general, cellulose acetate or paper filters remove tar and nicotine particles in this size range with an efficiency of 40–50%. Charcoal filters influence the retention of vapor-phase components and are made by adding up to 60 mg of activated charcoal in a segment of the cellulose acetate filter. Factors that influence the retention of vapor-phase compounds include the amount of charcoal used, the activity of the charcoal (based on surface area and pore volume), and the smoke velocity through the segment. Low-molecular-weight compounds with low boiling points (less than 150°C), which are not retained by the cellulose acetate filters, are partially removed by the charcoal filters. Removal efficiencies vary with the compound but are reported to be from 30 to 90%. As an example, benzene may be removed at an efficiency of 67%. Vented filters are designed to improve filter efficiency by decreased smoke flow through and increased residence time in the filter. However, vent holes can readily be covered by the fingers of the smoker, who may be inclined to do this in order to get the maximum amount of nicotine during smoking.

Under similar smoking conditions, filtered cigarettes will have lower MS yield relative to nonfiltered analogues. SS yields will not vary much, since they are reflective of tobacco weight burned during smolder. In general, more tobacco is consumed during smolder than during puffing. However, SS generally contains more alkaline and neutral compounds. SS smoke contains less or equal amounts of acids, phenols, and phytosterols than MS. Differences are due to temperature and mechanisms of chemical transfer (release) from the unburned tobacco. The approximate chemical composition of MS is given in Table 10–1. The relative concentration of specific constituents in MS versus SS tobacco smoke is shown in Table 10–2.

The pH of cigarette smoke influences the degree of protonation of the active addictive chemical, nicotine. The free-base form of nicotine is favored at a higher pH (more basic) and is more rapidly absorbed into the bloodstream than the mono- or diprotonated salt forms of nicotine that exist at lower pH (more acidic). Tobacco blends with a high sugar content produce a more acidic smoke; a basic cigarette smoke can be achieved by addition of ammonia.

TOXICITY OF TOBACCO SMOKE

The health effects of tobacco smoke in humans are well known from both clinical and epidemiological studies; such information is summarized in later chapters of this report. Animal studies of the toxicity of tobacco smoke are reviewed in the present section. The purpose of this review of animal models of tobacco smoke toxicity is to determine the potential usefulness of such models for assessing the toxicity of new and



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