Clearing the Smog: Fighting Air Pollution in Mexico City, Mexico, and São Paulo, Brazil

June 23, 2007

by Eliza Barclay


Richard Lord, www.rlordphoto.comLidia Reyes spent much of 2006 in the hospital. Her children, ages 7, 13, and 17, suffered several asthma attacks during the year, and needed to be hospitalized between five and eight days each time. The family lives in an industrial suburb of Mexico City and does not have access to health insurance.

Mexico City’s dirty air, particularly ozone—a ground-level pollutant that forms when nitrogen oxide emissions and hydrocarbons react with sunlight—most certainly played a role in the Reyes children’s asthma attacks. According to Disease Control Priorities in Developing Countries, 2nd ed. (DCP2), a comprehensive public health guide launched in April 2006, major pollutants emitted by the burning of fuels and other substances have been associated with increased respiratory and cardiovascular illness and death.

Mexico City is the third largest urban area and has one of the worst air pollution problems in the world. According to a study by Nobel Prize winner Mario Molina, the city's residents lose 2.5 million working days every year due to health problems caused by particle matter, such as soot. Other Latin American capitals such as São Paulo, Brazil, also rank among the world’s worst in terms of outdoor air pollution.

But these megacities and other capitals in the region have made significant strides in reducing air pollution levels, a testament to good policies and relatively simple technological fixes. While much work still lies ahead, Mexico City and São Paulo offer encouraging examples of the opportunities for megacities in the developing world to improve air quality.

What is Air Pollution?

Air pollutants are divided into the categories of suspended particulate matter (PM), which can take the form of dusts, fumes, mists, and smokes; gaseous pollutants like sulfur dioxide (SO2) and nitrogen dioxide (NO2); and odors. Motor vehicles, one of the major sources of pollutants in megacities, can emit PM, nitric oxide and NO2 (together referred to as NOx), carbon monoxide (CO), organic compounds, and lead, depending on the type of fuel available.

One of the more threatening and prevalent pollutants in Mexico City and São Paulo, particulate matter suspended in air is considered a health threat at 2.5 micrometers in diameter, (PM2.5), and most dangerous at 10 micrometers in diameter (PM10).

In 2005, the World Health Organization (WHO) produced a new set of air quality guidelines, which indicated that higher than normal levels of PM, ozone, NO and SO can lead to a one percent increase in total death rates. The guidelines identify levels for each pollutant above which increased mortality responses are expected based on current scientific findings.

“In fact, the risks are higher than we thought,” said Tord Kjellstrom, a visiting fellow at the National Centre for Epidemiology and Population Health at the Australian National University and a DCP2 author.

Mexico City

Mexico City is situated atop a high plain at an altitude of 7,200 feet (2200 meters), and is surrounded on three sides by mountains. Smog hangs over the city much of the year, and thermal inversions keep polluted air close to the ground during the winter. About 21 million people live in the metropolitan area, and about 6 million cars are in circulation.

Thirty to 50 percent of the time, Mexico City’s PM10 level exceeds levels recommended by the WHO. And the metropolis did not meet acceptable air quality standards for ozone limits 209 days in 2006. Yet this was a major improvement over 1994, when 340 days did not meet the standards.

According to Dr. Juan Luis Jose Sienra, an asthma expert and a practitioner at the National Children’s Hospital in Mexico City, about 9 percent of Mexico City residents have asthma. “Pollution doesn’t cause asthma, but pollen and ozone can make it worse,” said Dr. Sienra.

São Paulo

São Paulo is the largest city in South America and the fourth largest city in the world with a population of 18.6 million. The city is home to a fleet of approximately five million cars and one million motorcycles.

According to a 2004 study by the state environmental agency, Companhia de Tecnologia de Saneamento Ambiental (CETESB), although the levels of primary air pollutants have decreased over the last 20 years, the agency still records days with high levels of nitrogen dioxide, carbon monoxide, suspended particulate matter, and ozone. In the last five years, ozone has become the most problematic pollutant with a high frequency of heavy pollution levels.

Brazilian researchers have found a direct link between respiratory illnesses and pollution. A 2000 study at the University of São Paulo on respiratory diseases in children and outdoor air pollution found that children’s daily hospital admissions for respiratory disease and pneumonia showed significant increases associated with ozone (5-8 percent), NO2 (9 percent), and with PM10 (9 percent). Consistently, the number of admissions for pneumonia was greater than for all other respiratory diseases combined. Also, infants (children under 1 year) had higher rates than older children.

The study concluded that given the present concentrations of air pollution in São Paulo and the large population potentially exposed, attention should be directed to minimize such effects. São Paulo’s recent implementation of policies to reduce pollution led to marked drops in illness and death rates.

“We have seen that the decrease of the mortality rate across time is highly associated with the decrease of air pollution across time,” said Dr. Paulo Saldiva, an environmental health expert at the University of São Paulo. “This decrease is expressed in levels of particulate matter.”

Still, Dr. Saldiva noted that the São Paulo officials still do not consider the health costs associated with pollution when developing transportation and urban policies.

Transportation Policies Reduce Pollution

Increased control of emission sources and adequate urban planning are both necessary in order to keep pollution at safe levels. Both Mexico City and São Paulo have been able to effectively reduce air pollution by implementing transportation policies that restrict driving and alter the content of fuels. According to Dr. Enrique Loyola, an epidemiologist at the Pan American Health Organization, PM10 levels in Mexico City dropped 24 percent between 2000 and 2004, while PM10 in São Paulo decreased 21 percent over the same time period.

“What’s most important is to detect the problem. Mexico has admitted to the problem and begun to resolve it,” said Dr. Sienra.

In 1986, São Paulo implemented a program called Proconve where drivers are prohibited from using their vehicles one day a week based on their license plate numbers. The program has been credited with reducing gaseous pollutants by 94 percent, according to the CETESB, with the most marked results seen after 2000.

In 2003, São Paulo instituted its first program, called Promoto, to regulate emissions from motorcycles. Motorcycles emit seven times as much PM as cars because catalytic converters and other pollution control technology are not required for motorcycle engines. Promoto required manufacturers to reduce emissions by 50 percent on bikes made in 2005 and to reach the same emission standards as cars by 2009.

Mexico City began a program similar to Proconve in 1989 called Hoy No Circula, which means, “today don’t drive.” The city government also worked to close an oil refinery and factories inside the city limits that heavily contributed to emissions.

Both countries eliminated the lead content in gasoline in the 1990s, a significant measure to end lead pollution from combustion of leaded fuels. Emissions standards for vehicles in both cities are on their way to meeting the standards set in the United States and Europe.

Reducing the sulfur content in diesel fuel is another crucial step for megacities in the developing world, and ultra-low sulfur diesel will soon be available in Mexico City and São Paulo. Mexico’s state oil company PEMEX has announced plans to begin producing ultra-low sulfur diesel by late 2007, while Brazil’s Petrobras will make it available in major cities in by 2008.

Ethanol – Friend or Foe?

Brazil also now replaces 20 to 25 percent of its gasoline with ethanol, a cleaner-burning fuel made from sugar cane, which researchers say may help reduce air pollution.

While researchers are just beginning to document how ethanol mitigates urban air pollution, they have also noticed that the burning of the green sugar cane after the harvest is creating its own pollution problems. This may offset most of the benefits gained from substituting alcohol for gasoline, according to Dr. Saldiva.

“The problem is that we’ve seen that there is a serious impact on the communities near where the sugar cane is grown from the burning of the green cane after harvesting,” said Dr. Saldiva. “Those other cities submitted higher levels of pollution than the big cities like São Paulo.”

Saldiva believes Brazil needs to find another way to clear the green cane to eliminate that source of pollution. Then, he says, the true environmental and health benefits of ethanol will be evident.

Many environmental health experts, like Dr. Kjellstrom of the National Australian University, are optimistic that policymakers in megacities will continue to work to clean up air pollution. He says that the recent increased attention to climate change and cutting carbon emissions can have many positive benefits for health as well.

“You cut carbon emissions from vehicles and power plants and you get the added benefits of less pollution, fewer traffic accidents, reduced noise, and more people walking to use public transportation,” said Dr. Kjellstrom. “If you do the calculation for large cities, you’re talking about really big stuff in terms of health benefits.”

Eliza Barclay is a freelance journalist based in Mexico City. She can be reached at



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