Whether the weather is cold or hot, one thing is usually certain in this northern Mexico City suburb: It snows.

Snow, at least, is the word that residents use for the fine white powder that blankets their streets and houses every morning. And it is a fitting description for what scientists say is a lingering cloud of wind-borne chemicals, raised from a giant caustic-soda plant beyond the horizon, that hangs above the working-class neighborhood."It even kills the trees," said Pedro Quesadas Herrera, a 40-year-old accountant, pointing to lifeless shrubs on the block where he lives with his wife and four daughters. "There's no way to get away from it."

Amid the bare concrete houses and flat, rubble-strewn fields of Ecatepec, many adults and children have developed respiratory problems, and dozens bear large patches of rough, discolored skin on their faces and arms.

The neighborhood air is so foul with sharp, industrial odors that it burns a visitor's eyes and throat.

Yet by Mexico City standards, Ecatepec's toxic snowfall is just one symptom of a deepening environmental crisis that has left the Mexican capital with the most polluted air of any major metropolitan area in the world.

Trapped by unfavorable geography and explosive population growth, Mexico City has what scientists say is an unequaled collection of atmospheric poisons. It is overwhelmed by industrial emissions, smoke, vehicle exhaust and human waste.

Saturated with contaminants, the gray-brown blanket of pollution that covers the Mexico City valley now routinely exceeds - by as much as four times - the maximum exposure limits established by the World Health Organization.

Infectious diseases like salmonella and hepatitis can be contracted simply by inhaling bacteria suspended in the air. And doctors say that even mild outdoor exercise, like jogging, has now become a calculated risk.

As pollution problems have worsened, the Mexican government, pushed by Mexico City's mayor, Manuel Camacho Solis, has adopted increasingly stringent measures to force reduction in harmful emissions.

Since late 1989, every automobile in the city has been required to remain idle one business day a week. This winter, the authorities banned traffic in 50 square blocks around the central plaza, Zocalo, and have begun shutting down dozens of industries for violating pollution laws.

Those efforts have been redoubled as environmental concerns have become central to the proposed free-trade agreement grouping Mexico, the United States and Canada. Last fall, the Mexican government announced a $2.5-billion, four-year plan to rescue the capital's air quality. In March, President Carlos Salinas de Gortari ordered the shutdown of the city's giant state oil refinery.

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."