadmium)
- radioactive polonium 210 (210Po)
Types of Tobacco
The main commercially grown species of tobacco used in cigarette products in the United States, western Europe, and Japan is Nicotiana tabacum. Within this species, sub-types of tobacco include bright (Virginia, flue-cured), Burley, Maryland, and oriental (aromatic) tobaccos. Cigarettes made in the United Kingdom and Finland primarily use bright tobaccos. Both varieties of cigarettes, when ignited, create smoke particles containing nicotine. Such "mainstream" smoke particlesminus water and nicotineare the "tar" produced by burning cigarettes.
Cigarettes are made from sheets of reconstituted tobacco. Special solutions are used to keep the tobacco mixture intact, and chemicals known as humectants are added to maintain tobacco moisture. In addition, trade secret flavorants are included to make the product taste better during smoking. Such tobacco additives, when burned, may yield undesirable compounds. For example, the burning of licorice flavorant may produce chemicals known as polycyclic aromatic hydrocarbons (PAHs), and the burning of sugar may increase tar and nicotine in the smoke. Coumarin, a known cancer-causing substance in animals, has in the past been used as a tobacco additive. It is also likely that commercial tobaccos contain up to a few parts per million of the pesticides DDT and DDD, as well as the agricultural chemical maleic hydrazide.
Approximately 400 to 500 separate gaseous substances are present in the smoke of a nonfilter cigarette. The major elements of cigarette vapor include:
Other noteworthy substances include:
- nitrogen oxides
- hydrogen cyanide
The particles of cigarette smoke contain at least 3500 individual compounds such as
- tobacco alkaloids (nornicotine, anatabine, anabasine)
- polycyclic aromatic hydrocarbons (PAHs; e.g., benzo(a)pyrene, B(a)P)
- aromatic amines
- tobacco-specific nitrosamines (TSNAs)
Tobacco-specific nitrosamines (TSNAs) are formed during tobacco curing and processing. TSNAs are chemicals that are suspected of causing lung cancer in humans. In rodent studies, regardless of the where or how it is applied, the TSNA known as NNK produces lung adenomas—benign tumors of epithelial (surface cell) tissue and lung adenocarcinomas—malignant epithelial tumors with gland-like characteristics. The TSNA known as NNAL also produces lung adenocarcinomas in rodents, although it is a more powerful pancreatic carcinogen (substance that causes cancer of the pancreas) in rats.
TSNAs in Cigarette Smoke
Some of the TSNAs found in cigarette smoke particles are
- N-nitrosonornicotine (NNN)
- 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK)
- N-nitrosoanatabine (NAB)
Other TSNAs include
- 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanol (NNAL)
- N-nitrosoanatabine (NAT)
- N-nitrosoanabasine (NAB)
- 4-(N-methyl-N-nitrosamino)-4-(3-pyridyl)-1-butanol (iso-NNAL)
- 4-(N-methyl-N-nitrosamino)-4-(3-pyridyl)butyric acid (iso-NNAC)
The U.S. National Academy of Science estimates that a pack-a-day smoker is exposed to about 17 micrograms (mg) of cancer-causing TSNAs daily.
Approximate TSNA Levels in Cigarette Smoke
- NNNNonfilter: 278; Filter: 209
- NATNonfilter: 236; Filter: 172
- NABNonfilter: 30; Filter: 21
- NNKNonfilter: 156; Filter: 156
- Total TSMAsNonfilter: 700; Filter: 558
Manufacturers can reduce the levels of TSNA in cigarette smoke by using lighter tobacco blends and by selecting parts of the tobacco plant that are low in nitratea forerunner of TSNAs. However, tobacco blends with low amounts of nitrate may have higher levels of polycyclic aromatic hydrocarbons (PAHs) in mainstream smoke.
Polycyclic aromatic hydrocarbons (PAHs) also are thought to be major contributors to lung cancer risk in smokers. PAHs are "procarcinogens" that are metabolized, or broken down by the body into reactive substances. For example, the chemical benzo(a)pyrene is changed into a compound that is known to react with human genetic material (DNA) and form DNA "adducts." It is thought that such adducts may cause problems with lung cell reproduction that eventually may lead to lung cancer. Lung cancer patients who still smoke have higher levels of PAH-DNA adducts than smokers without lung cancer.
PAH Levels in Cigarette Smoke
The International Agency for Research on Cancer (IARC) has declared that some PAHs in tobacco smoke show "sufficient" evidence of cancer-causing effects (carcinogenicity) in laboratory animals. Such PAHs include
Other Chemicals in Cigarette Smoke
Other chemicals in tobacco may damage the lungs. Tobacco contains at least 30 metals, although the most toxic of thesenickel and cadmiumare present in only small quantities. Most metals found in tobacco come from the soil, fertilizers, or agricultural sprays. The element polonium 210 (210Po), which is a radioactive compound, also has been identified in the particulate portion of cigarette smoke (0.03–0.07 pCi per cigarette).