Millions of people in Mexico City are choking on the worst air quality in 14 years, as cars and trucks add to the pollution with their outdated emissions controls. And it’s not as though Mexico lacks access to the best technology.

The nation’s export-oriented factories, the focus of a $20 billion investment boom, ship motor vehicles to countries with strict pollution limits. Yet Mexico’s requirements are less stringent so cars, pickups and commercial trucks sold domestically don’t live up to the same emissions standards as the models made for foreign buyers.

Mexico’s weaker pollution controls are worsening the thick layer of smog basting the capital, which got so bad last week that city officials banned 40 percent of all vehicles from the streets. Transportation accounts for almost half of Mexico City’s air pollution, which according to government data kills as many as 2,700 people every year.

“We produce some of the best vehicles sold in the U.S. or Europe,” said Edmundo Molina, an energy researcher at the Monterrey Institute of Technology. “The paradoxical thing is why we use technology here that’s not as good compared with other countries. And the reason has to do with regulation.”

While low-polluting fuel is already sold in Mexico City, it has yet to be made available nationwide by Petroleos Mexicanos, the state-owned oil company known as Pemex. That makes it uneconomical to introduce vehicles with more modern emissions controls, according to the Mexican Automobile Industry Association.Boosting tax breaks on new models would also help modernize the fleet of heavy vehicles, according to Miguel Elizalde, head of the country’s trade association for bus and truck makers. The average age of heavy vehicles in Mexico is 17 years, he said.