In the final example study that we reviewed, Bellander et al. (2001) had sufficient source emission and environmental measurement data to calibrate and validate predicted levels of NO2 in the environment of Stockholm, Sweden, for at least a portion of the exposure period in an epidemiologic study of lung cancer (1955–1990). They also validated their predicted location of residence in Stockholm for each participant in the study by cross-checking results using external geocoding service companies. The resolution and precision of this exposure assessment process resulted in the capability to detect a wide range of individual long-term average exposure and to detect risk of lung cancer to average traffic level exposure to NO2 within a 95% confidence limit. The procedures and results of these studies clearly indicate the need for expertise in environmental science and related disciplines in epidemiologic studies involving pollutant emissions.