Parliament building at sunset amidst pollution and smog in New Delhi, Delhi, India, Delhi. (Photo Credit: Getty Images)
For one to two days during wildfire season, cities like New York, Chicago, San Francisco, Sacramento and Seattle earn the dubious distinction of being the most polluted in the world, according to real-time data from air quality monitoring company IQAir.
However, in most cases, the air in these cities is usually not that polluted. So which cities have the worst air quality consistently and why?
Air pollution is often measured by the concentration of particulate matter (PM), or the mixture of solid and liquid droplets suspended in the air. Hundreds of cities have air that is considered unhealthy for just one day a year by the U.S. Environmental Protection Agency, according to the latest World Health Organization (WHO) data on fine particulate matter PM2.5, or inhalable particles smaller than 2.5 micrograms.
The IQAir report found that the worst air quality in 2022 was in Lahore, Pakistan. Hotan in China came in second, followed by the suburbs of Delhi, India. In 2021, the three most polluted cities were in India; among the world’s capitals, Delhi ranked first, followed by Dhaka, Bangladesh, and N’Djamena, Chad. Each of these cities averaged more than 90 micrograms per cubic meter of PM2.5 throughout the year, nearly 20 times higher than recommended levels.
While these rankings highlight some pockets of air pollution, it’s impossible to pinpoint which cities actually have the worst air. The quality of the sensors used varies, and many African countries are missing from the list because they don’t provide any data at all. The WHO only uses high-quality sensors, but it doesn’t even attempt to rank cities; a spokesperson told Live Science that data-collection procedures vary widely.
Even the PM2.5 metric itself has its limitations. Inhaling fine particles can cause health problems, and very few cities meet the WHO’s strictest PM2.5 guidelines. But the chemical composition of those particles can also affect health. A WHO spokesperson said the study is not conclusive, but it’s possible that cities with the same overall PM2.5 levels could have different health outcomes depending on the source of the pollution.
There are several reasons why some cities have higher PM2.5 levels than others. One is geography. Hotan is located next to the Taklamakan Desert and is often subject to dust storms. N'Djamena is also close to the southern edge of the Sahara Desert.
Sourse: www.livescience.com
