Mexico City, one of the most polluted cities on Earth, faces a record wave of deadly smog this winter when thermal inversions convert the atmosphere into a "toxic gas chamber," ecologists warned.

Residents of 7,300-foot high Mexico City have come to dread winter months when cold air traps pollutants and leave the city under a ghastly brown pall.Ecologists said Friday that the valley, surrounded by towering volcanoes and home to an estimated 20 million people, will see the worst winter's smog ever. They warn that people might faint - or even fall dead - right in the street, much like the small birds that somtimes fall out of trees when the air gets particularly foul.

The ecologists said this winter will be particularly dangerous because of predicted low temperatures and the failure of government officials to regulate emmissions from industry and, more importantly, the city's 3.5 million vehicles - said to account for 80 percent of the capital's smog.

"This winter is going to be hell," said Alfonso Cipres, president of the Mexican Ecological Movement. "We are going to live in a toxic gas chamber."

Cipres, in a view shared by many Mexican environmentalists, said thermal inversions this winter will become more frequent and last longer. An inversion occurs when heavy cold air from the upper atmosphere traps warmer air below it, sealing the pollution within the walls of the Valley of Mexico that surrounds the capital.

Although there have been few inversions so far this fall, smog levels are already reaching dangerously high levels. Normally, severe inversions and the resulting heavy pollution do not begin until December, lasting through mid-March.

But anyone living in Mexico City knows the smog season already is well underway: the sky is thick with fumes that burn the eyes, rasp the throat and choke the lungs.

"We are already facing a truly critical situation. During each of the last 15 days, ozone levels were five times emergency levels," said Jorge Gonzalez Torres, of the Mexican Green Party, an environmental activist group. "What can we expect when the thermal inversions begin in earnest?"

"This winter our health will be at great risk. Ozone levels have become extremely serious already, and the inversions we've had are lasting five or six hours instead of the normal three hours," Gonzalez said.

"We believe the inversions can cause death, even though (the Ecology Ministry) would have us believe they do not," he added.

Sedue spokesman Alfonso Martin told United Press International that while he respects "the oppinion of our fine ecologists," he is concerned that they may be "unduly alarming" the public.

"It's difficult to predict thermal inversions, we have no crystal ball," Martin said. But when pressed on the issue, he conceded that "we could be in for a very harsh winter."

"We are definitely worried. That's why we began a permanent campaign to alert the population . . . so they will contribute to fighting the smog," he said.

The campaign includes radio, television and newspaper ads that explain what thermal inversions are and how they affect pollution levels. The ads also urge people to tune-up their cars and refrain from driving on particularly bad days.

Sedue and the city government in August began an unprecedented program of mandatory vehicle inspection, although the program is too new to have produced results. City officials are not even sure how many cars were inspected to date.

"We're very skeptical about the program," Cipres told UPI. "How are they going to force millions of drivers to show up for inspection?"

Gonzalez was even more blunt: "We ecologists have tried everything. We marched, protested and denounced. But as long the population doesn't join us, we will remain a little group that does nothing but shout while things just get worse."