The South Asian countries of Pakistan, India and Bangladesh are among the most polluted countries in the world while Australia, New Zealand and Iceland are among the least polluted countries, a new study by IQAir had determined.

IQAir, a Swiss technology company that studies air quality and pollution and specializes in protection against airborne pollutants, released its seventh annual World Air Quality Report Tuesday, which highlighted "alarming trends of the world’s most polluted countries, territories, and regions in 2024."

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All but one of the world’s top 20 most polluted cities last year were in Asia, with the majority of these cities – 13 – in India, the world’s most populous country. Neighboring Pakistan followed with four of the most polluted cities, with China and Kazakhstan having one each.

Data from more than 40,000 air quality monitoring stations across 8,954 locations in 138 countries, territories and regions were analyzed by IQAir’s air quality scientists for this year's report, IQAir said.

(milehightraveler/ E+/ Getty Images)

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"Air pollution remains the greatest environmental threat to human health," the report said, adding 99% percent of the global population lives in areas that do not meet recommended air quality guideline levels, according to the World Health Organization (WHO).

Air pollution is also the second leading global risk factor for death, and the second leading risk factor for deaths among children under five, following malnutrition, because of its significant impact on respiratory and developmental health, as per the WHO.

"Air pollution remains a critical threat to both human health and environmental stability, yet vast populations remain unaware of their exposure levels," Global CEO of IQAir Frank Hammes said in a statement. "Air quality data saves lives. It creates much needed awareness, informs policy decisions, guiding public health interventions, and empowers communities to take action to reduce air pollution and protect future generations."

Only 17% of global cities meet WHO air pollution guidelines, with 126 countries and regions out of 138 exceeding the WHO annual PM2.5 guideline value of 5 micrograms/cubic meters. Here's a look at which countries and regions are the most polluted and which are the least, including in the United States.

Which U.S. cities are the most polluted?

While Ontario, California, was the most polluted city in the U.S. in 2024, the most polluted major U.S. city was Los Angeles, according to the report. In fact, the top five most polluted cities in the U.S. were in California.

Here a list of the 10 most polluted cities in the U.S.:

(Reuters)

• Ontario, California

• Bloomington, California

• Huntington Park, California

• San Bernardino, California

• Fontana, California

• Visalia, California

• Mission, Texas

• Glendora, California

• Hanford, California

• Conroe, Texas

New Delhi, India, Nov. 14, 2024. (REUTERS/Anushree Fadnavis)

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What is the least polluted U.S. city?

Seattle, Washington was the cleanest major city in the U.S., though Waimea, Hawaii was the least polluted regional city in the U.S., which overall ranked 116 on the World Air Quality Report 2024.

Here a list of the 10 least polluted cities in the U.S.:

• Waimea, Hawaii

• Ocean View, Hawaii

• Seaside, California

• Stanwood, Washington

• Mountain View, Hawaii

• Lander, Wyoming

• Astoria, Oregon

• Tillamook, Oregon

• Silverdale, Washington

• Kihei, Hawaii

Which countries are the most polluted?

The twenty most polluted countries in the world are:

• Chad: More than 18 times higher than the WHO PM2.5 annual guideline.

• Bangladesh: More than 15 times higher than the WHO PM2.5 annual guideline.

• Pakistan: More than 14 times higher than the WHO PM2.5 annual guideline.

• Democratic Republic of the Congo: More than 11 times higher than the WHO PM2.5 annual guideline.

• India: More than 10 times higher than the WHO PM2.5 annual guideline.

(buzbuzzer/ E+/ Getty Images)

• Tajikistan

• Nepal

• Uganda

• Rwanda

• Burundi

• Nigeria

• Egypt

• Iraq

• Ghana

• Indonesia

• Gambia

• United Arab Emirates

• Bahrain

• Uzbekistan

• Qatar

What is the most polluted city in the world?

Byrnihat in eastern India was the most polluted metropolitan area of 2024, with India home to six of the nine most polluted global cities. The region of Central and South Asia, meanwhile, is home to the top seven most polluted cities in the world.

Here's a list of the 10 most polluted cities in the world:

New Delhi, India, Nove, 14, 2024. (REUTERS/Anushree Fadnavis)

• Byrnihat, India

• Delhi, India

• Karaganda, Kazakhstan

• Mullanpur, India

• Lahore, Pakistan

• Faridabad, India

• N'Djamena, Chad

• Loni, India

• New Delhi, India

• Multan, Pakistan

Which countries are the least polluted?

Only seven countries met WHO's annual average PM2.5 guideline. These include:

• Australia

• Bahamas

• Barbados

• Estonia

• Grenada

• Iceland

• New Zealand

What is the least polluted city?

The cleanest cities in the world in 2024 include:

A beach at Tres Palmas in Rincón, Puerto Rico. (Shawn Harrison/ USGS)

• Mayaguez, Puerto Rico

• Nieuwoudtville, South Africa

• Dexter, Oregon, U.S.

• Haines, Alaska, U.S.

• Chu, Kazakhstan

• Hawaiian Ocean View, Hawaii, U.S.

• Waimea, Hawaii, U.S.

• Volcano, Hawaii, U.S.

• Emu River, Australia

• Kapaau, Hawaii, U.S.

Monitoring air quality

While there has been notable progress in expanding air quality monitoring across various countries, regions, and territories over the past 12 months with low-cost air quality monitors — used by citizen scientists, researchers, community advocates, and local organizations — proven to be effective tools for data collection, "considerable gaps still exist in government-operated regulatory systems in many parts of the world," IQAir said. For example, in Africa, real-time, publicly accessible air quality monitoring data is so scarce that there is only one monitoring station for every 3.7 million people.

(Sanchit Khanna/ Hindustan Times/ Getty Images)

"The World Air Quality Report underscores how this urgent, data-driven approach is crucial in driving policy changes and collective efforts to reduce pollution for future generations," IQAir said in its report.

Saman Shafiq is a trending news reporter for USA TODAY. Reach her at sshafiq@gannett.com and follow her on X and Instagram @saman_shafiq7.

This article originally appeared on USA TODAY: Report reveals 2024's most polluted cities, countries: Which US cities made the list?

Reporting by Saman Shafiq, USA TODAY / USA TODAY.

USA TODAY Network via Reuters Connect.

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