President Trump is, once again, falsely claiming that America has the cleanest air in the world: On Wednesday he tweeted that the US has the "purest air on the planet."

US air is not the cleanest in the world. In fact, it's gotten dirtier on Trump's watch.

Plus, air doesn't belong to any one country, making it tough to determine whose is cleanest. If there were a prize for cleanest air, it would probably go to Australia or New Zealand.

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President Trump has once again falsely claimed that the US has the cleanest air in the world.

"Who's got the world's cleanest and safest air and water? AMERICA!" Trump tweeted on Wednesday, as CNN kicked off a 7-hour-long session of "climate crisis" town halls with 10 Democratic presidential candidates.

"I want crystal clean water and the cleanest and the purest air on the planet – we've now got that!" Trump added.

This isn't the first time that Trump has falsely touted America's air as number one — he tweeted a similar statement in October 2018, along with a map from a World Health Organization report. The statement was not true then, and it definitely isn't now.

Air in Australia and New Zealand is far cleaner than in the US

The Environmental Performance Index, a metric from environmental scientists at Yale and Columbia that ranks 180 countries around the world, puts the US in 10th place when it comes to overall air quality (Australia is first).

In terms of PM 2.5 pollution — a measure of ultra-fine particulate matter in the air — the country with the world's cleanest air is New Zealand, while the US ranks seventh on that list. Meanwhile, the cleanest cities in the world (in terms of particulate concentrations) are in Sweden.

Air-pollution expert Gabriele Pfister, deputy director at the National Center for Atmospheric Research (NCAR) Atmospheric Chemistry Observations and Modeling Lab, told Business Insider that a single country could never have the "cleanest" air on Earth, because we all live under the same sky.

Using the WHO map to brag about the US' air quality, as Trump did in October, is misleading, Pfister added.

That's because the map only reflected estimates of how much ultra-fine PM 2.5 pollution people are breathing around the world, and that's just one component of dirty air.

"America doesn't have the cleanest air in the world just because it's white on this map," Pfister said.

As a category, PM 2.5 includes dust and other tiny particles floating around that are smaller than 2.5 microns wide (sometimes referred to as "PM 2.5" pollution). Common sources of these pollutants include emissions from cars and other vehicles, as well as heating oil, coal, secondhand smoke, chemicals from power plants, smoke from cooking, and even natural sources like forest fires.

Pfister said it's no surprise that the US have fewer of these small pollutants than some developing countries, where more people cook over open flames.

PM 2.5 particles are 30 times skinnier than a strand of human hair. Because they're so small, they can easily travel into the lungs — a serious problem for people who already have breathing and heart issues.

When the air is hazy, breathing it in can have dangerous, long-term effects on the brain. Heavy exposure over time can also lead to reduced lung function and even death.

US air is cleaner than it used to be, but there is still work to do

According to the EPA, aggregate emissions of six of the most common air pollutants in the US have dropped 73% since 1970, when the agency got serious about cleaning up the air.

But the work isn't done. In 2014, the EPA estimated that between 50,000 and 120,000 people in the US die prematurely every year because of bad air. The agency also notes on its website that "despite great progress in air quality improvement, approximately 111 million people nationwide lived in counties with pollution levels above the primary U.S. National Ambient Air Quality Standards in 2017."

This 2018 EPA map shows the counties that still don't meet the Clean Air Act standards:

US EPA

"PM 2.5 is one component of air pollution," Pfister said. "If you look at ozone pollution, you will actually see that the US has quite a bit more ozone pollution than many regions in the developing world ... that has to do with the ozone chemistry."

She worries about the air she breathes in Colorado, she said, as more oil and gas operations open up near schools, hospitals, and people's homes.

"Specifically in the western US, we have leveled off. We are not clean," she said.

Trump is making it more difficult for Americans to breathe

Despite Trump's claims about the US' air quality, air in the country is actually getting dirtier and more dangerous to breathe under his administration. An Associated Press report in June analyzed federal Environmental Protection Agency (EPA) data and found that "there were 15% more days with unhealthy air in America both last year and the year before" than there were during the period from 2013 to 2016.

"There were noticeably more polluted air days each year in the president's first two years in office than any of the four years before," the AP said.

The American Lung Association's "State of the Air" report for 2019 found, similarly, that more than four in 10 Americans live in counties that got at least one "F" for unhealthy air.

"That's 7 million more than last year's report," the report said.

Air pollution is deadly — it kills tens of thousands of people in the US every year. Yet Trump has rolled back at least 10 air-pollution and emissions rules while he's been president, according to the New York Times.

If you're wondering how clean the air you're breathing right now is, check out the National Weather Service air quality forecast, or use the EPA's interactive air quality map.

Update: This story was originally published in October 2018. It has been updated with Trump's latest comments on US air quality.