CIENEGA DE FLORES, Mexico - When an American replaces the battery in a car, likely as not the old battery will be shipped to Mexico rather than trucked to a modern U.S. recycling plant.

U.S. recyclers have some of the world's top technology for safely breaking apart batteries to smelt the lead for reuse. But U.S. recycling plants are closing down or standing idle.

Plants in Mexico are not.

Mexico has won a leg up for a reason: Its lead emissions standards are one-10th as stringent as U.S. standards. Mexican factories can ignore strict U.S. regulations that cap harmful lead emissions onto factory floors and into the air.

The result has been an ever-increasing surge in the trade of used batteries across the border. One watchdog group estimated that in 2011, the dead batteries headed to Mexico would have filled 17,952 tractor-trailers. And the trade keeps growing, the result of a stark regulatory gap that has left Mexico at risk of becoming a "pollution haven," according to a Montreal-based commission that investigates environmental issues under the North American Free Trade Agreement, the economic accord between the U.S., Mexico and Canada.

"Raising the bar is what it's all about," said Irasema Coronado, the executive director of the Commission for Environmental Cooperation, the trilateral NAFTA board, adding that she hopes the three nations "are able to harmonize how they are going to deal with these batteries."

"I do reiterate that all of us who own a car in North America own this problem," she said.

It's not just environmentalists worried about the issue. U.S. smelters and recycling firms that don't have operations in Mexico also are demanding that the environmental bar be raised for companies in Mexico and, to a lesser extent, Canada.

In late December, one company, Dallas-based RSR Corp., urged that the three nations take "robust and immediate action to halt battery exports."

"Allowing the tsunami of lead acid batteries to continue to be exported to Mexican facilities sentences those working in or living near these facilities to a lifetime of increased lead burden in their bodies," RSR chief executive Robert E. Finn said in a Dec. 21 letter to the Commission for Environmental Cooperation.

Even as recycling has become the norm, the United States has seen huge consolidation of lead smelting and refining. In little-more than four decades, the number of smelters in the U.S. has fallen from 154 to 14. At the top of the heap is Johnson Controls Inc., a diversified industrial conglomerate based in Milwaukee that is the world's largest supplier of automotive batteries.

Johnson Controls has two huge recycling plants in Mexico, one in Cienega de Flores, about 10 miles north of Monterrey, an industrial hub, and another newer facility in Garcia, on the city's western outskirts. The two plants receive nearly three-quarters of the spent batteries sent to Mexico for recycling.

U.S. watchdog groups say the Johnson Controls plant in Cienega de Flores emitted more than six metric tons of lead into the air in 2010, 33 times the level of emissions expected for a much cleaner plant it opened in Florence, S.C., in September. Mexican activists say they are concerned about what they see as a double standard but have been unable to gather data from the company.