2.2.2 |. PM10 and PM10–2.5

We collected gravimetric PM₁₀ and PM_2.5 samples using RTI MicroPEMs v 3.2A (RTI International) fitted with PM₁₀ and PM_2.5 inlets and 25 mm 3 μm pore size Teflon filters. MicroPEMs were enclosed in cages hung at the child’s breathing height (approximately 1 m from the floor) from an IV pole in the sleeping area. Although MicroPEMs are designed as personal exposure samplers, one study showed good agreement between MicroPEMs deployed as gravimetric area samplers and ambient monitoring stations at PM₁₀ concentrations ranging from 20 to 140 μg/m³.⁸⁰

Pre-deployment flow rates were set to 0.5 L/min using a TSI 4140 flow meter (TSI Inc.), with flow rates checked again using the TSI at the field laboratory after the equipment was picked up from the home. If both pre-and post-deployment TSI flow rates were available, we used their mean as the primary flow rate estimate. Missing pre-deployment flow rates (~4% of samples) were imputed using the target flow rate (99% of non-missing observations were 0.5 L/min). Missing post-deployment flow rates (19% of sleeping area samples) were imputed using the median of the non-missing observations. Temperature and relative humidity (RH) in the sleeping area were also recorded every 5 min during MicroPEM deployment using a HOBO^® Onset^® U12–011 logger (Onset Computer Corporation). For 15.5% of samples, we did not have temperature/RH HOBOs available; in these cases, we imputed the missing data using the MicroPEM temperature and a linear regression equation fit to the HOBO vs. MicroPEM measurements from homes with both instruments deployed (Pearson ρ = 0.87, p < 0.01, n = 118).

We processed MicroPEM files logging timestamps, temperature, nephelometer flow rates, and other data using the rtimicropem package, with further processing in R, version 3.6.1 (R Foundation for Statistical Computing). Technicians inspected plots of pump inlet and orifice pressures to evaluate potential pump malfunctions or filter overloading; overloading was assumed if inlet pressure reached five inches of water while the orifice pressure simultaneously destabilized and the flow rate rapidly increased. In these cases, the sample duration was cut off at the date/time just before this transition occurred. For sample durations <10 days, raw data files and field notes were examined for error messages and possible explanations, including tampering or loss of electrical or battery power. A subset of files from MicroPEMs that malfunctioned or had overloaded filters (~5% of the MicroPEM data) were sent to RTI for a reliability check.

Sample air volumes were calculated by multiplying the mean flow rate by the sample duration. Filters were conditioned and weighed to the nearest 0.5 μg following standard procedures^81,82 in a temperature and humidity controlled laboratory at UW (Seattle, WA), with filter mass changes divided by sample volumes to calculate PM_2.5 and PM₁₀ concentrations. Quality assurance/quality control (QA/QC) procedures were described previously⁷² and included laboratory and field blank filters, reweighing of filters with negative mass changes, and flagging filters with small holes or tears. We used the method detection limit (MDL), defined as three times the median absolute deviation of the field blanks mass changes, to blank correct the primary samples. Expressed as concentrations, the PM_2.5 and PM₁₀ MDLs were 0.2 μg/m³ and 0.1 μg/m³, respectively, at an assumed flow rate of 0.5 L/min and 14 days sample duration; no measurements were below their respective MDLs. One pair of co-located PM₁₀ samples showed good agreement between measured concentrations (22.60 μg/m³ and 20.0 μg/m³); precision of two pairs of co-located PM_2.5 measurements was also good and is reported in Ref.72 PM_10–2.5 concentrations were calculated using the subtraction method, that is, subtracting the co-located blank-corrected PM_2.5 concentration from the blank-corrected PM₁₀ concentration. Following,⁸³ the PM_10–2.5 MDL was defined as the square root of the sum of the squares of the PM_2.5 and PM₁₀ measurements, or 0.2 μg/m³; one observation below this value was replaced with the MDL.