Tobacco smoke is an aerosol produced by the incomplete combustion of tobacco during the smoking of cigarettes, pipes, or cigars. Temperatures in burning cigarettes range from about 400 ℃ between puffs to about 900 ℃ during a puff. During the burning of tobacco (itself a complex mixture), thousands of chemical substances are generated by combustion, distillation, pyrolysis, and pyrosynthesis. Tobacco smoke is used as a fumigant and inhalant.
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Composition
The particles in tobacco smoke are liquid aerosol droplets (~ 20% water), with a mass median aerodynamic diameter (MMAD) that is submicrometer (and thus, fairly "lung-respirable" by humans). The droplets are present in high concentrations (some estimates are as high as 10¹⁰ droplets per cm³). Most cigarettes today contain a cigarette filter, which can reduce "tar" and nicotine smoke yields up to 50% by several different mechanisms, with an even greater removal rate for other classes of compounds (e.g., phenols).
Tobacco smoke may be grouped into a particulate phase (trapped on a glass-fiber pad, and termed "TPM" (total particulate matter)) and a gas/vapor phase (which passes through such a glass-fiber pad). "Tar" is mathematically determined by subtracting the weight of the nicotine and water from the TPM. However, several components of tobacco smoke (e.g., hydrogen cyanide, formaldehyde, phenanthrene, and pyrene) do not fit neatly into this rather arbitrary classification, because they are distributed among the solid, liquid and gaseous phases.
Tobacco smoke contains a number of toxicologically significant chemicals and groups of chemicals, including polycyclic aromatic hydrocarbons (benzopyrene), tobacco-specific nitrosamines (NNK, NNN), aldehydes (acrolein, formaldehyde), carbon monoxide, hydrogen cyanide, nitrogen oxides, benzene, toluene, phenols (phenol, cresol), and aromatic amines (nicotine, ABP (4-Aminobiphenyl)). The radioactive element polonium-210 is also known to occur in tobacco smoke. The chemical composition of smoke depends on puff frequency, intensity, volume, and duration at different stages of cigarette consumption.
Between 1933 and the late 1940s, the yields from an average cigarette varied from 33 to 49 mg "tar" and from < 1 to 3 mg nicotine. However, in the 1960s and 1970s, the average yield from cigarettes in Western Europe and the USA was around 16 mg tar and 1.5 mg nicotine per cigarette. Current average levels are lower. This has been achieved in a variety of ways including use of selected strains of tobacco plant, changes in agricultural and curing procedures, use of reconstituted sheets (reprocessed tobacco leaf wastes), incorporation of tobacco stalks, reduction of the amount of tobacco needed to fill a cigarette by expanding it (like puffed wheat) to increase its "filling power", and by the use of filters and high-porosity wrapping papers. The development of lower "tar" and nicotine cigarettes has tended to yield products that lacked the taste components to which the smoker had become accustomed. In order to keep such products acceptable to the consumer, the manufacturers reconstitute aroma or flavor.
Safety
Tobacco smoke, besides being an irritant and significant indoor air pollutant, is known to cause lung cancer, heart disease, chronic obstructive pulmonary disease (COPD), emphysema, and other serious diseases in smokers (and, according to some, in non-smokers as well). The actual mechanisms by which smoking can cause so many diseases remain largely unknown. Many attempts have been made to produce lung cancer in animals exposed to tobacco smoke by the inhalation route, without success. It is only by collecting the "tar" and repeatedly painting this on to mice that tumors are produced, and these tumors are very different from those tumors exhibited by smokers. Tobacco smoke is associated with an increased risk of developing respiratory conditions such as bronchitis, pneumonia, and asthma. Tobacco smoke aerosols generated at temperatures below 400 ℃ did not test positive in the Ames assay.
In spite of all changes in cigarette design and manufacturing over the last 50 years, the use of filters and "light" cigarettes has neither decreased the nicotine intake per cigarette, nor has it lowered the incidence of lung cancers (NCI, 2001; IARC 83, 2004; U.S. Surgeon General, 2004). The shift over the years from higher- to lower-yield cigarettes may explain the change in the pathology of lung cancer. That is, the percentage of lung cancers that are adenocarcinomas has increased, while the percentage of squamous cell cancers has decreased. The change in tumor type is believed to reflect the higher nitrosamine delivery of lower-yield cigarettes and the increased depth or volume of inhalation of lower-yield cigarettes to compensate for lower level concentrations of nicotine in the smoke.
In the United States, lung cancer incidence and mortality rates are particularly high among African American men. Lung cancer tends to be most common in developed countries, particularly in North America and Europe, and less common in developing countries, particularly in Africa and South America. Over 85% of all lung cancers are attributed to cigarette smoking; however, only a fraction of long-term cigarette smokers develop lung cancer. Cancer of the lung is occasionally observed among nonsmokers, indicating a genetic susceptibility in lung tumorigenesis. The incidence of lung cancer seems to have increased in dogs as well as in humans and cigarette smoking can hardly be incriminated.
Tobacco polyphenols (e. g., caffeic acid, chlorogenic acid, scopoletin, rutin) determine the taste and quality of the smoke. Freshly cured tobacco leaf is unfit for use because of its pungent and irritating smoke. After fermentation and aging, the leaf delivers mild and aromatic smoke.
Plant smoke in general has a bacteriostatic effect and is one of the oldest methods of preserving meat (along with salting, adding sugar, and drying).
Nicotine hypothesis
According to the mainstream "nicotine hypothesis", nicotine is the main biochemical reason for tobacco smoking. Nicotine is similar to acetylcholine, an important neurotransmitter. The uptake of nicotine may be facilitated by other chemicals present in tobacco smoke. However, the nicotine theory cannot explain the following contradictions:
Other possibilities are various neurotransmitter-like psychoactive aromatic amines (amphetamines, indole-based synthetic cannabinoids, ergine-type alkaloids), synthesized randomly by pyrolysis and pyrosynthesis during smoking and found in various plant smoke condensates. Nicotine itself may serve as a template for further psychoactive compounds.
Peak plasma levels of nicotine occur 2–10 min after smoking tobacco. Nicotine undergoes a large first-pass effect during which the liver metabolizes 80–90%. Protein binding ranges from 4.9% to 20%. The apparent volume of distribution varies between 1 and 3 l/kg. The metabolites include isomethylnicotinium ion, nornicotine, cotinine, and nicotine-1-N-oxide. Nicotine passes into breast milk in small quantities. In addition to nicotine, tobacco smoke contains small amounts of the alkaloids anatabine, anabasine, nornicotine, N-methylanabasine, anabaseine, nicotine N'-oxide, myosmine, β-nicotyrine, cotinine, and 2,3'-bipyridyl.
The nicotine molecule contains two nitrogen atoms with basic properties. The nicotine molecule can thus add one proton to form a monoprotonated species or two protons to form the diprotonated species. Nicotine in tobacco smoke exists in either a protonated or unprotonated form, depending on the presence of natural acids and bases, the tobacco blend, tip ventilation, and the use of additives. Whereas protonated nicotine is positively charged and not volatile, unprotonated nicotine ("free-base nicotine") from tobacco smoke is volatile and readily passes into lipid cell membranes. More unprotonated nicotine in a puff delivers more nicotine to the brain. The same holds true for other (tobacco and non-tobacco) alkaloids. The approximate dividing line between dominance by protonated forms or by the unprotonated form is a pH of 8. At pH 8, the two fractions are present in equal parts. At any lower pH, the fraction of protonated nicotine is greater.
More unprotonated nicotine in tobacco smoke increases smoke "strength", "impact", or "kick". However, the irritation and harshness of smoke at higher pH makes it harder for smokers to inhale it. Cigarette design ensures that the smoke has enough unprotonated nicotine to rapidly transfer nicotine into the brain but not so much of it as to be too harsh for the smoker. Variability in the acid-base nature of tobacco leaf is considerable. Flue-cured ("bright") tobacco typically produces acidic smoke, whereas air-cured ("burley") tobacco typically produces basic smoke. The natural acids in tobacco smoke (e.g., formic acid, acetic acid, and propionic acid) can protonate nicotine, while the natural bases (e.g., ammonia) tend to neutralize the acids and keep more nicotine in the unprotonated form. Acidic tobacco additives such as levulinic acid are used as "smoothing" agents to reduce smoke harshness.