National Geographic : 1987 Apr
A filthy haze chokes Mexico City on a winter morning in 1986 (below). Vehicle exhaust, factory smoke, and smoldering refuse heaps befoul the air inhaled by 17 million people. Breathing this air has been compared to smoking two packs of cigarettes a day. By contrast, the sky was clear in September 1985 (right) when winds washed out the city's vast bowl-shaped valley. Even the peaks of near by volcanoes were visible -a cause for celebration.