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Mexico City officials have cancelled classes for millions of students as smoke from brush fires continues to choke the city of 9 million.

A light rain overnight has done little to cut the pollution, which remained at about 1 ½ times acceptable limits.

The city declared a partial driving ban, but activists of the Citizen Observatory on Air Quality called Thursday for officials to limit polluting activities like trucks and construction sites.

The activists said the city should include extremely small particles as a cause for imposing emergency measures. Such particles are frequently found in smoke, diesel exhaust and dust. Emergency measures are currently imposed mainly for ozone levels.

The group said "forest fires are unfortunately going to be an ever more frequent problem as a result of global warming."