SOOTY, yellow air often hangs over El Paso, Texas, like a dirty blanket. Filled with carbon monoxide, ozone, and particulates, the foul atmosphere obscures the city's skyline and blocks the view of Ciudad Juarez, just across the Rio Grande in Mexico.El Paso's severe pollution, which has worsened during the past 10 years despite new air quality regulations, is just one of many environmental problems along the Mexico-United States border. The growing contamination of air, water, and soil now poses a potential threat to President Bush's plans for a free-trade treaty with Mexico. Environmentalists, led by critics like Mike McCloskey, chairman of the Sierra Club, and Joan Claybrook, president of Public Citizen, a consumer advocacy group, are threatening to haul Bush into court. The charge: failure to heed environmental laws in trade talks with Mexico and other nations. Lori Wallach, an attorney for Public Citizen, complains that the president is crafting trade deals best suited to big business, not to the health and safety of workers on both sides of the border. Tensions between the White House and its critics intensified during the past week when the administration unilaterally ruled that trade agreements are exempt from the National Environmental Policy Act of 1970 (NEPA). Mr. McCloskey and Ms. Claybrook insist that trade agreements, like other federal actions, require compliance with NEPA. Claybrook says: "Consumer and environmental dangers in our time know no national boundaries.... International agreements must be the new battleground for consumer and environmental law enforcement." William Reilly, administrator of the Environmental Protection Agency (EPA), disputes charges that the Bush administration is avoiding issues like water and air pollution in talks with Mexico. "I don't think that there has been any previous moment in the history of our two countries when there has been such sustained and intensive and high-level attention to the environmental problems that affect our border," he told an Aug. 1 press conference. The EPA and the Mexican environmental agency, SEDUE, have crafted a 10-part plan to address problems. But Mike Clark, president of Friends of the Earth, scoffs at EPA's border plan as weak and unenforceable. "This is a docile dog," Mr. Clark says. "It has no teeth. It cannot bite. It is simply a way to evade the fact that we have good laws on the books, and [Bush] does not want to enforce them." At the heart of this debate is NEPA, the "magna carta" of conservationists. Again and again since the 1970s, NEPA has forced federal officials to give greater weight to environmental issues. NEPA, for example, requires government agencies to issue environmental impact statements (EIS) before sanctioning major offshore oil drilling projects. Specifically, NEPA requires that "all agencies of the federal government," to "the fullest extent possible," take the environment into account when they begin any major action "significantly affecting the quality of the human environment." NEPA says all federal agencies must issue a "detailed statement" on "the environmental impact of the proposed action." The statement must include "any adverse environmental effects which cannot be avoided should the proposal be implemented." Those are tough terms. Sometimes they force US officials to admit that what they propose will damage the nation's ecosystem. Why is the Bush White House refusing to conduct a NEPA study? "We don't think we're required to," says Tori Clark, an official with the office of US trade representative. Mr. Reilly says a NEPA impact statement would only "duplicate" much of what the administration is already doing with its own plans. Reilly says he was advised by the State Department, the Justice Department, and the Council on Environmental Quality that NEPA "does not apply in this instance." Furthermore, he says, "what we are doing is considerably in excess of what we would do" under NEPA. In a draft entitled, "Integrated Environmental Plan for the Mexico-U.S. Border Area: First Stage, 1992-1994," EPA singles out numerous problems that must be addressed by trade negotiators. These include Tijuana wastewater, which spills into San Diego; Mexican sewage that flows into Imperial County, Calif.; and Rio Grande water pollution. In the 1992 federal budget, Bush calls for $112 million to alleviate border environmental problems, including $100 million for a treatment plant in San Diego to clean effluent from Tijuana. But critics are unimpressed. Ms. Wallach asks: "Why are they in such a hurry? Everything is just zoom, zoom, zoom before everyone has a chance to look at the down side of a trade agreement." Wallach says environmentalists want a trade agreement that highlights "health and safety, clean food, pollution control, sustainable development, workers' rights to organize, general human rights, job security, fair trade in agriculture, and equal environmental standards on both sides of the border." "The administration is either opposed to, or is ignoring, all those things," she charges.