Description: Background Combustion-generated fine particulate matter (PM 2.5 ) is associated with cardiovascular morbidity. Both traffic-related air pollution and residential wood combustion may be important, but few studies have compared their impacts. Objectives To assess and compare effects of traffic-related and woodsmoke PM 2.5 on endothelial function and systemic inflammation (C reactive protein, interleukin-6 and band cells) among healthy adults in Vancouver, British Columbia, Canada, using high efficiency particulate air (HEPA) filtration to introduce indoor PM 2.5 exposure gradients. Methods We recruited 83 healthy adults from 44 homes in traffic-impacted or woodsmoke-impacted areas to participate in this randomised, single-blind cross-over intervention study. PM 2.5 concentrations were measured during two consecutive 7-day periods, one with filtration and the other with ‘placebo filtration’. Endothelial function and biomarkers of systematic inflammation were measured at the end of each 7-day period. Results HEPA filtration was associated with a 40% decrease in indoor PM 2.5 concentrations. There was no relationship between PM 2.5 exposure and endothelial function. There was evidence of an association between indoor PM 2.5 and C reactive protein among those in traffic-impacted locations (42.1% increase in C reactive protein per IQR increase in indoor PM 2.5 , 95% CI 1.2% to 99.5%), but not among those in woodsmoke-impacted locations. There were no associations with interleukin-6 or band cells. Conclusions Evidence of an association between C reactive protein and indoor PM 2.5 among healthy adults in traffic-impacted areas is consistent with the hypothesis that traffic-related particles, even at relatively low concentrations, play an important role in the cardiovascular effects of the urban PM mixture. Trial registration number http://www.clinicaltrials.gov (NCT01570062).