The air over the Chesapeake region travels here from a much larger area than the watershed itself, and the extent of that airshed varies on any given day according to the current weather pattern. Unfortunately, much of our air originates in areas of the Midwest dominated by polluted emissions from powerplants and motor vehicles. We generate plenty of air pollution from the same sources within our region, but the amount from outside sources essentially doubles it. This heavy air pollution load causes human health problems. In addition, pollutants washed out of the air by rain, especially oxides of nitrogen, cause serious problems for the Chesapeake Bay and its tidal tributaries.