Product formation mechanisms inside a burning cigarette

Abstract Inside a burning cigarette a large variety of chemical and physical processes are occurring in an oxygen-deficient, hydroge-rich environment, with temperatures up to 950°C. The physical processes occurring during combustion are rationalised and a picture is presented of the major mechanistic regions inside the cigarette during puffing and natural smouldering. There are two major regions inside the burning zone of the cigarette where products are released: a heat producing combustion zone, and a pyrolysis/distillation zone just downstream of the combustion zone. The vast majority of organic smoke products are formed in the pyrolysis/distillation region. The location of these zones has been determined by mapping internal gas concentrations, and by measuring internal density and temperature changes. Product formation routes have traditionally been unravelled using isolated pyrolysis experiments. Recent work with radioactively labelled nicotine has shown that the results from such experiments can be very different to those obtained from the dynamic conditions inside the cigarette. However, attempts have been made in recent years to design pyrolysis experiments which simulate more closely the real conditions inside the burning zone of the cigarette. The oxides of carbon are formed by both combustion and thermal decomposition of tobacco constituents. The major features of these two mechanisms are discussed. Many studies have also deduced that a significant proportion of carbon monoxide is formed by the carbonaceous reduction of carbon dioxide. The occurrence of this reaction is used to interpret the effect of ventilation on the ratio of carbon monoxide/carbon dioxide yields, and the relative values of this ratio in mainstream and sidestream smoke. The generation of mainstream and sidestream smoke is reviewed, together with factors that contribute to the relative deliveries for particular smoke components. The most critical factor is the mechanistic origins of the component. Finally, recent mathematical models of some of the processes contributing to smoke formation are discussed. It is likely that this technique will be used increasingly in the future to test our ideas on smoke formation mechanisms.