Air pollution in tropical cities is rapidly increasing and will lead to hundreds of thousands of preventable deaths if stronger regulations are not implemented, according to a new study.
Some 180,000 premature deaths in large tropical cities in 2018 alone were attributed to increased exposure to pollutants since 2005, according to researchers at University College London.
That number, they noted, is made all the more alarming by the fact that nearly three-quarters of megacities, those with 10 million people or more, are expected to be in the tropics by the end of the 21st century.
In many of them, pollutants have increased between 8% and 14% year on year, which is three times the national or regional rates of increase. And the vast majority appeared to come from industrial and residential sources, not agricultural practices like biomass burning, which have historically caused air pollution in tropical regions.
“What really surprised us was the size of the trends we were seeing,” said Eloise Marais, an associate professor of geography at University College London and co-author of the study, published Friday in the journal Science Advances.
“Because air quality is degrading so rapidly and population is also increasing rapidly, we estimate very, very sharp trends in urban population exposure to air pollution, with implications for urban public health.”
The researchers used data collected by NASA and European Space Agency satellite instruments to calculate concentrations of several pollutants known to be harmful to humans: fine particles, known as PM2.5; nitrogen dioxide; ammonia; and volatile organic compounds (VOCs), a group of chemicals that includes formaldehyde and benzene. The data covered 46 tropical cities in Africa, the Middle East and Asia that are expected to have populations of 10 million or more by 2100.
They found annual increases of up to 14% nitrogen dioxide, 12% ammonia, 11% VOCs and 8% fine particles. These numbers, combined with public health risk assessment models, made it possible to estimate how many premature deaths would be associated with these increases.
From 2005 to 2018, nitrogen dioxide, which has been linked to lung cancer and heart disease, increased significantly in 34 of the 46 cities. Among the hardest hit are Chittagong, Bangladesh, where concentrations have tripled, and four cities —Luanda, Angola; Dhaka, Bangladesh; Antananarivo, in Madagascar; and Hanoi, Vietnam—where concentrations more than doubled.
In many cities, concentrations of some pollutants have decreased while others have increased. But Jakarta, Indonesia, was the only one that saw a statistically significant improvement in overall air quality as a result of government policies.
The improvement is relative because Jakarta still has a serious air pollution problem, but trends pointed to the effectiveness of policies aimed at reducing pollution. The city has emission standards for vehicles, the researchers noted, and has seen a reduction in nitrogen dioxide, which is associated with vehicle exhaust. But it has no limits to biomass burning, such as burning land to clear it after harvest, and has seen increased concentrations of ammonia, which is associated with these agricultural activities.
Overall, however, the researchers found that most of the increase in pollution was driven not by the burning of biomass, but by sources such as traffic and fuel burning — a distinction the researchers were able to make because the burning of biomass shows up in observations of satellite in intense but relatively short bursts, usually with a seasonal pattern. Other human activities produce less intense but more sustained pollution.
“Open burning of biomass for land clearing and agricultural waste disposal has dominated air pollution in the tropics in the past,” Karn Vohra, a researcher at University College London and lead author of the study, said in a statement. “Our analysis suggests that we are entering a new era of air pollution in these cities, with some showing one-year degradation rates that other cities have experienced in a decade.”
The study does not answer the question of which activities, specifically, are most responsible.
“The driver of these trends is anthropogenic activity, but this is very broad — there is so much anthropogenic activity that takes place within a city,” Marais said, adding that more scientific research is needed to identify the biggest contributors.
Then, she said, it would be up to policymakers to do cost-benefit analyzes and determine the most effective and cost-effective ways to reduce pollution.
