The intervention Window-mounted HEPAirX air filtration units (Air Innovations, Inc., North Syracuse, NY, USA) equipped with MERV 17 filters (rated to remove ≥99.97% of particles ≥0.3 μm in diameter) were used. These units can operate at ~10 exchanges/hour in a 28.3 m3 (103 ft3) room and have user-controlled air heating and cooling elements. The units were installed preferentially in living rooms of apartments (N=16), where participants spent much of their day. Eight units were installed in bedrooms due to space restrictions or because living room windows did not accommodate the HEPAirX unit. To maximize particle removal, the HEPA units were operated at the highest possible fan speed and the vents were blocked so that there was no flow of outdoor air through the unit into the apartment. Also, participants were asked to keep windows closed as much as possible during the study period to minimize infiltration from outside. Filters were changed in each apartment after 21 days (HEPA for sham or vice versa). A new HEPA filter (MERV 17) was used in each apartment. The sham filter was an empty, perforated sheet metal box that was the same size and shape and had the same appearance as the metal frame around the HEPA filters. The HEPAirX sounded the same regardless of sham or HEPA filtration. A sign written in English and Spanish was placed on the HEPA-unit cover asking participants not to tamper with or expose the filter.