End-expiratory carbon monoxide levels in healthy subjects living in a densely populated urban environment.

Carbon monoxide (CO) has a high affinity for haemoglobin and is a common cause of poisoning in industry and the home. Exhaled CO levels in patients with respiratory disease have been reported but exhaled CO in a large cohort of healthy subjects grouped by age and gender has not been reported. Exhaled CO levels and spirometry lung function data were recorded from 1032 subjects at a university campus and two commercial plazas. Subjects were also asked to complete a respiratory symptom questionnaire. Ninety-eight subjects reported respiratory disease and were excluded from the study. Non-smoking male subjects (n=508) had higher exhaled CO levels (4.36+/-2.54 ppm) [range 0-21 ppm] compared with female (n=348) subjects (3.72+/-2.12 ppm) [range 0-14 ppm] (p<0.0005), and older subjects (>60 years) had lower exhaled CO levels compared with young subjects (<22 years) (p=0.018). Over 13% of non-smokers had an exhaled CO greater than 7 ppm. Smokers showed significantly higher exhaled CO levels compared with non-smokers (p<0.0005) and smokers who complained of frequent cough and sputum production had higher levels of exhaled CO compared with smokers without such complaints. Smoking history (pack-years) was directly related to age (r=0.59) but correlated inversely with forced expiratory flow in the 1st second (FEV1) (r=-0.29) and peak expiratory flow rate (PEFR) (r=-0.25) (p<0.05). If a city's micro environmental CO concentrations and human activity patterns is available, regular monitoring of exhaled CO in healthy subjects has the potential to be used as a functional index of air pollution.