Communities impacted by fine-particle air pollution (particles with an aerodynamic diameter less than 2.5 mm; PM2.5) from forest fires and residential wood burning require effective, evidence-based exposure-reduction strategies. Public health recommendations during smoke episodes typically include advising community members to remain indoors and the use of air cleaners, yet little information is available on the effectiveness of these measures. Our study attempted to address the following objectives: to measure indoor infiltration factor (Finf) of PM2.5 from forest fires wood smoke, to determine the effectiveness of high-efficiency particulate air (HEPA) filter air cleaners in reducing indoor PM2.5, and to analyze the home determinants of Finf and air cleaner effectiveness (ACE). We collected indoor/outdoor 1-min PM2.5 averages and 48-h outdoor PM2.5 filter samples for 21 winter and 17 summer homes impacted by wood burning and forest fire smoke, respectively, during 2004 - 2005. A portable HEPA filter air cleaner was operated indoors with the filter removed for one of two sampling days. Particle Finf and ACE were calculated for each home using a recursive model. We found mean Finf ± SD was 0.27 ± 0.18 and 0.61 ± 0.27 in winter (n =19) and summer (n =13), respectively, for days when HEPA filters were not used. Lower Finf ± SD values of 0.10 ± 0.08 and 0.19 ± 0.20 were found on corresponding days when HEPA filters were in place. Mean ± SD ACE ([Finf without filter -Finf with filter]/ Finf without filter) in winter and summer were 55 ± 38% and 65 ± 35%, respectively. Number of windows and season predicted Finf (P<0.001). No significant predictors of ACE were identified. Our findings show that remaining indoors combined with use of air cleaner can effectively reduce PM2.5 exposure during forest fires and residential wood burning. © 2008 Nature Publishing Group. All rights reserved.