At present, roughly half of the world's population lives in urban centers. There are now more than 20 cities with a population of over 10 million inhabitants, compared to less than 5 about 50 years ago. This tendency toward urbanization is expected to continue, particularly in the developing world. A consequence of this growing trend is that millions of people are being exposed to harmful levels of urban air pollutants caused mainly by emissions from motor vehicles and from industrial and domestic activities involving the combustion of fossil fuels. The driving force for the design and implementation of emission control strate­ gies aimed at improving air quality has been the protection of the health of the population in urban centers. There are, however, other consequences of the pres­ ence of air pollutants besides the direct effect on human health. Reduced visibil­ ity, damage to monuments and buildings, and many other such consequences indirectly affect our quality of life. Another set of consequences involves damage to ecological systems. In fact, the nature of "photochemical smog" was first uncovered in the 1950s in connection with observations of its harmful effects on crops and plants in the vicinity of Los Angeles.

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Historical Summary of the Geology, Climate, Hydrology, Culture, and Natural Resource Utilization in the Basin of Mexico * Socioeconomic Change and its Impact on Forest Resources in the Basin of Mexico * Physical Geography, Geology, Hydrogeology, and Forest Soils of the Basin of Mexico * Forests in the Basin of Mexico * The Climate of the Mexico City Air Basin * Air Pollution Levels and Trends in the Mexico City Metropolitan Area * Emissions Trends and Pollution Control Strategies in the Mexico City Metropolitan Area * Heavy Metals in Forest Soils, Vegetation and Drainage Waters in the Basin of Mexico * Oxidant Exposure and Effects on Pines in Forests in the Mexico City and Los Angeles, California Air Basins * Decline of Sacred Fir in the Desierto de los Leones National Park * Histological Symptoms of Air Pollution Injury in Foliage, Bark, and Xylem of Abies religiosa in the Basin of Mexico * Lichens and Air Pollution in the Forests Surrounding Mexico City * Nitrogen and Sulfur Deposition in the Mexico City Air Basin * Effects of Oxidants in Ambient Air on Annual Crops in the Basin of Mexico * Summary of Air Pollutant Impacts on Forests in the Mexico City Air Basin * Resources at Risk and Research Needs