On Rio Grande, pollution knows no borders. Maintaining high air-quality standards is especially tough along the US-Mexican border. Mexico has more pressing economic concerns, but officials are making some effort to keep the air clean.

When a representative of an electronics plant in Ciudad Ju'arez - just across the Rio Grande from El Paso - indicated that his company properly disposed of its hazardous wastes, Howard Applegate was impressed. The professor emeritus of civil engineering at the University of Texas at El Paso knew from his work on environmental issues that most hazardous materials are neither easily nor cheaply discarded, if disposal is done right. His interest piqued, he inquired further.

But the answer Dr. Applegate got left him shuddering.

``He told me they stored their [waste] PCBs until they got enough, at which time they sent a guy out to the parking lot with half a 50-gallon oil drum and a blow torch to incinerate it,'' Applegate recalls. ``We live with the lovely thought that they [PCBs] are being transported across the border in the wind. And they don't have a green card.''

El Paso is not the only city along the 1,254-mile border between Mexico and the United States facing serious pollution problems. Concerns over air and water pollution from Brownsville, Texas, west to San Diego, have been wrestled with - in some cases quite successfully - for several decades.

Yet, as a developing nation with protracted economic difficulties, Mexico's approach to environmental issues is different from that of its wealthy, post-industrial neighbor to the north. ``We get concerned because we can't see 40 miles down the road,'' says Manuel Aguirre, regional director of the Texas Air Control Board, in an analysis of the region's pollution problems. ``They're looking at, down the road, can I stay alive next week?''

Mexico has worked to assume its burden in relieving border environmental concerns, US local and federal environmental experts say. That is especially true since the US and Mexico signed an environmental agreement in 1983. But these experts add that the region's environmental problems are not likely to receive top billing while Mexico's economy remains weak.

``We see the haze rising over Ju'arez, and we know it's from burning tires and railroad ties, and who knows what else treated with what chemicals,'' says UTEP's Applegate. ``But you can't expect them to stop burning that stuff when they have no other way to keep warm.''

El Paso is especially susceptible to pollution problems that are unique to a region where the developed and third worlds rub shoulders. Part of the problem is geographical: Nearby mountains encourage air inversions that trap dirty air above the city. But most important is the population distribution, experts say. The El Paso-Ju'arez metropolis is a city of 1.5 million people. Yet only the one-third of the population on the US side of the border falls under US pollution regulations.

El Paso's air quality is ranked among the worst in the nation, according to the Environmental Protection Agency (EPA). The city is falling below national standards in three of the six categories - ozone, carbon dioxide, and particulates - regulated by the EPA.

Cars are a particular problem, for which both sides are responsible. ``They have older cars burning dirtier gas,'' says Applegate, ``but we have two-thirds of the region's cars.'' Mexicans are much less likely than Americans to keep their cars clean and tuned up because there are no required inspections, and a tune-up in Mexico can easily cost two weeks' wages.

While continuing to work with state and local authorities to improve air quality, the EPA is not planning any sanctions against El Paso such as those it has issued against other cities.

``The EPA recognizes that attainment of these standards is especially difficult in some areas of the border, and El Paso is the prime case,'' says James Yarbrough, a scientist with the EPA's Dallas office, ``... this is something that has to be addressed at the federal level.''

The roots of international cooperation lie in the International Boundary and Water Commission, which celebrates its centennial next year. Established to settle territorial and water-rights disputes, the commission works to address water pollution problems through binational cooperation. Under the commission's guidance, joint sewage treatment plants have been built, and programs for desalinizing agricultural runoff have been developed.

This year the US Congress authorized further steps to battle pollution of the Rio Grande. Of major concern is Nuevo Laredo, across the border from Laredo, Texas, where raw sewage surpassing 25 million gallons a day pours into the river.

Another developing source of concern is the maquiladoras - industrial plants in Mexico that are owned and operated by foreign companies, usually American. In the past few years the number of ``maquilas'' has mushroomed to more than 1,300. They employ more than 355,000 Mexicans. The great majority of these plants are located along the border. Those plants working in such fields as electronics, furniture finishing, and auto parts manufacturing and assembly use large amounts of hazardous materials.

Most of it is transported into Mexico from the US. ``The amounts are staggering,'' says Richard Bath, a political scientist at UTEP. ``And there's little question that most of it is being disposed of one way or another, and illegally, in Mexico.''

By international agreement, hazardous materials imported into Mexico for the maquilas are to be exported back to the US after use for proper disposal. But this does not appear to be happening.

One study by the Border Ecology Project in Naco, Ariz., found no records of chemical shipments between Arizona's Cochise County and Agua Prieta in Mexico. ``There's incredible ignorance on both sides of the border on how to deal with hazardous wastes,'' says Dick Kamp, the project's director. ``As far as what's going where and how it's being used, no one knows anything.''

Mr. Kamp says the EPA claims to have no evidence that large quantities of hazardous materials are being stored. ``But that's absurd,'' he says. ``We found one company holding on to 5,500 gallons going back 16 years.''

As a step toward rectifying that ``ignorance,'' the EPA and SEDUE, its Mexican counterpart, last month sponsored a conference in Tijuana to acquaint maquila operators with regulations governing the transportation and disposal of hazardous wastes. Kamp says he is particularly encouraged by EPA negotiations with Mexico for on-site company inspections.

``The maquila people want to know what the rules are concerning these materials,'' says Dr. Bath, who attended the conference. ``The question is whether they adhere to them or not.''