All these antipollution measures have helped stem the increase of global pollution emission levels
. Between 1970, when the Clean Air Act was passed, and 1995, total emissions of the major air pollutants in the United States decreased by nearly 30 percent. During the same 25-years period, the U.S. population increased 28 percent and vehicle miles traveled increased 166 percent. Air pollution control is race between the reduction of pollution form each source, such as a factory of a car, and the rapid multiplication of sources. Some in American cities is expected to increase again as the number of cars and miles driven continue to rise. Meanwhile, developing countries are building up their own industries, and their citizens are buying cars as they can afford them. Ominous changes continue in the global atmosphere. New efforts to control air pollution will be necessary as long as these trends continue.