Time course of end-expired carbon monoxide concentration is important in studies of cigarette smoking.

1. Fifteen asymptomatic habitual smokers each smoked one of their usual cigarettes, not having smoked for 2 h. End-expired carbon monoxide concentration (EECO) was measured with an Ecolyzer 2000 series analyser before smoking (pre-S value), 1 min after finishing smoking (post-S value) and then at intervals up to 1 h. 2. The mean EECO boost (increase) over all subjects declined biphasically after smoking, with an initial fast phase from 1 to 5 min, and then a slow phase from 5 to 60 min. EECO fell by as much in the first 5 min as in the next hour. 3. Post-S EECO was related to pre-S EECO (r = 0.89, P less than 0.001), but EECO boost was not related to pre-S (r = 0.00). EECO boost was unaffected by the sampling manoeuvre. 4. EECO measurements in epidemiological and smoking studies should not be made for at least 5 min after a cigarette is finished.