Inventories are estimates a collection ofair pollutant emission estimates based on an emission factor (i.e., grams CO per km of vehicle travel) and an activity indicator (ie.e, km ov vehicle travel per year). The product of the emission factor and the activity inducator is the number of grams of pollutant emitted per year (usually expressed in metric tons). Corrective factors to account for emission control systems, seasonal variations in activity or future growh rates are often applied to the base emission estimates. Inventories typically change over time becauses of improvements in the data or changes in source operating conditons. A complete inventory includes all motor vehicle sources, industrial sources and areas sources (space heating, open burning, street dust, agricultural or forestry burning, etc.).