There are four general categories for sources of air pollution. These are mobile sources (cars, trucks, buses, planes, and trains), area sources (agricultural areas, cities, wood burning fireplaces), stationary sources (power plants, oil refineries, industrial facilities, factories), and natural sources (wildfires, volcanoes, wind-blown dust). Industrial activities and driving cars and trucks are the greatest sources of air pollution in Colorado. Sources of air pollution are diverse, change, and sometimes it’s not very clear exactly where the pollution comes from, especially for pollutants that are created in the atmosphere instead of directly emitted. Other sources of pollution can be small but because there are thousands of them, they collectively produce a lot of pollution. Each fireplace that emits smoke, each can of solvent in a garage that is not tightly capped, or each spill of a few drops of gasoline as we fill the tank of our car add up and contribute to poor air quality.