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Air pollution is affected by change in altitude. Cities with surface elevations above 1500 meters have atmospheric pressures which are approximately fifteen percent (15%) below pressures at sea level. Consequently, mobile sources designed to operate at pressures of one atmosphere perform less efficiently at high altitudes and emit greater amounts of hydrocarbons and carbon monoxide than those designed to operate at the lower atmospheric pressures. The net result is a photochemical smog problem which is further enhanced by the increased solar radiation of higher altitudes. The most significant effect of air pollution at high altitudes is upon human health. This is due primarily to the inhalation of carbon monoxide at the reduced oxygen concentrations of high altitudes. Particularly susceptible is the fetus exposed to hypoxia and elevated carboxyhemoglobin levels. There is insufficient evidence to support significantly increased ecological effects at high altitudes. Reduction in visibility is being observed in the vicinity of large metropolitan areas and near large industrial complexes at high altitudes.
U.S. EPA. ALTITUDE AS A FACTOR IN AIR POLLUTION. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/9-78/015 (NTIS PB285645), 1978.