Four years after a U.S. court ordered a huge ore smelter near El Paso to reduce its emission of toxic lead dust, Mexican authorities say that the plant still poses a health hazard, particularly to local children.

On the U.S. side of the border, however, plant and environmental officials express doubt about Mexico's own determination to deal with the problem. Monitoring devices donated to Mexico are going unused, they say, and the Mexican government has done little to end illegal squatting in polluted areas.

The smelter, owned by Asarco, Inc., an American company, has been in operation for 90 years and is one of the largest in the country, with more than a thousand employees.

A decade ago, according to a study city by local authorities, the plant was emitting some 3,000 pounds of dust a day, two-thirds of its lead dust. Large quantities of the dust were found as far as seven miles away, settled on rooftops, household utensils and in yards. The company paid neighboring American farmers for pollution damage to their crops.

Since, then, according to plant manager William Kelly, the company has spent close to $50 million on improvements and antipollution devices, and increased the height of its stack from 300 feet to 820 feet.

Dust emission is now 300 pounds a day, Kelly says, with 25 per cent lead content. Furthermore, he says, the company has installed monitors and keeps meteorologists on 24-hour duty to determine when weather conditions force a shut down.

Most of these steps followed a 1972 court decision after the state of Texas andd the city of El Paso brought suit against Asarco. The company was fined $90,000 and ordered to pay medical expenses of individual es of individuals affected by the pollution, to continue checking children in the area for signs of mineral poisoning and to install pollution-control devices.

No damages were paid to Mexico according to Mexican authorities.

Two years after the court's decision, a Mexican government survey of 724 children living near the plant found that more than 10 per cent of them had more lead in their blood than the U.S. Surgeon General's maximum limit of 30 micrograms per 100 milliliters.

The particles are too large to be inhaled, and the age group most affected seems to be the 3-to 7-year-olds who put things in their mouths.

In El Paso, the director of the county health department, Dr. Bernard Rosenblum, said that many children have suffered irreversible damage, although none are known to have died from lead poisoning.

"On X-rays you could actually see the deposits of lead in the bone marrow," Dr. Rosenblum said. "Some of the children had weakness of the wrist muscles and serious learning disability."

The head of air programs at the Dallas branch of the Environmental Protection Agency, Jack Devita, says that "To our knowledge, Asarco is in compliance with court orders and has no serious emission problem."

Kelly, the plant manager, says: "Another two years and we'll have halted most emission," adding: "You can't take a 90-year-old plant and revamp it quickly."

On the U.S. side of the border, the company has razed the shacks of "Smeltertown," the highly contaminated area near the plant where many of its workers used to live. It also scooped off one foot of to-soil in the most affected areas.

On the Mexican side, however, within a mile or two of the smelter, hundreds of families continue to live in shantytowns built on top of several feet of highly contaminated dust spewed out by the chimney since 1887.

A high official in Mexico's environmental protection department, asked about the need to scoop away the contaminated soil, said: "Those people are squatting there illegally in the first place, and should be moved altogether."

American officials also cast doubt on Mexico's determination to face up to the problem. Mexican officials say they are waiting for the U.S. government to make the first move in launching joint studies - agreed on at a December 1975 meeting - to evaluate the present contamination.

And El Paso's Dr. Rosenblum said the city had given authorities in near by Ciudad Juarez some of its used air pollution measuring equipment, four high-volume samplers.

Until recently, he said, El Paso had done the laboratory testing on the samples collected by the Mexicans, who lacked the equipment for the analysis.

"But for the last few months, the Mexicans have not been sending them over," he said. "The engineer who should collect the samples and turn them over to us says he has not been getting any travel expenses, so he has told me he is no longer bothering to pick them up."