FOR decades the March 18 Oil Refinery in northern Mexico City belched out enough smoke, gases, and dirt to produce close to 5 percent of the pollution fouling the air of this notoriously polluted city.
Then in 1991, President Carlos Salinas de Gortari, increasingly aware of the mounting toll the bad air was taking on residents' health and the Mexican capital's reputation, announced the refinery would be closed. Soon, in place of smokestacks, a park, baseball fields, and an ecological reserve will bloom.
The case of the March 18 Oil Refinery is one small chapter in the story of Mexico City's recent battle to clean up some of the worst air-quality conditions in the world. Begun in earnest during the six-year presidency of Mr. Salinas, who took office in December 1988, the campaign has yielded some important results. It includes everything from driving restrictions and the world's second-largest air-quality monitoring system to the planting of millions of trees.
According to an assessment of Mexico City air quality released last month, average levels of some of the worst airborne poisons, including lead and carbon monoxide, have been steadily reduced. At the same time, however, ozone - an eye and throat irritant and health hazard produced in a photochemical reaction to airborne hydrocarbons - has tended to stabilize at levels that carry the designation ``poor air quality.''
Some public officials are trumpeting the encouraging results. But even as they do, critics warn that what progress has been made will be lost if more comprehensive measures aren't taken: notably to improve traffic flow, to address the city's worsening public-transit woes, and to get tougher on industrial emissions.
``Mexico City can point to some important successes, but we also have grave problems to resolve before we can talk about clean air,'' says Luis Manuel Guerra, executive director of the National Autonomous Institute of Ecological Studies (INAINE), a nongovernmental agency.
Among successful measures, he counts the requirement of catalytic converters on all cars made after 1991 and the progressive switch to unleaded gas, which by the end of this year should account for about half of gas used.
``But with ozone, all we've seen is a stabilization of the crisis,'' Mr. Guerra says. ``Unfortunately it's stabilization at very high levels.''
Public officials argue that Mexico City should be allowed to tout its progress. It is the world's most populous megalopolis (population nearly 20 million) with all the acute economic challenges of any developing-world city, plus climatic conditions that exacerbate air-quality problems.
``In five years we've reduced lead levels 90 percent, sulfur dioxide levels have been within norms equivalent to those of the US EPA [Environmental Protection Agency] for 31 months running, and for carbon monoxide it's been 27 months,'' says Fernando Menendez Garza, executive director of the Metropolitan Commission for the Prevention and Control of Mexico Valley Pollution. ``I don't know of any other city whether in the industrialized or developing world that has controlled to this extent, and in a few years time its most dangerous pollutants.''
Another recent report, based on a five-year joint US-Mexico study of Mexico City air quality, concluded that reducing certain contaminants is not enough, since that means remaining pollutants simply react to each other in different ways often causing new problems.
Called the Global Air Quality Study and directed by the Mexican Petroleum Institute, the report acknowledges that the region's particular natural conditions mean that no one strategy for combating pollution can be uniformly implemented.
Mexico City is located in a valley surrounded by mountains that have been denuded of the trees that would act as natural pollution ``processors.'' The hills trap the pollutants of 2.6 million vehicles, 30,000 industries, and dust-producing, chemical-spraying farmers.
The combination of winds and mountains make for important shifts in pollutant location and concentrations during the day. In addition, the city's elevation at 7,000 feet above sea level means a thinner layer of oxygen and thus a less-efficient burning of fuels. Because of seasonal atmospheric conditions, air quality in the city is generally worst from October to January.
In an effort to reinforce their battle, officials are considering altering one of the showpieces of the city's antipollution effort - the famous ``Hoy no circula'' program designed to keep a significant fraction of the city's automobile fleet idle every weekday.
Begun in 1990, the program calls for every car to stay off the city's roads one day a week, according to the last number in its license plate. But after five years of experience, the Metropolitan Commission is expected to revise the plan this month to exempt all cars with catalytic converters.
The reason? Studies, including one by INAINE, found that ``Hoy no circula'' (called ``One day without a car'' in English) actually encouraged people to buy a second car to sidestep the restrictions. And the second car was often an older, more polluting model.
The government estimates that the revision would add up to 300,000 cars a day to city streets. INAINE, however, says there would be 800,000 extra cars, but they would be cleaner cars.
THIS kind of reasoning irks some environmental specialists who say too much emphasis is being put on simply cleaning up cars and not enough on providing an effective, long-term alternative to the automobile.
``The only way we can seriously clean up our air is to reduce our use of fuels, not just make the fuels themselves cleaner, and that means expansion and improvement of a public-transportation system that is completely inadequate for a city this size,'' says Laura Itzel Castillo Juarez, a national deputy and leader of the Democratic Revolution Party's environmental committee.
Ms. Castillo says her party has offered proposals to use urban planning and expanded public transportation to reduce the time the average resident spends in transit each day. ``Unfortunately,'' she adds, ``there doesn't seem to be too much interest in that kind of comprehensive solution.''
Sounding a similar note, Guerra says too much emphasis is placed on ``showy'' programs like ``Hoy no circula'' and a highly publicized ``every family plant a tree'' campaign and not enough on more mundane but effective measures - such as synchronizing traffic lights to help reduce the city's inordinately high per-kilometer consumption of fuel.
``Ours is a society that prefers cosmetics over really changing things,'' he says, ``but without a revolution in our transportation system we aren't going to be able to take our pollution levels much lower.''
Others say the focus on vehicle pollution is leaving unattended the problem of polluting industries.
``I see every day what industries are sending into the air and it's frightening,'' says Gabriela Elizondo, an environmental engineer and industrial consultant. ``The problem is that most of these companies are small and medium businesses, many of which are burning fuels in old systems or using new or toxic chemicals to try to compete and survive. Their last concern is how much pollution they are causing.''
The concentration of 20 percent of Mexico's industry in the Mexico Valley is another problem, she says. That's one reason Ms. Elizondo took the closing of the March 18 Oil Refinery as a hopeful sign. But little has been done in recent years to encourage a dispersal of Mexico City's industry, she adds.
Menendez counters that the valley's need is not fewer industries: ``We need the employment and revenues they generate,'' he says. But he adds that a ``significant effort'' must be made to inspect and clean up businesses.
Still, Menendez maintains that not enough attention is paid to the headway Mexico City has made in its antipollution battle. ``I'm the first person to say we started our efforts very late and that levels of pollutants like ozone need a lot more work,'' he says. ``But we are moving forward.''