Hydrogen peroxide in exhaled air: a source of error, a paradox and its resolution.

The concentration of hydrogen peroxide (HO) in exhaled air has been reported to be elevated in asthma and chronic obstructive pulmonary disease (COPD), but results are inconsistent and difficult to reproduce. As HO occurs in ambient air, we examined its association with exhaled HO in human subjects. Exhaled breath condensate (EBC) of 12 COPD patients and nine healthy control subjects was collected either with an inhalation filter (efficiency 81%) or without. Ambient air condensate (AAC) was collected in parallel and samples were analysed for HO. Additionally, ambient HO was recorded by an atmospheric measuring device (online fluorometric measurement). HO concentration in AAC was significantly higher (p<0.001) than in EBC. AAC variations were concordant with the data from the atmospheric measuring instrument. In both subjects' groups, the inhalation filter reduced HO values (p<0.01). Despite generally low levels in exhaled air, analysis by a mathematical model revealed a contribution from endogenous HO production. The low HO levels in exhaled air are explained by the reconditioning of HO-containing inhaled air in the airways. Inhaled HO may be one factor in the heterogeneity and limited reproducibility of study results. A valid determination of endogenous HO production requires inhalation filters.