Atmospheric sciences

Danger in the skies of São Paulo

Air pollution has declined in the last 30 years, but levels of some compounds are still higher than acceptable

Léo Ramos Chaves Aerial view of Ibirapuera Park, a region of heavy traffic on São Paulo’s south sideLéo Ramos Chaves

Researchers who have studied air quality in the São Paulo Metropolitan Region (SPMR) for the past 30 years say that the total concentration of pollutants emitted by cars, trucks, and industries has fallen and that city residents are now breathing cleaner air than in the 1980s. Air quality has improved thanks to decreased concentrations of the main components of pollution, including both gases and microscopic particles. The bad news is that despite the reductions, the levels of some pollutants are still almost twice as high as those accepted by the World Health Organization (WHO) (see graph).

This drop in the amount of pollutants released into the atmosphere is a consequence of public policies implemented to control vehicular pollution that led to the development of more efficient, less polluting engines (see Pesquisa FAPESP Issue No. 224). Part of the decline is also due to the combined effect of mixing ethanol with gasoline and the upsurge in the number of flex-fuel cars, which currently account for more than half of light vehicles in the SPMR. Since 1993, increasingly higher proportions of ethanol—which emits less carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) than gasoline—have been added to this petroleum by-product. Every liter of gasoline in Brazil now contains 27% ethanol. Two other factors helped improve air quality for São Paulo residents: the adoption of cleaner industrial production processes (for example, oil-fired boilers have been replaced with electric ones) and the relocation of many factories to other cities in the 1980s and 1990s.

From 1988 to 2015, cars circulating in São Paulo emitted 20 times less CO, NOx, and particulate matter, while HC emissions decreased 40-fold. There was also a threefold decline in emissions of these same pollutants by heavy vehicles, like trucks and buses—which were only regulated later—according to analyses by researchers at the Institute of Astronomy, Geophysics and Atmospheric Sciences of the University of São Paulo (IAG-USP), published in three articles released by the journals Atmospheric Environment, Atmospheric Chemistry and Physics, and Journal of Transport & Health in 2017. However, despite improvements to engines and the use of either pure ethanol or ethanol mixed with gasoline, data from Cetesb (Companhia de Tecnologia de Saneamento Ambiental), the agency that monitors air quality in the SPMR, show that light vehicles, like cars and motorcycles, and heavy vehicles continue to be the prime sources of air pollution in São Paulo.

Improvements in air quality have not been even greater in recent years largely because the number of vehicles in the region soared from one million in 2000 to nearly seven million in 2014. “Although vehicles are more efficient and pollute less, the fleet has grown a lot,” points out physicist Maria de Fátima Andrade, of IAG-USP, who led the three studies. With more cars on the streets, fuel consumption rose 25% in volume in less than a decade, from 5.5 billion liters per year in 2007 to nearly 7 billion in 2014, according to data from Brazil’s National Petroleum Agency.

Only one of the main pollutants assessed by the IAG-USP team showed no consistent reduction: ozone, a secondary pollutant produced when compounds released by engines react with sunlight and substances in the atmosphere. Concentrations of this gas, which irritates the airways and increases the risk of infection, declined through the mid-2000s but then began climbing again, although current levels still remain under those of the mid-1990s. In addition to the gases released into the air by vehicular exhaust, the fuel vapors that are released when gas is pumped at a service station seem to contribute significantly to the formation of ozone and other secondary pollutants. For every liter pumped into a fuel tank, a few milliliters escape into the atmosphere in the form of vapor. This may not seem like much but, according to Andrade, given the quantity of fuel that is pumped in the SPMR, it may ultimately have a significant impact, even more so than exhaust emissions from cars and trucks. “The scale of evaporative processes is not adequately taken into account,” says Andrade. “It’s possible that the impact of these vapors is being underestimated.”

What most worries experts today is the pollution component called particulate matter (PM)—a mixture of very small solid particles or liquid droplets—because it presents a major human health risk. PM is produced directly when engines burn fuel or forms in the atmosphere from certain gases. Particle size is inversely proportionate to the ability to trigger health problems: the smaller the particulates, the longer they remain suspended in the air and the greater their potential to wreak damage. Since they are so small, they easily penetrate the respiratory tract and accumulate there, where they can trigger pulmonary inflammation, exacerbate illnesses like asthma, and even cause problems in other organs.

In the SPMR, Cetesb monitors the concentration of both inhalable coarse particles (less than 10 micrometers in diameter, or PM10) and inhalable fine particles (less than 2.5 micrometers). Levels of both have been falling, although they remain above WHO guidelines. But Cetesb does not monitor the level of even smaller, or ultrafine, particles, which measure less than 50 nanometers in diameter. Vehicles running on gas produce larger amounts of ultrafine particles than those fueled by ethanol.

Published in July 2017 in the journal Nature Communications, a study done in collaboration with researchers from the USP Physics Institute (IF-UPS) reported a 30% hike in the concentration of ultrafine particles in the city of São Paulo from January to May 2011, a period when ethanol prices skyrocketed and drivers of flex-fuel cars began consuming more gasoline than ethanol. Pollution data were gathered at the IF-USP station and used to feed statistical models that took into account traffic, meteorological information, and consumer behavior. “Modeling indicated that when ethanol prices went back down, gas consumption dropped and the concentration of these particles measured in the atmosphere also decreased,” says physicist Paulo Artaxo, a professor at IF-USP and one of the authors of the study.

Léo Ramos Chaves One of the busiest thoroughfares in the city of São Paulo: Marginal do Rio Pinheiros AvenueLéo Ramos Chaves

From womb to old age 
According to pathologist Paulo Saldiva, of the USP School of Medicine (FMUSP), reducing the concentration of fine and ultrafine particles in the air is literally a matter of life or death. “People’s vulnerability to air pollution varies, and a portion of them die from health problems caused by pollution, especially those triggered by these tiny particles,” says Saldiva, leader of a study that estimated that around 3,500 people in the city of São Paulo die from pollution every year (see Pesquisa FAPESP Issue No. 129). “They accumulate in the pulmonary alveoli and from there can easily reach the central nervous system and other parts of the body,” says Saldiva, who also leads an FMUSP team that is currently investigating whether there is a link between prolonged exposure to pollution and the development of Alzheimer’s disease in humans. Some time ago, his group noted that the offspring of rodents exposed to urban environmental pollution had low birth weights because placental problems hampered the transfer of nutrients. The researchers also found indications that prolonged exposure to pollutants, especially fine and ultrafine particles, harms lung development.

In Saldiva’s opinion, there is an urgent need for initiatives to improve public transit, such as the adoption of cleaner fuels and new technologies that cut travel time, bringing pollution levels in line with WHO guidelines. “Prolonged exposure to pollution presents a risk that is hard for us to avoid, because it does to the body what a cigarette does to a smoker,” he says. “I decide whether or not to smoke, but I have no way of making this type of choice when it comes to breathing São Paulo air.”

1. Narrowing the uncertainties on aerosol and climate changes in São Paulo State: NUANCES-SPS (No. 08/58104-8); Grant Mechanism Thematic project; Program FAPESP Research Program on Global Climate Change (RPGCC); Principal Investigator Maria de Fátima Andrade (USP); Investment R$3,297,909.37.
2. GoAmazon: interactions of the urban plume of Manaus with biogenic forest emissions in Amazonia (No. 13/05014-0); Grant Mechanism Thematic project; Program FAPESP Research Program on Global Climate Change (RPGCC); Principal Investigator Paulo Eduardo Artaxo Neto (USP); Investment R$4,290,930.31.

Scientific articles
ANDRADE, M. F. et al. Air quality in the megacity of São Paulo: Evolution over the last 30 years and future perspectives. Atmospheric Environment. V. 159, pp. 66-82. June 2017.
PACHECO, M. T. et al. A review of emissions and concentrations of particulate matter in the three major metropolitan areas of Brazil. Journal of Transport & Health. V. 4, pp. 53-72. March 2017.
VARA-VELA, A. et al. Impact of vehicular emissions on the formation of fine particles in the Sao Paulo Metropolitan Area: A numerical study with the WRF-Chem model. Atmospheric Chemistry and Physics. V. 16, No. 2, pp. 777-97. January 2017.
SALVO, A. et al. Reduced ultrafine particle levels in São Paulo’s atmosphere during shifts from gasoline to ethanol use. Nature Communications. July 2017.