A review of pyrolysis studies to unravel reaction steps in burning tobacco

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The chemical constituents of tobacco smoke are generated in the burning zone of the cigarette where the processes of combustion, pyrolysis, distillation and aerosol formation occur. Tobacco itself consists of many different chemical components and consequently a large number of reactions occur in parallel as the cigarette burns. Relationships between tobacco components and smoke products are complex and difficult to unravel. Pyrolysis experiments have commonly been used to establish such relationships. However, unless they are performed under dynamic conditions that are relevant to those that occur during tobacco burning, results can be obtained which have little resemblance to those obtained during cigarette smoking. Bearing in mind this limitation, a variety of pyrolysis studies are reviewed which give insights into the mechanisms and reaction pathways occurring in the cigarette. The oxides of carbon are formed by thermal decomposition and combustion of tobacco constituents, and carbon dioxide is further converted to carbon monoxide by carbonaceous reduction. Pyrolysis studies used to elucidate these three processes are discussed. Pyrolysis results are presented which indicate that the majority of the so-called semi-volatile components of cigarette smoke are formed from tobacco at temperatures below 600° C. A literature survey of the tobacco component-semi-volatile product routes has been summarised. It indicates that their formation is complex and only partially understood. A few components (e.g. nicotine and other alkaloids) are transferred directlyfrom the tobacco; most are formed principally as a result of pyrolytic decomposition of many tobacco components in parallel.