There has been a very noticeable increase in air pollution during the past ten years over and downwind of the several large metropolitan areas of the United States such as the Northwest—Vancouver-Seattle-Tacoma-Portland; the West Coast from San Francisco-Sacramento-Fresno-Los Angeles; the Front Range of the Rockies from Boulder-Denver-Colorado Springs-Pueblo; the Midwest—Omaha-Kansas City-St. Louis-Memphis; the Great Lakes area of Chicago-Detroit-Cleveland-Buffalo; and the Northeast—Washington-Philadelphia-New York-Boston. The worst accumulation of particulate matter occurs at the top of the inversion which commonly intensifies at night at levels ranging from 1000 to 4000 ft or so above the ground. This dense concentration of air-suspended particles is most apparent to air travelers. Thus, it has not as yet disturbed the general public except during periods of stagnant weather systems when the concentration of heavily polluted air extends downward and engulfs them on the highways, at their homes and in their working areas.

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