Smoking is associated with an accelerated loss of lung function and inhalation accelerates the decline further. Exhaled CO reflects the exposure of smoke to the lungs.

To investigate whether self-reported inhalation and type of cigarette influenced the level of exhaled CO and whether CO could provide additional information to usual measures of smoking regarding prediction of present lung function and decline in lung function over an extended period of time.

Cigarette smokers from the Copenhagen City Heart Study with valid measures of lung function and exhaled CO; in total 3738 subjects, 2096 women and 1642 men.

Subjects not inhaling had slightly lower exhaled CO values than those inhaling, but substantially higher values than non-smokers (P<0.001). Smokers of plain cigarettes had slightly lower CO values than smokers of filter cigarettes (P<0.001). Increasing CO levels were correlated to a lower FEV₁%pred and to an accelerated decline in lung function. However, in multiple linear regression analyses these correlations were not significant.

Inhalation and type of cigarette affects exhaled CO levels. CO measures have no predictive value regarding neither present lung function nor decline in lung function with time in a population survey setting.