Mechanism - Smoking

Chemical carcinogens

Smoke, or any partially burnt organic matter, contains carcinogens (cancer-causing agents). The potential effects of smoking, such as lung cancer, can take up to 20 years to manifest themselves. Historically, women began smoking en masse later than men, so an increased death rate caused by smoking amongst women did not appear until later. The male lung cancer death rate decreased in 1975 — roughly 20 years after the initial decline in cigarette consumption in men. A fall in consumption in women also began in 1975 but by 1991 had not manifested in a decrease in lung cancer related mortalities amongst women.

Smoke contains several carcinogenic pyrolytic products that bind to DNA and cause genetic mutations. Particularly potent carcinogens are polynuclear aromatic hydrocarbons (PAH), which are toxicated to mutagenic epoxides. The first PAH to be identified as a carcinogen in tobacco smoke was benzopyrene, which has been shown to toxicate into an epoxide that irreversibly attaches to a cell's nuclear DNA, which may either kill the cell or cause a genetic mutation. If the mutation inhibits programmed cell death, the cell can survive to become a cancer cell. Similarly, acrolein, which is abundant in tobacco smoke, also irreversibly binds to DNA, causes mutations and thus also cancer. However, it needs no activation to become carcinogenic.

There are over 19 known carcinogen in cigarette smoke. The following are some of the most potent carcinogens:

*Polynuclear aromatic hydrocarbons are tar components produced by pyrolysis in smoldering organic matter and emitted into smoke. Many of them are highly carcinogenic and mutagenic, because they are toxicated to mutagenic epoxides, which are electrophilic alkylating agents. The first PAH to be identified as a carcinogen in tobacco smoke was benzopyrene, which been shown to toxicate into a diol epoxide and then permanently attach to nuclear DNA, which may either kill the cell or cause a genetic mutation. The DNA contains the information on how the cell function; in practice, it contains the recipes for protein synthesis. If the mutation inhibits programmed cell death, the cell can survive to become a cancer cell, a cell that does not function like a normal cell. The carcinogenity is radiomimetic, i.e. similar to that produced by ionizing nuclear radiation. Tobacco manufacturers have experimented with combustionless vaporizer technology to allow cigarettes to be consumed without the formation of carcinogenic benzopyrenes. However, such products have become increasingly popular, with world wide markets claiming a safer smoke. No conclusive evidence has shown to prove or disprove health claims.

*Acrolein is a pyrolysis product that is abundant in cigarette smoke. It gives smoke an acrid smell and an irritating, lachromatory effect and is a major contributor to its carcinogenity. Like PAH metabolites, acrolein is also an electrophilic alkylating agent and permanently binds to the DNA base guanine, by a conjugate addition followed by cyclization into a hemiaminal. The acrolein-guanine adduct induces mutations during DNA copying and thus causes cancers in a manner similar to PAHs. However, acrolein is 1000 times more abundant than PAHs in cigarette smoke, and is able to react as is, without metabolic activation. Acrolein has been shown to be a mutagen and carcinogen in human cells. The carcinogenity of acrolein has been difficult to study by animal experimentation, because it has such a toxicity that it tends to kill the animals before they develop cancer. Generally, compounds able to react by conjugate addition as electrophiles (so-called Michael acceptors after Michael reaction) are toxic and carcinogenic, because they can permanently alkylate DNA, similarly to mustard gas or aflatoxin. Acrolein is only one of them present in cigarette smoke; for example, crotonaldehyde has been found in cigarette smoke. Michael acceptors also contribute to the chronic inflammation present in tobacco disease.

*Nitrosamines are a group of carcinogenic compounds found in cigarette smoke but not in uncured tobacco leaves. Nitrosamines form on flue-cured tobacco leaves during the curing process through a chemical reaction between nicotine and other compounds contained in the uncured leaf and various oxides of nitrogen found in all combustion gases. Switching to Indirect fire curing has been shown to reduce nitrosamine levels to less than 0.1 parts per million.

Radioactive carcinogens

In addition to chemical, nonradioactive carcinogens, tobacco and tobacco smoke contain small amounts of lead-210 (210Pb) and polonium-210 (210Po) both of which are radioactive carcinogens. The presence of polonium-210 in mainstream cigarette smoke has been experimentally measured at levels of 0.0263–0.036 pCi (0.97–1.33 mBq), which is equivalent to about 0.1 pCi per milligram of smoke (4 mBq/mg); or about 0.81 pCi of lead 210 per gram of dry condensed smoke (30 Bq/kg).

Research by NCAR radiochemist Ed Martell determined that radioactive compounds in cigarette smoke are deposited in "hot spots" where bronchial tubes branch. Since tar from cigarette smoke is resistant to dissolving in lung fluid, the radioactive compounds have a great deal of time to undergo radioactive decay before being cleared by natural processes. Indoors, these radioactive compounds linger in secondhand smoke, and therefore greater exposure occurs when these radioactive compounds are inhaled during normal breathing, which is deeper and longer than when inhaling cigarettes. Damage to the protective epithelial tissue from smoking only increases the prolonged retention of insoluble polonium 210 compounds produced from burning tobacco. Martell estimated that a carcinogenic radiation dose of 80–100 rads is delivered to the lung tissue of most smokers who die of lung cancer.

Smoking an average of 1.5 packs per day gives a radiation dose of 13-60 mSv/year, compared with living near a nuclear power station (0.0001 mSv/year) or the 3.0 mSv/year average dose for Americans

Nicotine

Nicotine that is contained in cigarettes and other smoked tobacco products is a stimulant and is one of the main factors leading to continued tobacco smoking. Although the amount of nicotine inhaled with tobacco smoke is quite small (most of the substance is destroyed by the heat) it is still sufficient to cause physical and/or psychological dependence. The amount of nicotine absorbed by the body from smoking depends on many factors, including the type of tobacco, whether the smoke is inhaled, and whether a filter is used. Despite the design of various cigarettes advertised and even tested on machines to deliver less of the toxic tar, studies show that when smoked by humans instead of machines, they deliver the same net amount of smoke. Ingesting a compound by smoking is one of the most rapid and efficient methods of introducing it into the bloodstream, second only to injection, which allows for the rapid feedback which supports the smokers' ability to titrate their dosage. On average it takes about ten seconds for the substance to reach the brain. As a result of the efficiency of this delivery system, many smokers feel as though they are unable to cease. Of those who attempt cessation and last three months without succumbing to nicotine, most are able to remain smoke free for the rest of their lives. There exists a possibility of depression in some who attempt cessation, as with other psychoactive substances. Depression is also common in teenage smokers; teens who smoke are four times as likely to develop depressive symptoms as their nonsmoking peers.

Although nicotine does play a role in acute episodes of some diseases (including stroke, impotence, and heart disease) by its stimulation of adrenaline release, which raises blood pressure, heart rate, and free fatty acids, the most serious longer term effects are more the result of the products of the smouldering combustion process. This has enabled development of various nicotine delivery systems, such as the nicotine patch or nicotine gum, that can satisfy the addictive craving by delivering nicotine without the harmful combustion by-products. This can help the heavily dependent smoker to quit gradually, while discontinuing further damage to health.

Nicotine is a highly addictive psychoactive chemical. When tobacco is smoked, most of the nicotine is pyrolyzed; a dose sufficient to cause mild somatic dependency and mild to strong psychological dependency remains. There is also a formation of harmane (a MAO inhibitor) from the acetaldehyde in cigarette smoke, which seems to play an important role in nicotine addiction probably by facilitating dopamine release in the nucleus accumbens in response to nicotine stimuli. According to studies by Henningfield and Benowitz, nicotine is more addictive than cannabis, caffeine, ethanol, cocaine, and heroin when considering both somatic and psychological dependence. However, due to the stronger withdrawal effects of ethanol, cocaine and heroin, nicotine may have a lower potential for somatic dependence than these substances. About half of Canadians who currently smoke have tried to quit. McGill University health professor Jennifer O'Loughlin stated that nicotine addiction can occur as soon as five months after the start of smoking.

Recent evidence has shown that smoking tobacco increases the release of dopamine in the brain, specifically in the mesolimbic pathway, the same neuro-reward circuit activated by drugs of abuse such as heroin and cocaine. This suggests nicotine use has a pleasurable effect that triggers positive reinforcement. One study found that smokers exhibit better reaction-time and memory performance compared to non-smokers, which is consistent with increased activation of dopamine receptors. Neurologically, rodent studies have found that nicotine self-administration causes lowering of reward thresholds—a finding opposite that of most other drugs of abuse (e.g. cocaine and heroin). This increase in reward circuit sensitivity persisted months after the self-administration ended, suggesting that nicotine's alteration of brain reward function is either long lasting or permanent.[citation needed] Furthermore, it has been found that nicotine can activate long term potentiation in vivo and in vitro.[citation needed] These studies suggest nicotine’s "trace memory" may contribute to difficulties in nicotine abstinence.[original research?]

The carcinogenity of tobacco smoke is not explained by nicotine per se, which is not carcinogenic or mutagenic. However, it inhibits apoptosis, therefore accelerating existing cancers. Also, NNK, a nicotine derivative converted from nicotine, can be carcinogenic.

It is worth noting that nicotine, although frequently implicated in producing tobacco addiction, is not significantly addictive when administered alone. The addictive potential manifests itself after co-administration of an MAOI, which specifically causes sensitization of the locomotor response in rats, a measure of addictive potential.

Genetic

Aespective of smoking status or quantity smoked.

Another study related to genetic changes in smokers was conducted by Wan L Lam and Stephen Lam from the BC Cancer Agency, in 2007. The study revealed that cigarette smoke can turn on or off some of the genes, which otherwise would remain inactive or active respectively. Some changes on genetic level could be reversed after the smoking was quit, yet others could not. Examples of reversible genes involved the so-called xenofobic functions, nucleotide metabolism and mucus secretion. Smoking turns off some DNA repair genes that cannot be reversed. It also switches off some genes responsible from protection from cancer growth in the body.