Section IV 
pairs of smokers, each pair smoking a similar number of cigarettes but with 
different levels of CO (one high-CO-level subject and one low-CO-level 
subject). The CO boost per cigarette was found to be significantly different 
for the matched pairs of smokers. The CO boost for the high-CO group was 
6.9 ppm per cigarette and for the low CO group 4.4 ppm. 
The study found no differences between the high-CO and low-CO groups 
in terms of number and duration of puffs. Given the significant differences 
in CO levels, the researchers speculate that the difference may reside in puff 
intensity, puff volume, or inhalation characteristics. These influences on CO 
levels are relevant to low-nicotine yields and changes in smoking behavior; 
Herning and colleagues (1983) reported that CO boost appears correlated to 
blood nicotine levels. 
In an earlier study. Burling and coworkers (1983) found that a smoker's 
CO level is influenced by factors other than the FTC-determined CO yield 
of cigarettes. The researchers reported that the CO level is significantly 
related to interpuff interval, cigarette duration, time since last cigarette, and 
self-rated estimate of depth of inhalation. This research underscores the 
likelihood that CO levels may be determined by multiple factors, not just 
stated yield. However, the finding suggests that, when numbers of cigarettes 
are held equal, a person smoking cigarettes with a higher CO yield will likely 
have higher CO levels than a person smoking cigarettes of lower CO yield. 
Furthermore, Wald and colleagues (1984) reported that smokers of filter 
cigarettes have a 60-percent higher intake of CO than do those who smoke 
nonfilter cigarettes. 
Russell and colleagues (1982), in a study of long-term switching to low- 
tar, low-nicotine cigarettes, observed complete compensation as measured by 
CO uptake, and Robinson and coworkers (1983) reported that COHb levels 
did not change significantly after smokers switched to cigarettes with 15- and 
72-percent lower CO deliveries. 
Robinson and colleagues (1984) examined exposure among 22 smokers 
of high-nicotine cigarettes who switched to cigarettes of similar nicotine 
yield but with reduced 5 delds for tar, CO, and hydrogen cyanide. Cotinine 
levels remained about the same; however, although reductions of 40 to 
50 percent in CO and HCN were expected, the measured reductions were 
5.3 percent for expired-air CO, 12.2 percent for COHb, 2 percent for saliva 
thiocyanate, and 1 percent for plasma thiocyanate. 
Darrall (1988) found that a 50-percent blockage of ventilation holes 
produced small changes in tar and nicotine yields but greater changes in CO. 
Nil and coworkers (1986), in a study of 117 regular smokers, reported that 
the CO boost of cigarettes appeared to remain steady among smokers despite 
controlled switching to cigarettes of higher or lower yields. 
Fischer and colleagues (1989), in an investigation of six different 
cigarette brands (filter and nonfilter and very low to medium tar yields), 
found that puff volume and puff frequency, the key determinants of total 
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