Which U.S. Cities Have the Worst Pollution?

The most polluted major U.S. city in 2024 was Los Angeles, according to a new report by IQAir, a Swiss company that tracks global air quality. Overall, IQAir found Ontario, California, was the most polluted city in the U.S. Other California cities—Bloomington and Huntington Park—also made the list of the worst air pollution in North America. 

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. IQAir looked specifically at fine particulate matter, or PM2.5, one the most dangerous pollutants made up of combustion of fossil fuels, dust storms, and wildfires.  

The research found Seattle was the cleanest major U.S. city. Globally, Mayaguez, Puerto Rico, was the cleanest metropolitan area of 2024. 

Overall, the U.S. saw a significant reduction in PM2.5 levels last year, with the annual average dropping 22% from 2023. 

Globally, only 17% of cities meet the World Health Organization (WHO) air pollution guideline. Seven countries met the WHO annual average PM2.5 guideline of 5 µg/m3, including Australia, Bahamas, Barbados, Estonia, Grenada, Iceland, and New Zealand. 

The five most polluted countries in 2024 were: Chad, Bangladesh, Pakistan, Democratic Republic of Congo, and India. Overall, the region of Central and South Asia was home to the top seven most polluted cities in the world. India was home to six of the nine most polluted global cities. 

IQAir said considerable gaps still exist in government-operated regulatory systems in many parts of the world. Low-cost air quality monitors—used by citizen scientists, researchers, community advocates, and local organizations—have proven to be effective tools to address these data gaps. These monitors have proven to enhance the availability of crucial data on air pollution levels worldwide. 

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

To download the 2024 World Air Quality Report, click here.