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Clearing the Smoke: Assessing the Science Base for Tobacco Harm Reduction
parameters and reporting of values is covered in other chapters of this report.
Mainstream smoke is pulled through the mouth end of the cigarette and then through a “Cambridge filter pad.” Aerosol particles in the smoke larger than 1 µm in diameter are trapped with 99% efficiency. The material is referred to as cigarette smoke condensate or total particulate matter (TPM). “Tar” is the weight of TPM minus nicotine and water. The material that passes through the filter pad is the gas or vapor phase of cigarette smoke. In general, the vapor phase consists predominantly of compounds with a molecular weight <60 and the particulate phase consists of compounds with a molecular weight >200.
The yields of MS increase with successive puffs as the cigarette is machine-smoked due to the decrease in filtration provided by the cigarette rod itself. However, smoking behavior studies coupled with yield measurements suggest that yields remain consistent from first puff to last when assessed under real-life smoking conditions.
When tobacco is heated, moisture and volatile material are distilled, and combustion leads to the generation of volatile gases and the residual, carbonized char. Char reacts with oxygen in the air during puffing and smoldering, producing volatile gases (carbon dioxide, carbon monoxide, and water) and the inorganic material known as ash.
The highest temperature reached during the burning of tobacco is approximately 800°C in the center of the burning zone during smolder. During puff, a solid-phase temperature of approximately 910°C is reached at the burning zone periphery, while the gas temperatures are lower. They vary between 600 and 700°C as the puff progresses. After the puff ends, solid-phase temperatures rapidly cool to approximately 600°C. This greatly influences particle formation, particle size, and toxicant formation. These temperatures contrast with that achieved with a newly marketed cigarette-like device, Eclipse, that combusts differently than conventional cigarettes and aerosolizes nicotine and glycerin.
The chemical nature of MS changes as smoke ages. The burning zone generates a highly concentrated vapor that is drawn down the cigarette to form mainstream smoke. The vapor cools quickly (in milliseconds) due to diluting air. Less volatile compounds quickly condense, mostly in airborne state. A combination of physical size and concentration affects both thermal and mechanical properties, which influence the number of particles in smoke. Droplets of less than about 0.1 µm will attach to the tobacco through which they pass or to other particles, which continue on into MS. Particles with sizes around 1 µm are “filtered” out by depositing onto the tobacco surface.
MS is a highly concentrated aerosol mixture. Smoke particles are liquid, consisting of approximately 20% water by volume. The particles vary