The Mexican Environment Minister, Julia Carabias, says no significant rainfall is expected for at least another two weeks and warned that the worst pollution is still to come.

Since Tuesday, more than a third of cars have been ordered off the roads, factories have been forced to reduce output, and schoolchildren are not allowed to play outside.

The Mexican capital, one of the world's biggest and most polluted cities, is covered by a thick veil of smog, along with smoke from surrounding forest fires.

Helicopters are used in dumping water over the forest fires in the Chiapas state

The BBC correspondent in Mexico City says one of the greatest concerns at the moment is the high ozone level. The elderly are being advised to stay indoors for fear that it could cause respiratory problems, or worsen heart complaints.

Ozone is a colourless, highly reactive pollutant created by sunlight reacting with car and factory fumes.

Alert also in Texas

Much of Mexico's smoky haze originates from forest fires in Oaxaca, and the southern state of Chiapas.

Fifty firefighters have been killed in the line of duty in the first five months of the year. The Mexican army has also been mobilised in the environmental emergency.

This week the US government sent Mexico firefighting advisers, some equipment and at least $2m for the hire of private blaze-extinguishing helicopters.

The fires, aided by southerly winds, have carried soot particles north causing hazy skies in Texas and other parts of the United States.

The Texas health officials have suggested that people with respiratory problems avoid exercise and outdoor activities.