Challenges in estimating health effects of indoor exposures to outdoor particles: Considerations for regional differences

Abstract

Ambient air pollution is a leading environmental risk factor causing substantial losses of life and significant morbidity. Concentration-response (CR) functions used globally to estimate such effects are largely based on ambient epidemiology, using centrally monitored outdoor air quality as an exposure indicator and various indices of population health as an outcome. Similar common understanding is mostly missing regarding indoor exposures. Less studied are health impact modifying factors such as particle size, infiltration, time-activity and population differences. In this discussion paper we aim at looking at one of these, infiltration. The sensitivity of overall personal exposure to indoor exposures was quantified by a simple probabilistic time-activity model to calculate fractional exposures for indoor, outdoor and in traffic time-activity. To demonstrate the potential regional differences in epidemiological C-R relationships we re-analysed the ESCAPE results for natural-cause mortality, focusing on geographical grouping of the cohorts: pooled estimates were calculated for the Nordic, Central European and Southern European cohorts. When comparing the relative differences in the regional hazard ratio increments, the Central European value (7%) is 1.75 times higher than the Nordic one, and Southern European value (12%) 3 times higher, respectively. While towards the expected direction when aiming to explain these differences at least partly with differences in PM_2.5 infiltration, the differences are not statistically significant and only the Central European and the all cohorts combined estimates reach borderline statistical significance. As the analysis of PM_2.5 infiltration factors by similar regions yielded only 10-15% differences, it seems possible that that the available data could also accommodate other regional factors, such as those originating from regional differences in population and contribution of indoor sources of PM, time-activity, behaviour, or compositional differences in the particulate matter.

Keywords: Concentration-exposure relationship; Infiltration; PM(2.5); Toxicity.