It is now well established that smokers modify their smoking behavior to self-titrate circulating nicotine to a level appropriate to their need (49). This compensatory behavior is achieved through varying the number of puffs, puff volume and interpuff interval, and covering the cigarette filter to reduce ventilation by sidestream air. This plasticity of smoking behavior means that estimating exposure to nicotine and tar in cigarette smokers is not possible through the use of machine protocols to calculate yield estimates (50) or through the simple counting of number and strength of cigarettes smoked. Smokers are able to titrate not only how many cigarettes they smoke but what strength of cigarette they smoke and how they smoke them. Therefore, either biochemical measures of exposure or naturalistic measures of smoking topography are necessary if an acceptable level of measurement precision is to be achieved (51).
