The US does not have 'the world's cleanest and safest air,' as Trump has claimed. These are the 8 states with the worst air pollution.

The ALA report evaluates three types of air pollution: short-term particulate matter, year-long particulate matter, and ozone pollution. California ranked the worst across all of them.

California had the greatest number of cities — nine — on the list of the 25 cities most polluted by particulate matter in the short-term. (The report considers short-term PM to be pollution over a 24-hour period.) Those cities included Bakersfield, Fresno, Sacramento, and Los Angeles.

In terms of year-long particle pollution, California has six of the eight most polluted cities. And 10 California cities were among the top 25 for ozone pollution, with eight of the 10 most ozone-polluted cities located in the state.

Particulate matter (PM) refers to dust and other tiny particles floating in the air that are smaller than 2.5 microns wide — that's 30 times skinnier than a strand of human hair.

For the most part, this air pollution comes in the form of carbon dioxide (CO2) and methane. Common sources of particulate matter (sometimes referred to as "PM 2.5" pollution) include emissions from cars, the burning of coal, secondhand smoke, chemicals from power plants, smoke from cooking, and natural sources like forest fires.

Burning fossil fuels also sends carcinogenic particles into the air, like benzene (a chemical linked to childhood leukemia and blood disorders) and formaldehyde (a known carcinogen).

Ground-level ozone pollution, meanwhile, is the result of a chemical reaction in which sunlight interacts with pollutants emitted by cars, power plants, industrial boilers, and refineries.

Unlike the ozone that protects our planet from solar radiation, this type of ozone can trigger breathing problems, particularly for children and the elderly.

Factories and gas-powered vehicles can also produce other air pollutants, like nitrogen oxide, sulfur dioxide, and hydrocarbons. These chemicals can react with sunlight to create smog.

The report found that more than four in 10 Americans, approximately 43% of the population, live in counties that have experienced unhealthy ozone or particle pollution.

That means those counties received at least one "F" grade for unhealthy air.

Breathing polluted air can have dangerous, long-term effects on the brain.

A study involving New York City schoolchildren found that kids who breathe poor air are more likely to need academic intervention. Research in the US shows that rates of dementia and cognitive decline are higher in places with more air pollution as well.

Studies conducted in China and Canada, meanwhile, show that children who breathe poor air are more likely to have breathing difficulties and asthma. Heavy exposure over time can also lead to reduced lung function long-term as well, and even cause death.

In the US, over 6 million children had ongoing asthma in 2016.

Globally, asthma kills around 1,000 people every day, researcher Haneen Khreis wrote in The Conversation.

Overall, air pollution contributes to an estimated 8.8 million extra deaths globally every year.

A study published in March showed that nearly 800,000 Europeans died from air pollution-derived issues in 2015 alone.

Another state with particularly air pollution, according to the ALA report, is Oregon. The areas around Bend, Eugene, and Portland all made the list of the 25 cities with the most short-term particulate matter.

Crook, Jackson, Klamath, Lake, and Lane Counties all received failing grades for PM pollution.

Washington has similar issues — the state has four cities on the list of the 25 places with the most short-term particulate matter.

Those cities were Yakima, Spokane, Vancouver, and the Seattle-Tacoma area. Seven counties in the state received failing grades for PM pollution.

Despite its location outside the continental US, parts of Alaska are also among the most polluted in the country.

The city of Fairbanks is the most polluted city in the US for both short-term and long-term PM pollution. Anchorage is also among the 25 cities with the most short-term pollution.

Texas also ranks among the US' most polluted states, particularly in terms of ozone pollution. Houston, the Dallas-Fort Worth area, and the region around El Paso all made the list of the top 25 most polluted cities.

Houston and McAllen-Edinburg are also on the list of cities with the most year-round PM pollution.

More than a dozen counties in Pennsylvania received a failing grade for air pollution from the American Lung Association.

Philadelphia made the lists of the 25 cities list with the most ozone and year-round PM pollution. Pittsburgh had issues with both short- and long-term PM pollution. Lancaster and the Johnstown-Somerset areas, meanwhile, were also on the list of cities with the highest levels of year-round pollution.

The area of Illinois around Chicago and Naperville is also among the most polluted in terms of both ozone and year-round PM pollution.

In Indiana, meanwhile, the region around Indianapolis, Carmel, and Muncie was among the cities with the worst year-round particle pollution.

Six counties in Montana received failing grades for short-term PM pollution, with Missoula ranking as the fifth-worst city in the US in that category.

Three counties — Ravalli, Lincoln, and, Missoula — received failing grades for year-long pollution.

The American Lung Association also listed the six US cities that are cleanest in terms of ozone and particle pollution.

In alphabetical order, those cities are: Bangor, Maine; Burlington, Vermont; Honolulu, Hawaii; the area around Lincoln and Beatrice, Nebraska; the part of Florida around Palm Bay, Melbourne, and Titus; and Wilmington, North Carolina.

However, the overall number of Americans exposed to polluted air increased to nearly 141.1 million between 2015 and 2017. That's up from the total of 133.9 million between 2014 and 2016. This EPA map shows the counties that had not met the agency's Clean Air Act standards as of last year.

In 2014, the EPA estimated that between 50,000 and 120,000 people in the US die prematurely every year because of bad air.