Mexico City authorities have declared a new pollution alert and will require 40 percent of cars to keep off the streets Tuesday.

Under a rule in effect through June, one-fifth of the city's cars normally must stay at home on a weekday, with the day determined by license plate numbers. But on Monday smog rose above 1½ times acceptable limits, meaning another fifth of cars are ordered off the streets.

The alert marks the second time in less than a month that the alert level has been reached despite the temporary program aimed at improving the capital's dirty air.

Before the rule was implemented last month, newer or cleaner cars were exempt from the one-day driving ban.

Authorities have pledged to come up with now, tougher vehicle emissions tests.

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