Mexico's government announced Tuesday measures to reduce smog in the capital by preventing corruption at emission inspection centers that have allowed polluting cars to stay on the road.

The environment ministry unveiled the new rules three months after authorities issued the first air quality alert in 13 years in the greater Mexico City area.

Normally, cars that pass emissions tests are allowed to drive every day while those that fail have to stay off the streets at least once a week. But corruption has allowed polluters to stay on the road.

The smog got so bad in March that authorities responded by temporarily expanding the traffic restrictions, forcing all cars to stay home at least once a week. The extraordinary measure will end June 30.

President Enrique Pena Nieto, meanwhile, demanded tougher emissions controls for the 5.4 million vehicles that circulate in the metropolitan area.

The new measures, which will apply from July 1 and last at least six months, will require that inspectors check emission levels recorded by a car's onboard computer instead of measuring them through the tailpipe.

Cars made after 2006, or two thirds of those circulating in the capital, have such computers, which the ministry says are more reliable.

Older cars will still be inspected through their tailpipes, but a central monitoring system will be established to prevent inspectors from "cheating" or "manipulating the test," said deputy environment minister Rodolfo Lacy.

Environment Minister Rafael Pacchiano said environmental protection authorities will visit inspection centers to ensure they follow the rules.

The tougher emissions tests also set new limits for public transport buses and heavy trucks.