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The world is experiencing a megacity boom. According to the United Nations, there are currently around 30 urban areas in the world with populations above 10 million, mostly in Southeast Asia, Africa, and the Middle East. The UN expects the number of these so-called megacities to increase within the next decade, especially in China and India. As such metropolises grow in size and number, scientists are working to understand what impact they’ll have on the world at large—especially when it comes to air pollution. 

In a study published last week in Science Advances, a group of international researchers examined satellite air pollution data from 2005-2018 across 46 projected future megacity locations throughout the tropical regions of Asia, Africa, and the Middle East. Looking over month by month imaging of pollutants such as nitrogen dioxide, formaldehyde, ammonia, and fine particulate matter, the scientists watched as cities became more and more polluted.

“The result, in a nutshell, is that most of these fast-growing cities actually show increases in almost all of these pollutants for the entire record,” says study author Karn Vohra,  a geography research fellow at the University College London. “What surprised us was the size of these changes.”

[Related: Living in the same city doesn’t mean breathing the same air.]

Across 40 out of 46 of the cities they studied, urban population exposure to air pollution increased 1.5 to 4 fold between 2005 and 2018. Some of those results ran counter to existing studies on air quality, which focused on regional swaths or national data. Examining the data on this scale made it clear that certain air pollutant trends were two to three times steeper in megacities than in surrounding areas. For example, Vorha says,  recent studies have indicated a decline in nitrogen dioxide pollution across Africa due to less dependence on biofuel burning. But in the cities where most people live, this new study shows, pollution is actually getting worse. 

“​​We continue to shift air pollution from one region to the next, rather than learning from errors of the past and ensuring rapid industrialisation and economic development don’t harm public health,” study co-author Eloise Marais, an associate professor in physical geography at UCL, said in a release

Poor air quality can contribute to a plethora of health problems—from asthma to death. Throughout the timeline of their observations, the researchers were able to estimate the number of premature deaths caused by air pollution: 180,000 in 2018, representing a 62 percent rise from 2005. The most rapid increase took place in Dhaka, Bangladesh as well as a number of cities in India, including Mumbai, Bangalore, Kolkata, Hyderabad, Chennai, Surat, Pune, and Ahmedabad.

“If…these trends persist, it’s definitely going to get worse,” says Vohra. “Let’s say the air quality does not change. Even then, the population in these cities is expanding at a very dramatic rate. In that case as well, the premature mortality will increase.”

[Related: Better vehicle emissions standards may have saved thousands of Americans.]

That doesn’t mean all hope is lost. Urban residents can help by relying more on public transit to keep cars off the road, Vohra notes.

On an even bigger scale to protect residents, it’s crucial for policymakers and leaders of growing communities to develop strict pollution measures (and actively monitor them) as industry continues to trickle in. Throw in a movement towards cleaner energy sources like wind and solar, and these cities will already be on the way to less polluted air. 

“A lot of studies have predicted that these will be megacities, most people will be living in the tropics and will be living in these cities,” Vohra says. “If we know something like this is going to happen, we should definitely have strict pollution measures being implemented immediately.” 

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