322 



786 



GOR! AND LYNCH 



ExpirwJ Air 

 CO (ppm) 



I 



+-4^ 



t • • a M M tt 



FTC COtmg/cigt 



Fig. 7 Mean expired air carbon monoxKk (CO) vaJuo as a function of FTC CO yield of cigareites smoked 

 (95% confidence intervals). 



constant (Fig. 3). Daily nicotine intake per kilogram of body weight also displays a 

 parallel ceiling (Fig. 4), and the mean plasma nicotine ceiling itself is not significantly 

 exceeded even at high numbers of cigarettes smoked daily (Fig. 6). Expired CO levels 

 also reach a ceiling corresponding to that of nicotine intake (Fig. 7), suggesting that 

 either agent or both agents may be independently responsible for limiting intake. 

 However, previous experience (37) suggests that CO is unlikely to have a primary role 

 in the limitation of intake within the range of concentrations measured in this study. 

 The results confirm earlier partial findings for nicotine, cotinine, and CO (3. 4. 7, 9. 

 12,23,24.33.34). 



The parallel distributions of plasma nicotine and cotinine at all FTC levels argue 

 aaainst a metabolic explanation of the ceiling conditions observed, unless nicotine or 

 greater proportions of a different metabolii; were excreted at faster rates, as intake 

 Increases. This possibility has not been obsened experimentally (5, 9), and ii shoiild 

 be considered very unlikely, especially in vievi of the parallel and independent stationary 

 conditions of expired CO levels. Pharmacodynamic satiation and related behavioral 

 determinants are the most probable reasons for the observed ceilings. 



.According to this thinking, smoke inuke may be determined by individual nicotine 

 demand, and to a lesser exten: by the capaarv- of the cigarette to deliver nicotine. The 

 former would influence behavioral factors, such as number of cigarettes smoked, mod- 

 ulation of the nicotine bolus effect by depth of inhalation, puff volume and frequency, 

 and tampering with filtration and air dilution devices (22). .\t high FTC yields roughly 

 equivalent amounts of nicotine may be extracted by individual smokers from the daily 

 average cigarette. At low yields most smokers compensate upward, although previous 

 studies have shov^n that at yields below 0.2-0 5 mg FTC nicotine it \^ill be progressively 

 more difficult to physically extract smoke a.nd nicotine from the cigarette no matter 



