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Toxicant formation and thermochemistry

The nature and relative levels of tobacco-derived toxicants are based on well-established tobacco thermochemistry[1-3]. The different classes of tobacco constituents tend to decompose at different heating temperatures, releasing chemical compounds into the aerosol as they do so.

This is in contrast to the smoke produced by a burning cigarette, which contains a wide range of chemicals because its burning tip undergoes a very wide range of temperatures, from ambient to about 1000°C. Attempts to selectively modify this complex combustion process in conventional cigarettes have not been successful[4].

Thermochemistry of tobacco and the differing range of chemical compounds released through distillation, pyrolysis and combustion
Thermochemistry of tobacco and the differing range of chemical compounds released through distillation, pyrolysis and combustion

By heating tobacco at a specific temperature in the range seen in a burning cigarette,  Baker established that[1], at lower temperatures of between 100 and 200°C, water and volatile substances including flavour molecules loosely bound to the surface of the tobacco leaf are the first to be released through distillation and evaporation. Simple pectins and sugars in tobacco may also begin to decompose. Although the boiling point of nicotine is around 247°C, nicotine starts to vaporize and enter the aerosol at temperatures from 170 to 200°C[5-6]. At 300 to 400°C, some tar components start to form from the breakdown of cellulose and other structural components of tobacco, but it is not until temperatures of more than 400°C that the main pyrolysis occurs.  Leaf components such as amino acids and esters decompose between 400°C and 600°C, and carboxymethylcelluloses and carbonates between 600°C and 900°C, producing the bulk of the carbon monoxide, polycyclic aromatic hydrocarbons and other combustion products that form the tar in cigarette smoke.

To design a tobacco heating product that produces significantly fewer of the toxicants formed by pyrolysis and combustion reactions, tobacco in a tobacco heating product is typically heated to around 250°C only. This allows nicotine to vaporize out of the tobacco leaf, but pyrolysis and combustion does not occur. The fraction of toxicants and their levels in tobacco heating-device emissions will depend on the operating temperature and the tobacco blend used.

  1. Baker, R. 1987. A review of pyrolysis studies to unravel reaction steps in burning tobacco. Journal of Analytical and Applied Pyrolysis. 11(1987): 555-573.
  2. Burton, H., in Shafizadeh, F. (eds). 1976. Thermal Uses and Properties of Carbohydrate and Lignins. Academic Press, New York, London. 275.
  3. Czégény, Z., Blazsó, M., Várhegyi, G., Jakab, E., Liu, C. and Nappi, L. 2009. Formation of selected toxicants from tobacco under different pyrolysis conditions. Journal of Analytical and Applied Pyrolysis. 85: 47-53.
  4. Baker, R. 2006. Smoke generation inside a burning cigarette: Modifying combustion to develop cigarettes that may be less hazardous to health. Progress in Energy and Combustion Science. 32: 373-385.
  5. Perfetti, T., Norman, A., Gordon, B., Coleman III, W., Morgan, W., Dull, G. and Miller, C. 2000. The transfer of nicotine from nicotine salts to mainstream smoke. Beitrage zur Tabakforsch. International. 19: 141-160
  6. Seeman, J. and Carchman, R. 2008. The possible role of ammonia toxicity on the exposure, deposition, retention, and the bioavailability of nicotine during smoking.  Food and Chemical Toxicology. 46: 1863-1881
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