<P> Air pollution in Mexico City is a continuing concern for citizens, health experts, and environmentalists . The air pollution of the Mexico City Metropolitan Area, contained within the Valley of Mexico, is measured by the Índice Metropolitano de la Calidad del Aire (Metropolitan Index of Air Quality). </P> <P> It has been said that "Mexico City's air has gone from among the world's cleanest to among the dirtiest in the span of a generation ." Historic air pollution episodes of the 1950s led to acute increases in infant mortality . </P> <P> Much improvement was made in the city since 1992, when the United Nations named Mexico City "the most polluted city on the planet ." At the time pollution was thought to cause 1,000 deaths and 35,000 hospitalizations per year . </P>

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