The authorities in Mexico City have taken emergency action to cut dangerously high air pollution levels by ordering hundreds of thousands of cars off the roads and telling factories to work at thirty percent of normal capacity.

The alert was declared after amounts of harmful ozone rose almost three times above the highest level considered acceptable by other countries.

The BBC Correspondent in the Mexican capital says that even by Mexico City's standards the unseasonally high pollution levels have been quite literally breath-taking. Meteorologists say an unusually clear dry spell has drawn huge amounts of ozone into the Mexico City basin. Environmentalists say drastic action is needed to reduce the four million tonnes of toxic emissions released into the city's air each year.