It becomes clear to the user of this process that there are a number of uncertainties and simplifications involved in the described approach. To make a reliable estimate, it is critical to clearly understand the numbers of persons impacted by any emissions changes along with the changes in ambient levels of air pollution that these persons experience. In reality, not all of the persons impacted by an emissions change will see the same changes in ambient air quality levels, and different locations contain different numbers of people residing and working there. Thus, it is common to break an airshed into sub-regions and then analyze each sub-region for concentration changes in that sub-region and the numbers of impacted persons in the sub-region using the process previously described. The approach can be improved as more tools and data become available in the region. Again, care should be taken to make sure the values estimated make sense in the broader context, perhaps by comparing the values obtained in the pollution analysis with ones calculated in other regions or by other authors.