Despite such measures, daily concentrations of pollutants like ozone and nitrogen dioxide have continued to rise. In 1990, Mexico City exceeded maximum ozone limits four out of every five days of the year -- more than twice the level of Los Angeles.

Until now, renewed economic expansion has canceled out the hard-won gains of mandatory conservation programs, and well-intended decrees have been emasculated by lax or corrupt enforcement. But as Mexico City has labored through the worst season of contaminated air in its history, Government officials and environmental groups agree that the struggle against pollution has become a high-stakes battle for the city's survival.

"The citizen of Mexico City gets up every morning and the first thing he confronts is the pollution," said Homero Aridjis of the Group of 100, a group of writers and artists that has taken a leading role in anti-pollution efforts. "There is a contaminant for every hour, every activity. It has reached the level of an ecological catastrophe." Tons of Foul Dust, A River of Sludge

It takes just a glance from a high-flying aircraft to understand the causes of Mexico City's pollution crisis, which is defined, like the city itself, by a semicircle of volcano peaks that reach as high as 16,900 feet. "It is like a giant cup or bowl that is open only to the north, where the prevailing winds come from," an American scientist said. "So it acts like a catch basin, collecting every fume."

Carpeting the broad Mexico City valley is a seemingly endless sprawl of urban industries and ramshackle neighborhoods that have pushed the valley's population from 5 million to roughly 16 million in the last 40 years. That growth has made greater Mexico City the largest metropolitan area in the world, filling the valley with 2.8 million vehicles and tens of thousands of poorly regulated businesses -- from cement factories to public baths -- that foul the air with furnaces, incinerators, spray paint, solvents and open-air chemical stockpiles. They release a total of 4.35 million tons of pollutants into the city's atmosphere every year, Government statistics say.

The effect of those emissions is compounded by the city's high elevation, 7,280 feet above sea level. It reduces by 23 percent the oxygen needed for breathing and effective burning of fuel.

In the vast areas covered by urban squatters, where latrines and sewers are often nonexistent, that dust carries with it tons of dried fecal matter. A confidential United Nations study conducted in 1989 said that about 30 percent of Mexico City residents have no sewerage service, and their solid waste, deposited in open areas, contributes about 600 tons of fecal dust to the city's pollution each day.