Unusual smog alert for Mexico City

Authorities have issued a pollution alert for the Mexican capital and surrounding suburbs after ozone levels reached more than 150 per cent of acceptable levels, something rarely seen during the summer rainy season.

The alert triggered a reduction in the number of cars allowed to circulate on Friday and also put restrictions on some commercial and industrial activities.

The metropolitan Environmental Commission said in a statement posted online that a high-pressure system, weak surface winds and solar radiation were making it difficult for air contaminants to disperse.

The worst pollution was in a southern district of the capital where the ozone index hit 158, classified as "very bad".

Air quality in other parts of the metropolis ranged from "good" to "regular" to "bad".

In March, Mexico City saw its first pollution alert since 2005 during a week when ozone hit 203 per cent of acceptable levels.

It was seen as a setback after the metropolis had made strides in reducing the chronic smog of the 1990s by cleaning up or moving out factories, banning leaded petrol and imposing stricter car emissions standards.

Last year the Supreme Court overturned a rule forcing cars over eight years old off the streets at least six days a month, a decision that officials have said put an estimated 1.4 million vehicles back on the roads daily, many of them older and higher-polluting models.

With Thursday's action, 10 pollution alerts have now been issued this year. In the wake of the March smog event, officials also lowered the threshold at which they declare an alert from 200 on the index to 150.

Government records suggest pollution is still significantly better than it was two decades ago. The city's highest recorded smog level was 398, set in March 1992.

Smog buildups are unusual in Mexico City during the summer, when near-daily thunderstorms and strong winds typically flush out contaminants. However, there was an alert July 8, also during the rainy season.