Background: Carbon monoxide (CO) can be detected in exhaled air and is increased in asthmatic patients. However, it is uncertain whether exhaled CO is increased in patients with allergic rhinitis. Objective and methods: To study whether exhaled CO is increased in patients with allergic rhinitis, exhaled CO concentrations were measured on a CO monitor by vital capacity manoeuvre in 86 patients with seasonal allergic rhinitis during and out of the cedar pollen season. Results: During the season, exhaled CO concentrations were 3.6 ± 0.3 p.p.m. and decreased to 1.2 ± 0.1 p.p.m, out of the season. The values of exhaled CO out of the season were similar to those in age-matched non-smoking healthy control subjects (1.2 ± 0.1 p.p.m.). Exhaled CO concentrations were significantly higher in patients with symptoms than in those without symptoms (P < 0.01). Exhaled CO concentrations in patients did not differ significantly among oral and nasal exhalation, and oral exhalation with an expiratory resistance (P > 0.20). Conclusion: These findings suggest that allergic rhinitis increases the concentration of CO in exhaled air and increases in exhaled CO may be derived from lower airways.
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