The exposure metric choices have significant impact on the association between short-term exposure to outdoor particulate matter and changes in lung function: Findings from a panel study in chronic obstructive pulmonary disease patients

Background: The use of ambient air pollution data obtained from central air-monitoring stations as surrogates for participants' exposures to outdoor air pollutants in previous studies may have introduced bias in the estimation of exposure-response associations. Objectives: We investigated and compared the effects of short-term exposure to outdoor particulate matter (PM_out) and outdoor-originated equivalent personal PM (PM_eq) on lung function in chronic obstructive pulmonary disease (COPD) patients. Methods: A total of 33 doctor-diagnosed stable COPD patients were recruited and repeatedly measured for lung function (totally 170 measurements) in 2013-2014. Daily PM_out concentrations were obtained from central-monitoring stations, and daily time-weighted average PM_eq concentrations were estimated based on PM_out over the study. Associations of PM with lung function were estimated using mixed-effects models. Results: Interquartile range increases in PM_2.5out (111.0μg/m³, 5-day) and PM_10out (112.0μg/m³, 3-day) were associated with a 3.3% (95% confidence interval [CI]: -5.8%, -0.8%) reduction and a 2.1% (95%CI: -3.9%, -0.3%) reduction in forced vital capacity (FVC), respectively. Similar results were found for forced expiratory volume in 1s (FEV1). An interquartile range increase in PM_2.5eq (45.3μg/m³, 3-day), but not PM_10eq, was still associated with a 1.7% (95%CI: -3.3%, -0.1%) reduction in FVC. Conclusions: Our study may provide a novel approach to assess the association of ambient PM with health observations with improved accuracy.