California's cities have long been plagued by smog, but now some of the state's cleanest air is being polluted by smoke from climate change-fueled wildfires.

For six straight years, Santa Barbara, California, had been on the American Lung Association's list of the cleanest cities in America for short-term particle pollution, which the organization has tracked for two decades in its annual State of the Air report. But in this year's report, the city dropped from one of the country's cleanest to one of the top 25 most polluted—thanks in large part to the wildfires that have ravaged the state in recent years.

California's cities are infamous for their smog, the toxic mix of car exhaust and industry emissions that often gets trapped over cities built in the valleys between the Pacific Ocean and the mountains. The warm climate exacerbates the problem: High temperatures on sunny days fuel chemical reactions between air pollutants, producing ozone—the main ingredient in smog.

And the same atmospheric and topographical features that trap car exhaust and other fumes over California's cities can also trap wildfire smoke during fire seasons, which can be just as hazardous to human health as the air pollution from man-made activities.

Particulate matter—the tiny particles of soot and other materials that are small enough to enter the lungs and sometimes even the bloodstream—are the biggest threat to humans from wildfire smoke, according to Dr. Afif El-Hasan, a pediatrician and national spokesperson for the American Lung Association. But when wildfires burn through populated areas, the smoke they generate carries additional risks.

"There's a few things that are going on with the wildfires," El-Hasan says. "You have obviously the burning of the wood and the natural vegetation, but you also have, unfortunately, in some cases burning of homes or buildings or human-made infrastructure."

When homes and cars and other man-made infrastructure burn, it not only releases particulate pollution into the atmosphere, but the smoke is often laced with asbestos, heavy metals, and other hazardous chemicals.

Smog contains its own set of noxious chemicals. Breathing in either wildfire smoke or smog can exacerbate asthma and other respiratory illnesses and increase the risk of heart attacks, stroke, and cancers.

But wildfires also have "the added problem" of the leftover ash that remains even after fires are put out, according to El-Hasan. "Just because the fire is out, it doesn't mean that the air quality in the local area is going to automatically go back to normal," he says. A strong sea breeze can clear the air of smog, but the same wind might kick up ash in the days or even weeks after a fire has been put out, which can be just as threatening to human health—especially for those with respiratory problems.

The long-term effects of inhaling wildfire smoke are less well understood, but one thing is certain: Climate change has made California's wildfires more frequent and more destructive. The 2018 fire season was the deadliest and most destructive on record, charring nearly two million acres in a single year, and shattering the record set just the previous year. Climate scientists predict this trend will continue as global warming leaves the state warmer and drier.

While wildfires may be a greater threat for the western United States, much of the nation saw its air quality drop in recent years. For the third straight year, the number of Americans living in areas with unhealthy levels of ozone or particle pollution rose. In 2017, 125 million people were exposed to unhealthy air; in 2019, that number reached 141 million.

The new report, which analyzes pollution levels over a three-year period in order to most accurately represent average conditions, shows that ozone pollution worsened across the nation between 2015 and 2017—the three warmest years on record.

Many of the policies to tackle air pollution could also help address climate change: Using provisions of the Clean Air Act to rein in carbon emissions from oil and gas production, energy generation, and the transportation sector could help clean up the air and slow the pace of climate change. Unfortunately, nearly all of the regulations in place to do that are currently under attack by the Trump administration.

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