What are the chemicals in tar that are carcinogenic, and how do they cause cancer?

There are over 19 known chemicals in cigarettes that cause cancer. The most prominent of these are found in two groups: organic carcinogens and radioactive carcinogens. The organic carcinogens include polynuclear aromatic hydrocarbons, acrolein, and nitrosamines. The radioactive carcinogens comprise lead-210 and polonium-210, both of which decay into other radioactive carcinogens.

The first and most prominent polynuclear aromatic hydrocarbon identified was benzopyrene[1]. Benzopyrene is metabolized into another chemical and permanently attaches to DNA (see Question 14), either killing the cell or leading to genetic mutation, which can transform it into a cancer cell. A cancer cell doesn't reproduce and die naturally, but reproduces rapidly and will replace cells in a tissue, organ, and, if left unchecked, an entire body system before killing the organism.

Acrolein[2], the other chemical, gives cigarette smoke its characteristic odor and is irritating to the nose and lungs. It also binds permanently to DNA. Acrolein is 1000 times greater in cigarette smoke than benzopyrene. Nitrosamines[3] are a group of compounds found in cigarette smoke but not in uncured tobacco leaves. They form as a result of curing. Nitrosamines also are produced by some foods through grilling and frying, and there is a correlation between the amounts of certain foods (such as grilled red meats) that are eaten and incidence of colon cancer. The radioactive elements in tobacco are a result of their natural exposure to minerals in the soil, so their content varies widely with the soil content. Whether or not these radioactive chemicals are found in sufficient quantities to cause cancer remains open to debate. Some researchers argue that they are of sufficient quantity to account for most of the lung cancer related to smoking.

How does smoking alter DNA?

Deoxyribonucleic acid (DNA) is the molecular basis of heredity located in the cell nuclei. DNA is the material that makes up a person's genes, which is necessary for the construction, organization, and function of living cells, tissues, organs, and organisms. Many of humans' most basic traits are influenced by DNA to varying degrees, such as height, weight, types of intellectual and athletic skills, and personality traits. Additionally, DNA can influence the susceptibility to develop certain diseases, including addiction and cancer. No heritability from DNA is one-to-one. In other words, even where DNA plays a large role in the inheritance of a particular trait, such as eye and skin color, it is never 100%. DNA essentially sets boundaries between two extremes, but where a trait ultimately falls within those boundaries, in large part will be determined by one's environment. Environmental influences therefore still alter any particular trait. Given that even traits that are "hardwired" into DNA can change, when we talk about the traits that are less hardwired (such as the susceptibility to developing cancer or addiction), DNA is less of an influence than environment.

The term environment requires some discussion. Most people think of environment in terms of inheritance as the immediate surroundings. These may be family, geography, community, culture, and nationality. But environment also includes physical characteristics, from microscopic changes surrounding a cell to macroscopic changes such as air and water quality, and diet. Scientists are increasingly interested in how tobacco affects the micro-environment of the cell. They want to know what causes the DNA to change (or mutate) as a result of that environmental change. It is this idea that leads scientists to study all types of diet and air and water qualities in order to understand better how they impact the body and the cells more directly.

Smoking's impact on DNA is reciprocal. In other words, there are genetic factors that influence one's response to tobacco and tobacco in turn influences one's genes by altering them through genetic mutation. This impact is threefold:

Individuals can be genetically more susceptible to becoming addicted to tobacco.

Individuals can be genetically more susceptible to the various diseases tobacco causes.

Tobacco itself can directly alter DNA and cause the cell to mutate.

Three separate studies have identified a genetic link between the susceptibility to become addicted and increasing the likelihood of developing lung cancer by up to 80%. These genetic variations involve a region on chromosome 15, which codes for the nicotine receptors. Possessing a single copy of the mutation increases the risk of developing lung cancer by 30%. Possessing two copies increases the risk by 80%. Several studies have concluded that smoking damages the DNA in the cells, causing carcinogenic changes that contribute to the risk of developing cancer. Over 19 primary carcinogens have been identified in tobacco smoke, of which the major constituents were discussed previously in Question 13.

  • [1] A class of antianxiety medications that include the drugs commonly known as Valium and Xanax.
  • [2] Responsible for the gummy yellowish residue and acrid smell from burning cigarettes. It is considered carcinogenic and is toxic to the skin. It was used in chemical warfare during World War I.
  • [3] Nitrosamines are found in many foods, including beer, fish, and also in meat and cheese products preserved with nitrite pickling salt. They are also produced from grilling and frying food as well as from burning tobacco. Carcinogenic in a wide variety of animal species, a feature suggesting that they may also be cancer-causing in humans.
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