Residential areas are being increasingly impacted by wildfire smoke that causes hazardous local ambient air quality conditions. Poor outdoor air quality also exacerbates the quality of indoor air as smoke particles penetrate the building envelope or the heating, ventilation, and air-conditioning (HVAC) filtration systems. In this work, we investigate the impact of wildfire-affected poor ambient air quality on indoor air particulate matter during a wildfire episode in June 2015 in interior Alaska. We measured size-resolved (0.3–10 μm) particle number counts (PNC; numbers/cm3) and calculated particle mass concentrations (PMC; μg/m3) outside and inside of three buildings in Fairbanks, Alaska, during this summer wildfire event. For comparison, the measurements were repeated during a no-wildfire period in summer 2017. Our results show that the fire episode increased the total PNC by factors of 189.4–244 in the outdoor air and by 19.5–150 in the indoor air compared to the total PNC measured during a non-fire season. The PNC was primarily dominated by particles in the size range 0.3–1 μm (> 99%) at all locations during the fire season, whereas the PMC was dominated by particles in the size range from 2.5 to 10 μm (40–67%). The indoor to outdoor ratio (I/O) of PNC during the fire season was significantly lower for an unventilated building (I/O = 0.13 ± 0.001) as compared to those with active (filtered) ventilation (I/O = 0.76 ± 0.11 and 0.62 ± 0.02), suggesting that lower efficiency filters (< Minimum Efficiency Reporting Value or MERV rating 11) often used in residential and public buildings may not control the infiltration of smaller smoke particles during a wildfire event. Although this study had a small sample size, the limited data collected here indicates that sheltering in a closed, non-ventilated building may be an effective strategy to reduce exposure to particulate matter during wildfires, given that there are no significant indoor source(s) of particulates and that the air leakage is insignificant. Finally, this study also shows that particulate mass concentrations (μg/m3) may not fully describe the relative differences between indoor and outdoor air quality especially during wildfire episodes.