The study was initiated to evaluate effects of automotive emissions from attached garages on the indoor environment in Canadian single-detached houses and the impact on occupants' health. This work consisted of testing a total of 25 houses in the Ottawa area to characterize the extent of leakage between the attached garages and houses. The selected houses were from various age groups, styles and types of attached garages. The study found that most of the air leaks from the garage were found to be leading into the basement. In four houses leaks from the garage into the main floor were more significant than leaks into the basement. Although some leakage was detected from the garage into the second storey space of some houses, these leaks may not be significant routes of garage pollutant entry in the house depending on house pressures. The house/garage interface leakage, as characterized by a variation on the Equivalent Leakage Area (ELA), ranged from 4 cm2 to 400 cm2 with an average of 140 cm2. The interface leakage area averaged 13% of the overall house leakage area. This is almost proportional to the ratio of interface area to house envelope area. In other words, the house/garage interface is built roughly as leaky as the rest of the house envelope. The average monitored pressure difference between the house and garage during the summer measurement period was 0.5 Pa. The range of pressure was from -2.3 Pa to 4.6 Pa. winter pressures were much higher averaging 1.6 Pa with a range of 0.4 pa to 4.4 Pa.