REPORTING FROM MEXICO CITY -- When you're above it, it might look like brown smoke. From far away at ground level, it can shine like a sheet of white ice. And when you're under it, which is most of the time, you can't really see it at all.
It is the famous air pollution of Mexico City, causing countless cases of scratchy throats, stinging eyes and mucus that looks like it came from outer-space.
It is, in a word, nasty.
And while it may no longer be known as the worst in the world -- these days Beijing's pollution problem is attracting that sort of global attention -- Mexico City's smog becomes particularly intense at the start of every Christmas season.
The effect persists even after two decades of aggressive anti-pollution policies that have dramatically cut down smog overall in Mexico's gigantic capital.
Last Saturday evening, while the mayor inaugurated the annual ice-rink downtown to kick off the holiday cheer, the city's contamination monitoring system recorded a high of 136 points on its pollution scale in Mexico City's southeast quadrant, and graded the air quality over the entire metropolitan region of 20.1 million people as "Bad" (links in Spanish).
The IMECA, as the scale is known by its Spanish acronym, has hit "bad" levels across the city all week since. The air is expected to be just as "mala" through December and into the new year.
The phenomenon joins weather, cultural and geographic factors that combine like in no other place, explained Armando Retama, director of the city's sophisticated air-quality monitoring service, with two dozen stations recording pollution levels around the clock.
The Valley of Mexico, where the metropolitan area is located, is a bowl that sits at about 7,300 feet above sea level. That landscape produces what meteorologists call the thermal inversion effect, in which colder mountain air hangs over the valley and traps warmer air and pollution particles below it.
In the mornings, it's rise and shine for many of the estimated 4 million cars and trucks that operate here.
"As the day goes on, the sun heats up the floor, and the floor heats up the air above it," Retama said in an interview this week. The air and the toxins combine, creating a haze that often transforms into ozone in the afternoon.
The pattern hits Mexico City hard in December, when Christmas shoppers and tourists bring routine gridlock to city streets. Confounding the problem, fireworks and pyrotechnic displays, popular during the seasonal festivities, add smoke and ash to the equation.
"We're producing millions of kilos of contaminants daily, in a matter of hours," Retama said.
And you can feel it. Those particles can include a host of unfriendly pollutants: sulfur dioxide, nitrogen dioxide, carbon monoxide, ozone, even lead. Will it ever fully go away?
Not unless the people of the city stop driving automobiles, Retama said. Despite improvements in public transportation, such as the Ecobici and Metrobus projects, car exhaust continues to be the chief factor contributing to Mexico City's smog, he said.
So far this week, the IMECA scale has not topped 150, which would trigger a "pre-contingency" alert for the metropolis. That would lead to restrictions on certain manufacturing sectors and on outdoor activities at schools. Every hour, the monitoring-service director said, reports on pollution levels are sent to local government agencies.
Yet on Friday, the metropolitan environmental agency released a statement reminding residents of the Valley of Mexico to take precautions against contributing to the pollution problem. Locals were advised to minimize the use of automobiles, chimneys, fireworks and open fires.
"Oh yes, I feel it, especially in the past few days," said Alejandra Abrego, 24. "My nose is dry, I am coughing and sneezing a lot."
Abrego, an archeology student strolling downtown on Friday, said she grew up in Mexico City. In her view, the smog has actually gotten worse in recent years, not better. For a generation that didn't experience the worst of the smog in the 1980s, it might seem so.
"We need more efficient public transit, so that people can start leaving their cars," Abrego said.
-- Daniel Hernandez
Photo: A view of Mexico City at sunset, Dec. 6, 2011. Credit: Daniel Hernandez / Los Angeles Times