THE WORLD’S MOST POLLUTED CITY : Mexico City: A deadly fog blankets residents. Should we let our 3-year-old play outside? Parents and politicians search for a way out.

The first day of my baby’s life was declared an ozone emergency.

For weeks before and after the day in March he was born, the city was strangled in an official air emergency. Few days, if any, in the first three months of the year had offered air considered by any measure fit to breathe.

The ozone hazard was nothing unusual. If not ozone, it would have been carbon monoxide, carbon dioxide, sulfur dioxide, lead, dried sewage, smoke, soot, dust or in any combination among the hundred or so contaminants that blanket the valley on any given day.

We live in Mexico City, the most polluted city in the world. The smog levels exceed by far the limits set by every health organization.

Since his birth, my 6-week-old has been, in effect, smoking at least two packs of cigarettes a day. He is one of 20 million people who, with each breath, draw a deadly fog into their lungs.

For foreigners like us, this is, fortunately, a temporary danger. Although my husband and I are fascinated by Mexico--its history, ancestral pride and rich culture--we will at some point leave. We cannot stand the fear we feel each time our two small boys breathe.

Our obsession is the day’s air. Should we let our 3-year-old play outside? Can the baby safely take some sun? Should we risk getting caught in cloud-spewing traffic to get out of the city this weekend? How, as ostensibly responsible parents, can we justify living here?

Among the pollution-related illnesses suffered by hundreds of thousands, if not millions, in Mexico City: emphysema, pneumonia, bronchitis, asthma, cardiovascular complications, conjunctivitis, sinusitis, laryngitis, allergies and bloody noses. No one knows the pollution-linked cancer rate, but some experts set the annual number of pollution-triggered deaths at 5,000.

But that’s too general. As in a children’s game, let’s connect some contaminants with their illnesses. Carbon monoxide: headache, fatigue, slowing of reflexes, nausea, blurred vision, motor weakness. Ozone: eye irritation, tearing, headache, asthma, sinus inflammation, lowered immunological defense. Lead: irritation of the nervous system, anemia, neurological deficit, mental retardation.

“Children receive more because they breathe more times a minute,” said Dr. Victor Sanchez, a lung specialist. “If we continue this way, Mexico will have very few intellectuals.”

Our baby already has had conjunctivitis and a throat infection. Our eyes sting, our noses bleed, we tire easily and find ourselves breathless. Yes, we have the luxury of following doctor’s orders. “We advise parents to take their children and leave the city,” said Dr. Daniel Aguilar, president of the Society of Mexican Allergists and Immunologists. “Permanently.”

But millions of Mexico City residents don’t have that alternative. Their lives, their jobs, their futures depend on being here. Thousands of Mexicans arrive each day desperate for economic opportunities.

I feel guilty complaining. We won’t be here long enough for our children to believe that the sky is always gray, as many Mexico City youths believe. We have a house with a large garden, which allows us to believe our little pocket of air is somehow cleaner. Our kids have electric air purifiers blowing constantly in their rooms. We keep the children indoors during the smoggiest hours.

By contrast, at any intersection, impoverished children play on small grassy knolls as their parents hawk wares at stoplights. Babies in cardboard boxes lie at exhaust-pipe level. Entire families practically live in the streets, swallowing the fumes that billow from passing vehicles. For them, there is no choice.

If everyone--including the country’s and city’s highest leaders--agrees that the situation is critical, shouldn’t that prompt drastic action?

One hears all kinds of proposals, from the satiric (Why not outlaw breathing?) to the ridiculous (Why not build giant fans on the mountains that ring the city to help circulate fresh air?) to the desperate (Why not install booths on street corners for a 90-second fix of pure oxygen?).

A group of Japanese scientists seriously proposed drilling tunnels through the mountains to permit clean air to enter the city. But the most serious studies call for the severest measures. The head of the city’s clean-air program suggests imposing a 1,000% tax on gasoline, barring the 3 million cars moving through the city each day, distributing hundreds of thousands of bicycles and making public transportation available for everyone.

But in a city so huge, class-divided and densely populated, an efficient and comprehensive public transportation system is practically impossible, and too expensive.

The government could shut down many of the 30,000 factories within city limits that operate with few pollution controls. But industry is too powerful. So, to calm an increasingly restive public, the government shuts down one oil refinery, as promised years ago, plus 36 factories, a drop in the bucket. Such moves are clearly aimed at smoothing the path for a free-trade pact with the United States.

Millions are so busy surviving that dirty air is not an issue they can bother with. Millions more, faced with a crisis they can do nothing to solve, simply choose to forget about it. Who knows, maybe the wind will blow it all away.

Others have created shells for themselves, believing that by being exposed to the hazard day after day, year after year, their systems have somehow become immune. When my husband asked a taxi driver how he deals with the pollution, he pulled a tennis ball from his pocket. “I squeeze it at every intersection,” he said. “That way, my body is reoxygenated.”

Sometimes, while making sure that the air purifiers are switched on, or covering my baby’s head with a blanket to cross the garden on a bad day, or finding the car coated with soot several hours after washing it, I feel as if I’m in a science-fiction movie. But it’s hardly fiction any more.

For newcomers, it’s impossible to imagine a different Mexico City. But only three decades ago, British travel writer Sybille Bedford marveled at the then-crystal air. It was, she wrote, like “mountain streams, cool, fine, flowing, as though refreshed by some bubbling spring.” How sad that my children will only know that by one day reading Bedford.