Alex Molinaroli, a senior executive at Johnson Controls, said its plants in Mexico far exceeded that country’s regulatory standards and that they would be upgraded to meet the new American standards when they take full effect in 2013. “We don’t have a Mexican standard or a U.S. standard or a German standard,” he said. “We have our one standard globally, which today is being driven by the E.P.A.”

Although Johnson Controls has won praise from the E.P.A. for environmental innovation in the United States, its Mexican recycling plant does not face the same regulatory scrutiny.

Working in the Dark

Mexico does have some regulations governing lead exposure, and many plants hire doctors to monitor lead in the blood of workers. But the results are not made public or even disclosed to the workers themselves. If the levels come back high, employees are sent home for several days with an analgesic for the bone pain that typically accompanies adult lead poisoning, said Ms. Jacott, of Fronteras Comunes, who has spent two years interviewing workers. There are no requirements for monitoring lead levels beyond the factory.

Residents who live near the Acumuladores de Jalisco plant said they had been told by the government that the ground water was contaminated with lead, and they tick off maladies they attribute to lead exposure.

The men who disassemble the batteries end each shift covered in dust from their work and must shower and change before they leave, said the wife of one worker, who said that the factory doctor took good care of the men. “Anyway,” she said, “there are not many other jobs around here.”

Environmental advocates and domestic recyclers say the onus should be on the United States to make sure its old batteries do not become Mexico’s health problem. Some say a system is needed for inspecting foreign recyclers so they can be held to American standards. But one group, Slab Watchdog, has called on companies like Wal-Mart — which sells a huge share of the nation’s batteries and prides itself on environmentally friendly operations — to guarantee that their batteries are recycled domestically.

The company’s spent batteries, a spokeswoman said, now go to Johnson Controls — the company that last year sent by far the most batteries to Mexico.