Worst Smog Cities in Pictures: Air Pollution, Ozone, and Asthma

2021 marks the 51st year of America’s Clean Air Act, a law passed to improve air quality. But more than half a century later, many U.S. cities still have unhealthy levels of smog, according to the American Lung Association’s annual State of the Air rankings. Warming trends from climate change increase ozone and, along with frequent wildfires, worsen air quality.

Think of the Bay Area as a big bowl. Winds make it easy for polluted air to move from one valley to another. Vehicles driven by the nearly 9 million residents of San Francisco, San Jose, and Oakland play a role in the area’s dirty air. During warm months, residents can get daily updates on air quality from the Bay Area Air Quality Management District’s Spare the Air campaign.

Utah’s capital, along with most of the state’s population, is positioned in front of the Wasatch Mountains. In winter, temperature inversions -- a layer of high warm air aloft -- traps cooler air below. That means pollution created by the Salt Lake-Provo-Orem area’s 2.5 million residents has nowhere to go.

Colorado is working to meet a June 2021 EPA compliance deadline for lowering ground ozone levels. Strict oil and gas regulations helped, as did having fewer of the Denver-Aurora metro area’s 3.6 million residents on the road during the COVID-19 pandemic. But rising temperatures and drier weather have also led to a jump in wildfires throughout the state.

The San Diego-Chula Vista-Carlsbad metro area scored an F on this year’s ozone report, with more than 100 days of high readings. The toxic air is bad news for the metro area’s more than 300,000 kids and adults with some form of asthma.

The Sacramento Valley is shaped like a bowl. In summer, pollutants like exhaust from cars get trapped closer to the ground. The Sacramento-Roseville metro area, home to 2.6 million, earned a spot on the lists for highest ozone days, most unhealthy spikes in particle pollution, and annual particle pollution levels.

Arizona has less cloud cover than any other state. And Phoenix is the state’s second-sunniest city. More sunlight reaching the ground means more ozone. That could be why Phoenix moved up the list two spots from No. 7 last year. To clean things up, local leaders are working to curb the number of drivers on the road and to cap the use of wood-burning fire pits and fireworks.

The Fresno-Madera-Hanford metro area only has 1.3 million residents. They enjoyed the lowest number of ozone days since 2000, but stayed in the same spot on the list as last year. In addition to the number of cars on the road, geography is partly to blame. The bowl-shaped San Joaquin Valley traps vehicle emissions with no way for them to get out.

Things are looking up a bit in Visalia-Tulare County, which moved down the list one spot from last year’s second-place ranking for ozone. It stayed in third place, though, for year-round air particle pollution. Tulare County, located in California’s San Joaquin Valley, is home to only about 460,000 people but more than 1 million cattle. Industrial dairies play a big part in local air pollution, which includes high levels of methane emissions.

Unlike other cities in California’s smog-trapping San Joaquin Valley, Bakersfield can blame some of its air pollution on oil and gas production. It’s no surprise the area ranked first for particle pollution. One local company admitted to violating the Clean Air Act for greenhouse gas emissions. And the city has some 40,000 residents with heart disease. All that smoggy air can increase their odds of having a heart attack.

2021’s smog capital is the Los Angeles-Long Beach metro area, home to 18.7 million people. The city’s crowded freeways definitely play a role in creating ozone levels, but LA’s famous sunny weather doesn’t help. Sunshine ramps up smog levels, aggravating symptoms for the 1.5 million residents who have some form of asthma.

This California city once again claims the No. 1 spot when it comes to year-round particle pollution, or soot. High rankings are nothing new for Bakersfield, which led this category from 2016 to 2018. Locals point to oil and gas production facilities, along with its location in the San Joaquin Valley, which traps pollution and holds it in. Wildfires also raised soot levels across the Western U.S.

Five cities ranked highest in the U.S. for the three measures of clean air: ozone, year-round particle pollution, and short-term particle pollution in 2021. Three repeated this honor 2 years in a row. That’s good news for residents: People who live in cities with good air quality may live longer, healthier lives.

Located near the U.S.-Canada border, Burlington is home to 279,000 residents who enjoy fresh, grade-A air. An overall lack of heavy industry coupled with fewer automobiles -- two leading sources of air pollution -- could be why.

History-packed Charlottesville is home to a little over 200,000 people and the University of Virginia. You’ll also find the Shenandoah National Park, part of the Blue Ridge Mountains. Residents enjoy outdoor activities like hiking, rafting, and skydiving into the smog and soot-free air.

Nearly 200,000 people call this metro area home. Located on the edge of New York’s Finger Lakes region, it offers wineries, rolling hills, hiking trails, and plenty of fishing spots. You can even find the Victorian home sites that inspired much of Mark Twain's work. What you won’t find is a lot of dirty air. Elmira-Corning ranked 7th on this year’s list of cleanest cities for year-round particle pollution.

Once again, Honolulu took top honors for lowest annual particle pollution levels. That means nearly 1 million people who call this tropical city home are breathing some of the nation’s cleanest air. Credit goes in part to the weather -- year-round winds and rain play a role -- as does a lack of heavy industry.

Home to 297,000 residents, this Atlantic Ocean beach community had no unhealthy air quality days from ozone, or long- or short-term particle pollution. North Carolina is also focusing on clean power research, sinking billions into solar and wind energy projects.