MEXICO CITY — Authorities issued a phase 1 environmental alert for the Mexican capital and surrounding suburbs Thursday after ozone levels reached more than 150 Imeca points, something rarely seen during the summer rainy season.
The alert triggered a reduction in the number of automobiles allowed to circulate Friday and also put restrictions on some commercial and industrial activities.
The Megalopolis Environmental Commission (CAMe) 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 the Tlalpan borough of Mexico City, at 158 Imeca points, 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 environmental alert since 2005 during a week when ozone hit 203 Imeca points.
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 gasoline and imposing stricter automobile 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 environmental 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 to 150 Imeca points.
Government records suggest pollution is still significantly better than it was two decades ago. The city’s highest recorded smog level was 398 Imeca points 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.