To further evaluate the influence of asthma severity status on intervention efficacy, we conducted stratified analysis for intervention changes to dPFV according to baseline asthma severity (Table 3).
Inclusion of [PM.sub.2.5] and PMc in our primary models attenuated the estimate of the efficacy of the air filter in improving dPFV, relative to placebo (Table 4).
We observed an improvement in dPFV, specifically a 4.1-percentage point reduction for children in the filter arm, relative to placebo.
Comparisons of PM and other air pollutant exposures with dPFV among children with asthma are limited and have indicated that pollutant effects on dPFV may be stronger among children with mild asthma.
The beneficial impact of the air-filter intervention on dPFV was not corroborated with parallel findings in other pulmonary function measures, i.e., morning and evening percent predicted [FEV.sub.1] and PEF.
Although within-individual variation in effort is likely, we expect that this error would be random rather than systematic over the two-winter observation period, particularly when considering the most robust finding, change in dPFV, where the smallest unit of measure is percent difference in successive PEF measures in less than 24 h (i.e., evening to morning).