Air pollution monitoring Particle number concentrations (PNC) were measured continuously during the six-week trial in each apartment using water-based condensation particle counters (CPC; TSI Model 3783, d50 7 nm, maximum detectable particle >3 µm). The CPCs were installed in the same room as the HEPAirX unit and recorded 30-second mean PNC (one-minute mean PNC in the first five homes). Both outdoor and indoor PNC were measured; a solenoid valve switched every 15 minutes between two, 1-m-long conductive silicon inlet tubes: one pulled from indoors and the other pulled from outdoors. An in-line flow sensor logged different voltages depending on whether a flow was detected in the line (2.49 V with flow, ~1.00 V with no flow); these were used to identify indoor and outdoor sampling periods. Before the start of the intervention in each apartment, CPC flow rates were measured using a flow meter (TSI Model 4140) (no discrepancies were observed throughout the study). The CPC vacuum was also checked for leaks by placing a polyethersulfone membrane filter (rated at 99.96% removal efficiency for 0.45 µm particles) on the inlet to insure the CPC measured <100 particles/cm3. Sites were visited weekly for regular maintenance (flow checks, time resets, etc.) and to download data. Data flagged by the CPC as erroneous (typically <1% of all data per home) were removed from the data set. Consistent with manufacturer specifications, all CPCs performed within 10% of one another in laboratory side-by-side comparisons.