Smog in the megacity of more than 20 million people worsened in March, prompting authorities to issue the first alerts in a decade and impose new traffic restrictions through June.

The new rules keep one-in-five cars off the road every day, but the alert issued yesterday will double the ban on today. Some 5.5 million vehicles circulate on normal days. The Environmental Commission of the Megalopolis, which includes authorities from the capital and surrounding states, also ordered cement, chemical, pharmaceutical, oil and power companies to reduce emissions by as much as 40 percent.

The alarm was raised after ozone levels reached 161 points, above the 150 level. The threshold was lowered from 190 to 150 last month.

Before the new air quality alert was issued, the environmental prosecutor's office (Profepa) warned that it may shut down and fine five companies that did not allow authorities to check their emissions during a pollution emergency in the greater Mexico City area in March. Three of the companies are subsidiaries of US firms, while one is based on Canada.

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