h commented that low sensitivity of the assay, and wide spread of measured concentrations of H2O2 might mask subtle effects on airway inflammation. Similarly, Barregard and co-workers, though they describe statistically significant alterations in EBC malondialdehyde (another proposed biomarker of oxidative stress) after exposure to wood smoke, suggest that the results be interpreted with caution due to very low overall levels in EBC, with many measurements below detection limit of the assay . Assay variability when measuring H2O2 using fluorimetry and colorimetry is broad in the reported literature, with some studies reporting good agreement of results [36–38], while others show higher coefficients of variation [32, 39, 40].
EBS pH was measured immediately after collection, without argon degassing or standardisation for CO2 partial pressure. There continues to be vigorous debate about the merits of each of these approaches , however degassing is not a feasible approach when collecting and processing large numbers of samples in a short period.
The potential drawbacks of EBC analysis, including assay variability and very low biomarker concentration have been highlighted in a recent review article  and are addressed in the ATS/ERS taskforce position paper on EBC . At this stage, EBC collection and analysis appears to show utility in highly controlled research settings, but the current collection and analysis techniques are not suitable for application "in the field" for community assessment of effects of air pollution.
In summary, in this study we detected no significant effect of ambient wood-smoke particulate air pollution on lung function of healthy school-aged male students, but a small effect on cough. Small but significant effects of peak pollution levels were seen in students with asthma. Urinary 1-OHP shows potential as a biomarker of exposure to PAHs in wood smoke in this population; however measurement of EBC pH and hydrogen peroxide using current methods of collection and analysis appears not to be useful for assessment of population health effects of air pollution.
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