Smoking and Tobacco Control Monograph No. 7 
can measure the risks of cigarettes under the "natural" circumstances of use. 
However, the dynamic nature of the exposure (Figure 1) challenges the 
epidemiologic researcher to classify accurately the pattern of cigarette use 
when changes are made that may not be indexed by tar and nicotine yields 
measured with a smoking machine. 
In considering the health implications of the changing cigarette, 
the concepts of exposure and dose are fundamental. Exposure has been 
defined by the National Research Council (1991) as the amount of material 
potentially available for interaction with a human, that is, material in contact 
with a person at a boundary, whether that boundary be the skin, lung, or the 
alimentary tract. On the other hand, dose is the amount of material that 
enters the organism. Dose may be further classified as the internal dose 
(i.e., the amount of material deposited) or as the biologically effective dose 
(i.e., the amount of material delivered to some biologically relevant site). 
Changes in the cigarette can be interpreted as potentially leading to changes 
in exposure; the health consequences of changing exposure vary with any 
resultant changes in dose of components of cigarette smoke that cause disease. 
The physiological functioning of the lung is also relevant to 
understanding the linkages in changes in the cigarette to changes in 
exposure and dose. The lung is a complex organ with several different 
"compartments," including the upper airway that extends from the nose 
and mouth to the larynx; the airways of the lung itself, which include the 
trachea, bronchi, and bronchioles; and the parenchyma of the lung, which 
includes the interstitium and the airspaces, or alveoli. The lung behaves as 
a filter that absorbs and deposits gaseous and particulate components of 
smoke throughout its surfaces during the act of smoking. The sites and 
extent of deposition of inhaled mainstream components vary, depending 
on solubility and other characteristics of gas phase components and the 
sizes of the particles. Cigarette smoke is a dynamic mixture in the respiratory 
tract, changing with humidification of the mixture, growth of particles, 
and changing composition as components are selectively removed by the 
filtration process (U.S. Department of Health and Human Services, 1984). 
These physiological considerations imply that there is no simple relationship, 
linear or nonlinear, between reported tar and nicotine yields — a measure 
of exposure — and biologically effective doses of toxic smoke components 
delivered to the sites of injury in the respiratory tract. 
The measures of cigarette smoking used in epidemiologic research on 
smoking and health can be classified as estimating either exposure or dose. 
I he most widely used measures, for example, information on cigarette 
smoking (duration of smoking, number of cigarettes smoked per day, and 
type or brand of cigarettes smoked), are exposure measures. Biomarkers that 
can be inter{)reted as indicators of dose include levels of carboxyhemoglobin, 1 
nicotine, and cotinine (U.S. Department of Health and Human Services, i 
1990). Thus, for ei)idemiological i)urposes, researchers use exposure measures, ^ 
typically obtained by questionnaire, and dose measures, based on biomarkers. i 
l or exam[)le, cigarettes smoked j)er day is an exposure measure, whereas f 
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