Air pollution due to PM2.5 is of public concern in Korea. Ambient PM2.5 can penetrate indoors through the building envelope, affecting the indoor PM2.5 concentrations. Most people stay indoors for approximately 80% of every day, implying that their primary exposure to PM2.5 could be determined by the indoor air. This study aims to investigate the infiltration of ambient PM2.5 through the building envelop in apartment housing units in Korea. The on-site infiltration test method, by using a blower-door depressurization procedure, was suggested in order to maintain an identical indoor-outdoor pressure difference among the tests. On-site experiments were conducted in 11 apartment housing units to estimate the PM2.5 infiltration factors. The results showed that the average infiltration factor of all the test housing units was 0.65 ± 0.13 (average ± standard deviation), with a minimum of 0.38 and a maximum of 0.88. Furthermore, the results from the relation of the building airtightness data to the infiltration factors suggests that a leaky housing unit with high ACH50, or a high specific effective leakage area (ELA), would be more significantly influenced by the ambient PM2.5. The study demonstrated that the suggested infiltration test procedure was useful to assess the infiltration factors in conditions of controlled indoor-outdoor pressure differences in real housing units.