We obtained 2246 measurements of integrated 24-h exposure to PM2.5 using Harvard Personal Exposure Monitors (H-PEM) (Mesa Labs, USA) that housed ZefluorTM 37 mm PTFE filters (Pall Life Sciences, USA) and were attached downstream from a personal sampling pumps (Apex Pro and TUFF™, Casella Inc; USA) operated at 1.8 L/min (Demokritou et al., 2001). Air monitors were placed inside waistpacks that participants were asked to wear at all times possible and to keep within 2 m while sleeping, sitting, or bathing (Supplementary Fig. S2). In a subsample of exposure measurements (n = 1595, 76% of all measurements), we added a pedometer (HJ-321 Tri-Axis, Omron; Japan) to the waistpack to monitor compliance in wearing them. Participants with 24-h step counts of < 500 steps were considered potentially non-compliant in wearing the air monitor on that day (n = 47, 3%).

Pump flow rates were measured at the start and end of each sampling period using a rotameter that was field calibrated at the beginning and middle of each measurement campaign using a primary gas flow standard (mini-BUCK Calibrator M−5; A.P. Buck Inc.; Orlando, FL, USA). For quality control and to address potential contamination, we collected ~ 7% field blank filters that were placed inside identical H-PEMs and cyclones, subject to the same field conditions, and analyzed using the same protocol as the filter samples.

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