Recent prospective cohort studies have suggested that long -term exposure to low levels of particulate matter ( PM ) air pollution is associated with increased mortality due to, especially, cardio -pulmonary disease. Exposure to ambient air pollution was estimated mostly as city average concentrations, assuming homogenous exposure within the city. We used an ongoing cohort study -The Netherlands Cohort Study ( NLCS ) on diet and cancer -to investigate the relationship between traffic -related air pollution and mortality. The baseline data collection took place in 1986. A study was conducted to develop methods for exposure assessment and evaluate the contrast in exposure to air pollution within the cohort. Assessment of long -term exposure to two traffic -related air pollutants, Black Smoke ( BS ) and Nitrogen Dioxide ( NO 2 ) , consisted of separate estimation of regional background, urban background, and local traffic contributions at the home address. Interpolation of concentration data from a routine monitoring network was used to estimate the regional background concentration. A regression model relating degree of urbanization to air pollution was used to allow for differences between different towns / neighborhoods of cities. Distance to major roads was calculated to characterize local traffic contributions, using a Geographic Information System ( GIS ) . Interpolation resulted in reasonably precise regional background estimation when distant sites were not used and distance squared was used as the weight. Cross -validation showed that prediction errors were about 15% of the range in regional background concentration. Urban and local scales contributed significantly to the contrast within the cohort. Prediction errors for estimating the urban background were about 25% of the range in background concentrations. When the developed model was applied to the study cohort, there was substantial contrast in estimated exposure to BS and NO 2 . About 90% of the study population lived 10 years or more at its 1986 home address -supporting the use of the estimated concentration at the 1986 address as a relevant exposure variable.