The Los Angeles-Long Beach metropolitan area has once again ranked as the worst in the nation for ozone pollution, according to the American Lung Association’s 2019 “State of the Air” report. The report covers the period between 2015 and 2017, and it states that nearly 141.1 million Americans are breathing unhealthy levels of air pollution. That’s more than four in every 10 people in the United States—an increase since last year’s report.

Created with data compiled from the U.S. Environmental Protection Agency, the report ranks metropolitan areas according to ozone levels, short-term particle pollution, and year-round particle pollution. Californian cities are consistently home to the worst air quality in the country—with Los Angeles ranked worst for smog and within the top 10 for worst year-round and short-term particle pollution.

According to the report: “The three years covered in this report ranked as the hottest years on record globally. High ozone days and spikes in particle pollution zoomed, putting millions more people at risk and adding challenges to the work cities are doing across the nation to clean up.”

While recent wildfires and heat waves are partly to blame for Los Angeles’s poor air quality, the city also suffers from bowl-shaped topography that traps in pollutants, a sunny climate that naturally forms ozone, and extremely heavy vehicular traffic that causes photochemical smog.

Yet L.A. has also come a long way since the 1950s, when the city’s chokingly thick smog resembled Beijing’s air pollution. Thanks to improvements in technology and the enactment of the Clean Air Acts, Los Angeles has greatly improved its air quality, though it has yet to meet federal ambient air standards. Currently, California is adopting green energy incentives to reduce greenhouse gas emissions by 40 percent by 2030.

“More must be done to address climate change and to protect communities from the growing risks to public health,” according to the report. “This year’s report covered the three warmest years in modern history and demonstrates the increased risk of harm from air pollution that comes despite other protective measures being in place. The Clean Air Act must remain intact and enforced to enable the nation to continue to protect all Americans from the dangers of air pollution.”

Lead Photo by Dillon Shook via Unsplash