The purpose of this guideline is to outline procedures that can be employed by the air pollution data analyst to evaluate trends in air quality. Trends will be generally considered as the broad long-term movement in the overall time sequence of historical air quality measurements. It will be examined in two ways. First will be in the form of a trend line or curve over time. Second will be a statistical categorization of the general direction of the movement over time, i.e., upward, downward, or no change. Associated with the second approach can be estimates of the rate of change of deterioration of improvement in the air quality. Most trend analysis can be performed upon aggregate measures of air quality estimates such as averages.