In California, the majority of office and other nondomestic buildings use mechanical cooling and ventilation even when an optimized natural ventilation (NV) system could meet cooling and fresh air requirements. Unfortunately, in most large California cities, the outdoor environment is contaminated With noise, fine particles, heat, toxic gases or, in most cases, a combination of all four. This contaminated environment has a detrimental impact on naturally ventilated buildings due to their lack of filtration and outdoor noise attenuation systems. This paper presents a study on the impact of airborne particle pollution on the potential for NV cooling of office buildings in California. The study uses a multi-year database of measured hourly weather data and PM2.5 data for the five largest metropolitan areas in California, representative of 90% of the state's population. The analysis is performed in two stages with increasing complexity. The first stage is a statistical analysis that identifies coincidence between high PM2.5 and outdoor air temperatures that are suitable for NV. In addition, this phase includes mUltivariable correlation to identify particular weather events or time periods that affect PM2.5 levels. The second level of analysis is more complex, using building thermal simulation (EnergyPlus) to perform a detailed assessment of NV potential in the five urban locations, calculating NV flow rates, resulting indoor exposure to PM2.5 and supplemental HVAC system energy consumption. The results show that using NV in moments when the outside weather is favorable can result in HVAC energy savings of 25-80%. However, limiting NV use to moments with outdoor particle levels below 12 mu g/m(3) decreases this energy saving potential to 2060%. In addition, in the majority of the cities analyzed in this study, the use of NV leads to an increase in indoor exposure to PM2.5 of outdoor origin of 400-500%. (C) 2016 Elsevier Ltd. All rights reserved.