THE most rapidly growing large urban areas are now in less-developed countries. High fertility and declining mortality, development strategies, and substantial rural-to-urban migration produce vast urban areas with populations in excess of ten million. These megacities are marked by chronic housing shortages, unemployment, and inadequate social services, but less recognized and less studied is the environmental degradation often associated with these areas. Mexico City is a prototype of this megaurbanization, and this article focuses on one aspect of environmental degradation, air pollution.
The problem is relatively recent. In 1900 the city had approximately one-half million residents; by 1950, it had grown sixfold and was expanding rapidly. The pattern is consistent with a core-periphery dichotomy common in less-developed countries that has become known as the Mexican model of development (Ezcurra 1990; Mumme 1991). In this scheme the primary node wields disproportionate political and economic power that enables it to monopolize scarce resources. Second-order cities and rural regions lack crucial capital investments, a situation that results in a developmental gap that impels migration from the hinterlands. Subsequent political and resource allocations favor the already-dominant urban core and trigger additional cycles of migration and investment. Thus, even though one of four Mexicans already lives in Mexico City, the political, social, and economic primacy of this sprawling urban center provokes continued growth. With an estimated twenty-two million residents and a 4 percent growth rate, the urban area will soon add a million new residents each year and is projected to reach a population of thirty-three million by the end of the twentieth century (Ezcurra 1990, 585).
Geographical setting compounds the effect of population. Built on the ruins of the Aztec capital, Tenochtitlan, Mexico City was founded on a small island in one of five large, interconnected lakes. The elevated, mountain-rimmed site of the city initially proved fortuitous. The lakes provided transportation, water, and food; local forests supplied fuel and building materials; from the surrounding slopes came a variety of indigenous foodstuffs and European crops and livestock. Additionally, the climate of the highland site was free from most lowland tropical diseases. Still, the physical environment of the basin deteriorated. City-dwelling Spaniards and an expanding Mestizo population cut forests for fuel and building materials. The lakes ceased to be viewed as assets; instead, they were associated with devastating floods and were viewed as obstacles to urban expansion. For the next three hundred fifty years Mexico City grew slowly.
In the 1940s, the Mexican development model designated the city as the industrial focus of Mexico. Despite an interior location and meager natural resources, industries and population in the city expanded markedly. With growth came new and in some cases unique environmental issues. Demand for water produced the first widely observed environmental dilemma. Overpumping of aquifers caused surface subsidence in much of the downtown. By 1950 industrial and vehicular emissions limited the accustomed views of nearby snow-capped volcanoes. Two decades passed, however, before deteriorating air quality prompted systematic measurement of the problem (Bravo 1987; de Bauer and Krupa 1990). Another decade lapsed before environmental legislation was earnestly enforced. Until recently, the priorities for many Mexico City residents were employment and improved living standards; public policy generally mirrored this concern. To some extent, environmental concerns are still submerged by matters of day-to-day existence. But serious questions are increasingly raised about the Mexican capital's environmental future.
The article examines air pollution in the Valley of Mexico from a geographical perspective and explores …