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.