The effect of portable HEPA filter air cleaners on indoor PM2.5 concentrations and second hand tobacco smoke exposure among pregnant women in Ulaanbaatar, Mongolia: The UGAAR randomized controlled trial

Background Portable HEPA filter air cleaners can reduce indoor fine particulate matter (PM 2.5 ), but their use has not been adequately evaluated in high pollution settings. We assessed air cleaner effectiveness in reducing indoor residential PM 2.5 and second hand smoke (SHS) exposures among non-smoking pregnant women in Ulaanbaatar, Mongolia. Methods We randomized 540 participants to an intervention group receiving 1 or 2 HEPA filter air cleaners or a control group receiving no air cleaners. We followed 259 intervention and 253 control participants to the end of pregnancy. We measured one-week indoor residential PM 2.5 concentrations in early (~ 11 weeks gestation) and late (~ 31 weeks gestation) pregnancy and collected outdoor PM 2.5 data from centrally-located government monitors. We assessed blood cadmium in late pregnancy. Hair nicotine was quantified in a subset ( n = 125) to evaluate blood cadmium as a biomarker of SHS exposure. We evaluated air cleaner effectiveness using mixed effects and multiple linear regression models and used stratified models and interaction terms to evaluate potential modifiers of effectiveness. Results The overall geometric mean (GM) one-week outdoor PM 2.5 concentration was 47.9 μg/m 3 (95% CI: 44.6, 51.6 μg/m 3 ), with highest concentrations in winter (118.0 μg/m 3 ; 110.4, 126.2 μg/m 3 ). One-week indoor and outdoor PM 2.5 concentrations were correlated ( r = 0.69). Indoor PM 2.5 concentrations were 29% (21, 37%) lower in intervention versus control apartments, with GMs of 17.3 μg/m 3 (15.8, 18.8 μg/m 3 ) and 24.5 μg/m 3 (22.2, 27.0 μg/m 3 ), respectively. Air cleaner effectiveness was greater when air cleaners were first deployed (40%; 31, 48%) than after approximately five months of use (15%; 0, 27%). Blood cadmium concentrations were 14% (4, 23%) lower among intervention participants, likely due to reduced SHS exposure. Conclusions Portable HEPA filter air cleaners can lower indoor PM 2.5 concentrations and SHS exposures in highly polluted settings.