Mexico city has the planet's worst air pollution - only Tokyo is bigger.
Said to have the planet's worst air pollution, this megacity - only Tokyo is bigger - has ozone levels exceeding World Health Organisation standards 300 days a year.

Four million motor vehicles, most of them old and belching fumes, and the city's location in a mountain-surrounded basin, are primarily responsible for the bad air.

The government has taken innovative steps to deal with it - including incentives for cleaner fuels and smog control measures, requirements for catalytic converters and driving limitations.

Assignment Earth focuses on families with serious health problems resulting from the pollution.

We see how air quality affects people's lives, talk to health experts, government officials and clean-air campaigners to find out if there has been any real progress in cleaning up the air and what other cities can learn from the experience of Mexico City.

This episode of Assignment Earth aired from 31 March to 21 April 2007

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