Relation of expired carbon monoxide to smoking history, lapsed time, TLCO measurement and passive smoking.

We quantified the influence of lapsed time, measurement of gas-transfer factor (TLCO), and passive smoking on expired carbon monoxide (CO) levels, and then evaluated the accuracy of smoking histories against expired CO measurements in patients newly attending 'occupational' compared with 'general' chest clinics. Expired CO levels had an estimated average rate of decline of 3.4 ppm/h in the presumed absence of further smoking, though individual rates depended necessarily on the initial levels (2.1, 3.9, 5.7 and 7.5 ppm/h, respectively, when the initial levels were 10, 20, 30 and 40 ppm). TLCO measurement was associated with a median increase in expired CO of 4.0 ppm, but passive exposure to tobacco smoke in non-smokers had negligible effect. Expired CO levels indicative of current smoking (> 8 ppm) were noted much more commonly in the current cigarette smokers (88%) than those who claimed to be current non-smokers (6.0%), but without significant difference between the non-smokers attending the occupational and general clinics (6.6% vs 5.3%). We conclude that the lapse of 1 h and the measurement of TLCO exert mild but important influences on the expired CO level, but that passive smoking does not. 'Occupational' and 'general' patients give similarly false declarations of current non-smoking when presenting initially for clinical evaluation.