Impact of correcting peak flow for nonlinear errors on air pollutant effect estimates from a panel study

Air pollutant effects are commonly investigated using panel studies employi ng daily measurement of changes in peak expiratory flow (PEF), Variable ori fice PEF meters are inaccurate with a nonlinear relationship to actual PEF, The impact on a panel study of correcting these errors was examined. Twice-daily PEF readings were taken by 147 9-yr old children for 8 weeks an d corrected using an equation derived from the response of 32 Vitalograph m eters to a servomechanism-controlled pump. Pollutant effect estimates for c orrected and uncorrected readings were derived using a regression approach incorporating appropriate confounders. Correction impacted little on mean PEF values (333.1-334.2 L.min(-1)), but did alter effect sizes, Nonsignificant nitrogen dioxide estimates for the e ntire panel decreased by up to 73%, but, for symptomatic/atopic children, a significant 5-day mean result was lost (decrease in effect size from -2.53 to -0.90% per 10 parts per billion (ppb)) and lag 0 became significant (de crease from -0.51 to -1.22% per 10 ppb), Mass concentration estimates of pa rticles with a 50% cut-off aerodynamic diameter of 2.5 mu m moved in both d irections (-0.22 changed to 0.11% per 10 mu g.m(-1) lag 3 and -0.29 to -0.7 3% per 10 mu g.m(-3) for the 5-day mean). Correction of nonlinearity of peak expiratory flow meters influenced the ov erall outcome of this panel study, and the changes in effect estimates woul d be sufficient to alter the interpretation of some studies. For adults, a greater change in effect estimates may follow the larger correction require d for their usual peak expiratory flow range.