President Donald Trump has falsely said that the US has the purest air in the world, but i n fact, it ranks 10th.

According to the American Lung Association, the 25 US cities with the most air pollution are mostly clustered in 8 states.

Air pollution — which includes particle and ozone pollution — causes 8.8 million deaths annually worldwide and is linked to health problems like childhood asthma and cognitive decline in older adults.

These are the eight most polluted states in the US, based on the number of people at risk.

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"Who's got the world's cleanest and safest air and water? AMERICA!" President Donald Trump tweeted earlier this month.

Unfortunately, that's not true.

The US ranks 10th worldwide when it comes to overall air quality, according to The Environmental Performance Index, an analysis by scientists at Yale and Columbia. Australia comes in first.

What's more, air quality in the US has been declining after decades of improvement, according to reporting from the Associated Press. Using data from the Environmental Protection Agency (EPA), the AP found that there were 15% more bad air days per US city last year than the average from preceding years.

Read More: President Trump keeps falsely saying the US has the 'world's cleanest and safest air.' Actually, he's making pollution worse.

Of course, levels of air pollution are not equal across the US. According to the 2019 "State of the Air" report from the American Lung Association (ALA) — which compiled pollution data from 2015 to 2017 — the 25 US cities with the worst air pollution are mostly located in eight states.

California topped the list, followed by Oregon and Washington.

Los Angeles is the city with the worst ozone pollution — LA has topped that list in 19 of the last 20 of these reports. The area of California around Fresno, Madera, and Hanford was found to have the most year-round particle pollution, while Bakersfield, California was the city with the worst short-term particle pollution.