Ozone levels have been so bad in Mexico City this week that the authorities ordered two million cars — about 40 percent of the city's vehicles — to stay off the roads for three consecutive days.
The three-day ban ended on Friday and constitutes the third such period of emergency measures to be imposed since March, making this by far the worst bout of pollution in the city for well over a decade.
While even the worst days in recent weeks are still a long way from producing the constantly watering eyes and irritated throats that characterized the Mexican capital 30 years ago, they have, nevertheless, raised fears that the city could be heading back in that direction.
"The city was pulled back from the edge of environmental collapse in the 1980s by policies that were then insufficiently monitored and not updated," says Luis Manuel Guerra, a well-known environmental scholar and activist. "This, and the global context of climate change, is now pushing us back towards collapse."
Mexico City's air quality problem begins with its location in a high-altitude valley surrounded by mountains that encourages the production of smog, and then traps it. This is especially true in the hot and dry Spring months, before the seasonal rains begin, when more contaminants are generated and thermal inversion means it takes longer for them to disperse.
The first crisis was brought under control by significantly limiting industrial activity in the area, as well as the introduction, in 1992, of emissions tests for vehicles and a rolling program keeping some cars off the road each day known as Hoy no Circula, or Today You Don't Drive. Though the quality of the air in Mexico City never really got good, by the early 2000s it became common to see headlines about how the 20-million strong megacity had passed the baton for unliveable levels of pollution to the likes of Delhi and Beijing.
The critical situation this year, however, has highlighted the fact that those old programs just provided temporary respite. Nobody claims that the emergency measures imposed in the last few weeks — focused on doubling the number of cars normally kept off the road in a day under Hoy no Circula — constitute a long-term solution to the smog problem.
Ozone in the air in Mexico City is measured in IMECA, with phase 1 emergencies declared when any of the multiple monitoring points registers the gas at over 150 IMECA. Levels reached 168 on Monday, 164 on Tuesday, and 192 on Wednesday. The Wednesday level was just eight points from a phase 2 emergency in which half the cars would be ordered off the road, and the authorities are obliged to "consider" suspending activities in public offices and schools. Phase 1 measures were lifted on Friday after levels on Thursday fell below 150 again.
While still far from the 400 IMECA registered in the city a generation ago, the quantity of ozone in the air still easily tops the amount deemed to be a health risk by the World Health Organization that is equivalent to 100 IMECA. Each year, an estimated 20,500 people die because of respiratory diseases caused by air pollution in the Mexican megalópolis, the area comprising the capital and another 18 municipalities, according to the National Institute of Respiratory Diseases.
In recent weeks experts have highlighted several reasons for bad air quality, including the endemic corruption that allows many vehicles to pass emissions tests with the help of a bribe. Others point to the particularly toxic emissions from the rusty and inefficient old microbuses that chug around the city belching out black smoke behind them.
"Programs like Hoy No Circula have never worked, they may bring traffic down a bit but pollution levels have not changed," says physicist Héctor Riveros from Mexico's National Autonomous University and a member of the team that put the program together in the first place. "Suppose we get half a million cars off the streets, we would be sending 800,000 people to the public transport system, which pollutes between four and five times more than private cars."
Back in the 1990s the program also triggered the phenomenon of middle class families buying a second car to use on the days when their main vehicle was kept off the road. Anecdotal reports suggest that the temporary expansion of the program this Spring has sparked a new spike in the second-hand car market that could end up putting even more vehicles on the road.
According to Riveros, pollution only went down in the early 2000s after the Mexican state-owned oil company Pemex improved the quality of the fuel it sells, and when the government made it obligatory for all new cars to have a catalytic converter. But while the scientist says that improving the monitoring of fuel quality would help, he stresses that the "only real solution" is for people not to have to travel such long distances to work across the sprawling metropolis that expands further every year.
Meanwhile, the current smog crisis has also exposed the possibility that climate change may also have become an additional aggravator of Mexico City's problem.
Guerra, the environmentalist, says the direct cause of this year's air emergency lies in an atypical winter storm in February. The storm's 50-mile winds at first cleaned up the air so much that residents were treated to spectacular views of the volcanoes that ring the valley. It was a brief reminder of why Carlos Fuentes gave one of his most famous books the title La Región Mas Transparente, or the most Transparent Region.
But the smog never really went away. Instead it got pushed up against those mountains and, after a change of wind direction, came back to settle over the city again.
Guerra says that the February storm originated in Canada and was pushed south to Mexico City, where it wouldn't normally have reached, because of the complex periodic El Niño weather phenomenon that is related to ocean currents.
Scientists say that El Niño is probably getting more frequent and more intense because of climate change. It was linked to record-breaking global temperatures last year and, it now appears, is also at least partially to blame for the number of Mexico City dwellers currently suffering from asthma attacks triggered by the smog.
Guerra adds that rising temperatures in the valley are also ensuring that the "cap" keeping the smog from dispersing is becoming ever more impenetrable. He says that irrespective of when the emergency measures are lifted this time, he does not expect air quality to be good again until the rainy season gets going properly next month.
Jo Tuckman contributed to this report
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