The report said that climate change continues to make air pollution worse.

About nine million more people are breathing dirty air than in last year's report.

California once again had the nation's most polluted cities.

Nearly half of the nation’s population – about 150 million people – live with and breathe polluted air, "placing their health and lives at risk," according to a new report released Tuesday by the American Lung Association, a public health group.

The association's 21st annual “State of the Air” report said climate change continues to make air pollution worse, with many Western cities again seeing record-breaking spikes in particle pollution from wildfires.

“The report finds the air quality in some communities has improved, but the ‘State of the Air’ finds that far too many people are still breathing unhealthy air,” said American Lung Association president and CEO Harold Wimmer.

In fact, about 9 million more people are breathing dirty air than in last year's report.

The 2020 report covers the years 2016-18, so any pollution decreases this year due to the nationwide COVID-19 lockdown were not included.

According to the lung association, those are the three years with the most recent quality-assured air pollution data. In addition, those three years – 2016, 2017 and 2018 – were among the five hottest recorded in global history. When it comes to air quality, changing climate patterns fuel wildfires and their dangerous smoke, and lead to worsened particle and ozone pollution, the report said.

According to the report, unhealthy levels of air pollution places people at risk for premature death and other serious health effects such as lung cancer, asthma attacks, cardiovascular damage and developmental and reproductive harm.

"Air pollution is linked to greater risk of lung infection,” Wimmer said. “Protecting everyone from COVID-19 and other lung infections is an urgent reminder of the importance of clean air."

The report deals with the two main types of air pollution that plague the USA: smog (also known as ground-level ozone) and soot (technically known as "particulate matter").

Smog forms on warm, sunny days and is made worse by chemicals that exit vehicle tailpipes and power plant and industrial smokestacks. Warmer temperatures make ozone more likely to form.

Soot pollution is deadlier and more of a health hazard than smog, causing more premature deaths and lung cancer, the lung association said.

Megadrought emerging in the West:Might be worse than any in 1,200 years

Global warming:2020 expected to be Earth's warmest year on record, scientists say

Primarily because of its geography and weather, California once again had the nation's most polluted cities. Los Angeles and Visalia topped the list for smog; Bakersfield and Fresno led the way for soot pollution.

The Environmental Protection Agency, the federal group tasked with overseeing air quality in the U.S., said the lung association's report "paints a pessimistic picture based upon a problematic methodology."

In a statement obtained by USA TODAY, the EPA said that the agency's "methods for determining air quality, which are based on the Clean Air Act and the latest science, show continued improvements in measures of U.S. air quality in recent years and into the future."

The nation's cleanest cities, according to the report, were Bangor, Maine; Burlington, Vermont; Honolulu, Hawaii; and Wilmington, North Carolina. To make the list of cleanest cities, "a city must experience no high ozone or high particle pollution days and must rank among the 25 cities with the lowest year-round particle pollution levels," the report said.