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 ambient to 950 ℃, depending on the amount of oxygen present. 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.
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.
|Compounds||In processed tobacco, per gram||In mainstream smoke, per cigarette||IARC evaluation of evidence of carcinogenicity|
|In laboratory animals||In humans|
|Polycyclic aromatic hydrocarbons|
|Benzo(a)pyrene||0.1-90 ng||20-40 ng||sufficient||probable|
|N-Nitrosodimethylamine||0-215 ng||0.1-180 ng||sufficient|
|N-Nitrosonornicotine||0.3-89 μg||0.12-3.7 μg||sufficient|
|4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone||0.2-7 μg||0.08-0.77 μg||sufficient|
|N-Nitrosoanabasine||0.01-1.9 μg||0.14-4.6 μg||limited|
|Formaldehyde||1.6-7.4 μg||70-100 μg||sufficient|
|Acetaldehyde||1.4-7.4 μg||18-1400 μg||sufficient|
|Crotonaldehyde||0.2-2.4 μg||10-20 μg|
|Miscellaneous organic compounds|
|Ethyl carbamate||310-375 ng||20-38 ng||sufficient|
|Vinyl chloride||1-16 ng||sufficient||sufficient|
|Hydrazine||14-51 ng||24-43 ng||sufficient||inadequate|
|Arsenic||500-900 ng||40-120 ng||inadequate||sufficient|
|Nickel||2000-6000 ng||0-600 ng||sufficient||limited|
|Chromium||1000-2000 ng||4-70 ng||sufficient||sufficient|
|Cadmium||1300-1600 ng||41-62 ng||sufficient||limited|
|Lead||8-10 μg||35-85 ng||sufficient||inadequate|
|Polonium-210||0.2-1.2 pCi||0.03-1.0 pCi||sufficient||sufficient|
Tobacco smoke, besides being an irritant, is alleged 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 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).
According to the mainstream "nicotine hypothesis", nicotine is the main biochemical reason for tobacco smoking. The uptake of nicotine may be facilitated by other chemicals present in tobacco smoke. However, the nicotine theory cannot explain the following contradictions:
- Nicotine alone does not seem capable of totally substituting for cigarette smoke.
- Lettuce cigarettes reinforced with nicotine were not accepted more readily than non-nicotine cigarettes (however, marijuana cigarettes are an exception).
- Nicotine given either by ingestion or intravenously was incapable of completely suppressing smoking in humans.
- Animals will not self-inject nicotine or do so only indifferently, whereas they will self-inject cocaine and amphetamine very readily.
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.
- Robert Kapp (2005), "Tobacco Smoke", Encyclopedia of Toxicology 4 (2nd ed.), Elsevier, pp. 200–202, ISBN 0-12-745354-7
- Ken Podraza, Basic Principles of Cigarette Design and Function (PDF), Philip Morris USA
- The Health Consequences of Smoking: The Changing Cigarette (PDF), U.S. Dept. of Health and Human Services, p. 49
- K. Rothwell; et al. (1999), Health effects of interactions between tobacco use and exposure to other agents, Environmental Health Criteria (211), World Health Organization
- Michael A. H. Russell (1977), "Smoking Problems: An Overview", in Murray E. Jarvik; Joseph W. Cullen; Ellen R. Gritz; Thomas M. Vogt; Louis Jolyon West, Research on Smoking Behavior (PDF), NIDA Research Monograph (17), pp. 13–34
- T. C. Tso (2007), "Tobacco", Ullmann's Encyclopedia of Industrial Chemistry (7th ed.), Wiley, pp. 1–26, doi:10.1002/14356007.a27_123
- Murray E. Jarvik (1977), "Biological Factors Underlying the Smoking Habit", in Murray E. Jarvik; Joseph W. Cullen; Ellen R. Gritz; Thomas M. Vogt; Louis Jolyon West, Research on Smoking Behavior (PDF), NIDA Research Monograph (17), pp. 122–148