Natural ventilation is one of the most important green building features. Although the intention is often to reduce energy consumption, natural ventilation strategies may deteriorate indoor air quality (IAQ) when in close proximity to major roadways. This study employs a CFD-based air quality model to quantify the impact of traffic-related air pollution on the IAQ of a naturally ventilated building in a near-road environment. Our study found that the building envelope restricts dispersion and dilution of particulate matter (PM).