Over the last 50 years a substantial understanding of the physics and chemistry underlying cigarette combusion and the formation of tobacco smoke has been generated, along with characterisation of some of the main smoke toxicants [ 1-2 ]. The current state of knowledge in these areas has been summarised in a number of reviews and book chapters [ 3-7 ]. In a series of publications in the 1980's, Dr Richard R. Baker systematically established the distributions of combustion temperature, gas velocity and key smoke constituents inside a burning cigarette [ 8-11 ]. These have become the fundation of modern cigarette combustion science. Based on Dr Baker’s and other studies in the literature, the following is a schematic representation of the key thermophysical processes occurring inside and around a burning cigarette:
Briefly, when a smoker draws on a lit cigarette, the temperature of the cigarette coal rises rapidly from its resting (smouldering) temperature of around 600 °C. Peak puff temperatures at the periphery of the coal can exceed 900 °C during a 35 mL, 2-sec puff. The high temperature inside the coal causes an increase in the viscosity of the air flowing through and a concomitant increase in the resistance to the draw of air through the coal. This effect forces air to be drawn primarily into the periphery of the coal around the paper burn line, which causes more complete combustion in this peripheral region.
The depletion of oxygen due to combustion results in the formation of a region immediately behind the coal where the temperatures remain high enough for thermal decomposition of tobacco (the pyrolysis/distillation zone). Large amounts of volatile and semi-volatile smoke constituents are produced in this region. A small amount of air is drawn in along the tobacco rod through permeable cigarette paper and smoke temperature decreases rapidly to produce a supersaturated aerosol. The smoke thus formed during a puff is subjected to filtration by the remaining tobacco rod and cigarette filter, as well as dilution by any filter ventilation holes. Some proportion of the light gases (such as CO) will diffuse out of the highly permeable cigarette paper. The smoke that leaves the mouth end of the cigarette is called mainstream smoke. Between puffs, hot smoke escapes from the top of the cigarette and forms the sidestream smoke.
Some of our recent combustion studies have focused on understanding the interactions between a burning cigarette and cellulosic substrates that make up the bands used to manufacture cigaretttes aimed at reducing the ignition propensity of the cigarette as measured by the American Standard ASTM 2187-04 [ 12-14 ]. In addition, we have been performing pyrolysis experiments under simulated cigarette burning conditions to investigate both the mechanistic and also kinetic aspects of tobacco or tobacco ingredients combustion [ 15-21 ]. Re-newed attempts are also being made to model a burning cigarette using modern computational tools [ 22,23 ].
More than 5,000 smoke constituents have so far been identified in cigarette smoke [ 7 ]. Around 150 of these have been identified as smoke toxicants. Over the last 20 years researchers and public health organisations, including Health Canada, have drawn up toxicant lists comprising sub-sets of these constituents - the most widely cited being the ‘Hoffmann Analyte’ list of 44 constituents. Cigarette combustion science will remain to play an important role in understanding the formation of smoke constituents. We have research underway to seek to deternine which of these toxicants are likely to be most important in relation to the various smoking-related diseases.