IQAir, a Swiss company that monitors air pollution, found that seven of the 10 most populous U.S. cities have returned to pre-pandemic levels of air pollution.

Wildfires in the U.S. combined with the near-elimination of pandemic restrictions pushed air pollution levels in the U.S. back to pre-Covid levels in 2021, according to a report released Tuesday.

IQAir, a Swiss company that monitors air quality around the world, said in the report that the U.S. as a whole saw a 7 percent increase in fine particle air pollution in 2021 compared to the previous year. Seven of the top 10 most populous cities in the United States have returned to pre-pandemic levels of air pollution, with Dallas, Miami and Washington, D.C., as the exceptions.

Atlanta recorded a 33 percent increase in pollution, attributed to traffic returning and increased monitoring. Similarly, Minneapolis recorded a spike of 35.7 percent last year.

Christi Chester-Schroeder, an air quality scientist for IQAir, said the reason for the jump in Minneapolis was stagnant air, increased monitoring and wildfires in the western U.S.

“In July 2021, the measurement was three times what an average July would be for Minneapolis,” she said.

The report adds to growing concern about the immediate health fallout of growing air pollution, something health experts have increasingly warned about in recent years. The report estimated that the deaths of 40,000 children under the age of 5 around the world in 2021 were linked to air pollution.

The World Health Organization has estimated air pollution kills 7 million people worldwide every year.

IQAir’s report used air quality data from 6,475 cities in 117 countries, regions and territories provided by stations operated by governments, nonprofit organizations, research institutions, educational facilities and citizen scientists. The report measured fine particle air pollution, called PM2.5, which can consist of anything from soot, smoke and mold spores to pollen, sand and human hair.

The report highlighted an ongoing disparity in air quality monitoring around the world. The U.S. has the largest number of air quality monitors of any country, but only 13 out of 54 countries in Africa had sufficient public air quality monitoring data,the report found. The Latin America and Caribbean regions also lacked sufficient monitoring. Between 2020 and 2021, the number of air quality monitoring stations increased significantly in India, New Zealand and Canada.

The harsh effects of climate change were directly tied to smoke, sand and dust pollution in China, South Korea, India, Australia and the United States. Bangladesh received the worst average measurement of air pollution concentration over any other country.

Some parts of the world showed improvement. China reported its fifth-consecutive decline in national air pollution, though it still ranked 95th out of the 117 areas surveyed. The U.S. ranked 27th.

Fine particle air pollution can be directly linked to adverse health effects “such as stroke, heart disease, chronic obstructive pulmonary disease, and lung cancer,” the report stated.

Ananya Roy, an environmental epidemiologist and senior health scientist for the Environmental Defense Fund, a nonprofit environmental advocacy group, said the health consequences of air pollution can quickly become societal issues.

“It manifests in various different ways,” Roy, who is not affiliated with IQAir, said. “One is poor quality of life, missed school days, asthma exacerbations, which end up having huge out-of-pocket expenditures or catastrophic health expenditures, leading to hospitalizations or ER visits or the inability to go to work.”

Roy added that air pollution is also a driver of global warming.

“Many of the sources that result in fine particle air pollution are also responsible for greenhouse gasses that lead to climate change,” she said.

Health officials have been warning about the dangers of air pollution.

In September, the World Health Organization updated its air quality guidelines to limit fine particle air pollution. Though not legally binding, the WHO stated the guidelines “are an evidence-informed tool for policy-makers to guide legislation and policies, in order to reduce levels of air pollutants and decrease the burden of disease that results from exposure to air pollution worldwide.”

According to the IQAir report, Puerto Rico, the U.S. Virgin Islands and the French territory of New Caledonia in the South Pacific are the only places in the world that meet the updated WHO guidelines.

The WHO’s guidelines suggest annual concentrations of PM2.5 should not exceed 5 units per measurement. In the U.S., the Environmental Protection Agency’s primary standard is 12 units per measurement.

Chicago, Atlanta and Los Angeles recorded 2021 averages above the EPA standard, according to the IQAir report.

Roy said the availability of air pollution measurements, such as this report, can potentially help mitigate inequities around the world.

“It’s really important to track and measure and map air pollution across communities in order to find actionable solutions that can deliver clean air to the families living in these communities,” she said.