Measuring environmental exposures in people’s activity space: The need to account for travel modes and exposure decay
Authors: Wei Lai , Kwan Mei-Po , Vermeulen Roel , Helbich Marco 
Published: January 2023

AbstractBackgroundAccurately quantifying people’s out-of-home environmental exposure is important for identifying disease risk factors. Several activity space-based exposure assessments exist, possibly leading to different exposure estimates, and have neither considered individual travel modes nor exposure-related distance decay effects.ObjectiveWe aimed 1) to develop an activity space-based exposure assessment approach that included travel modes and exposure-related distance decay effects and 2) to compare the size of such spaces and the exposure estimates derived from them across typically used activity space operationalizations.MethodsWe used 7-day-long global positioning system (GPS)-enabled smartphone-based tracking data of 269 Dutch adults. People’s GPS trajectory points were classified into passive and active travel modes. Exposure-related distance decay effects were modeled through linear, exponential, and Gaussian decay functions. We performed cross-comparisons on these three functional decay models and an unweighted model in conjunction with four activity space models (i.e., home-based buffers, minimum convex polygons, two standard deviational ellipses, and time-weighted GPS-based buffers). We applied non-parametric Kruskal-Wallis tests, pair-wise Wilcoxon signed-rank tests, and Spearman correlations to assess mean differences in the extent of the activity spaces and correlations across exposures to particulate matter (PM2.5), noise, green space, and blue space.ResultsParticipants spent, on average, 42% of their daily life out-of-home. We observed that including travel modes into activity space delineation resulted in significantly more compact activity spaces. Exposure estimates for PM2.5and blue space were significantly (p<0.05) different between exposure estimates that did or did not account for travel modes, unlike noise and green space, for which differences did not reach significance. While the inclusion of distance decay effects significantly affected noise and green space exposure assessments, the decay functions applied appear not to have had any impact on the results. We found that residential exposure estimates appear appropriate for use as proxy values for the overall amount of PM2.5exposure in people’s daily lives, while GPS-based assessments are suitable for noise, green space, and blue space.SignificanceFor some exposures, the tested activity space definitions, although significantly correlated, exhibited differing exposure estimate results based on inclusion or exclusion of travel modes or distance decay effect. Results only supported using home-based buffer values as proxies for individuals’ daily short-term PM2.5exposure.Impact statementAccurately quantifying people’s out-of-home environmental exposure is vital for identifying disease risk and protective factors. Although many activity space-based exposure assessments exist, these approaches possibly lead to different exposure estimates. We methodologically and conceptually innovate by developing an activity space-based exposure assessment considering people’s travel modes and exposure-related distance decay effect. Our comparison with other activity spaces provides novel insights into dynamic exposure assessment approaches. Despite most epidemiological studies still considering people’s homes as the sole exposure location, our study is fundamental because people are typically exposed to multiple out-of-home environmental contexts.