Hundreds of thousands of acres in California are burning, part of a spate of fires that have forced thousands of people to evacuate and taxed the state’s overextended firefighting resources. Satellite imagery reveals that thick plumes of smoke streaming off each blaze have combined into a thick, smoky veil that covered parts of at least 10 states Thursday.

Air quality has plummeted in many areas, making it dangerous to breathe for some.

The out-of-control wildfires follow an unusually prolific outbreak of lightning over the weekend, much of which was dry lightning that occurs when rain from thunderstorms evaporates before it hits the ground. That sparked dozens of new blazes, which have combined to form giant complexes of flames, smack dab in the middle of a record-breaking heat wave gripping California at one of the driest times of the year.

Air-quality alerts were in effect for much of central California Thursday, where ground-level pollutants could cause “serious health problems, aggravate lung disease, cause asthma attacks and acute bronchitis, and increase risk of respiratory infections,” according to the San Joaquin Valley Air Pollution Control District.

The National Weather Service in Hanford, Calif., which covers the Golden State’s Central Valley, wrote that “particle pollution, mainly in the form of smoke and haze, will impact most of Central California for the next several days.”

They also warned that “ash from surrounding wildfires may fall in Merced County, especially Friday.”

Additional fires across the Four Corners and desert Southwest, including in Colorado and New Mexico, have also prompted air-quality alerts.

Medical experts have expressed concern that the degraded air quality could exacerbate the severity of covid-19 symptoms and prove especially problematic for vulnerable populations, such as people with asthma. Even in areas far away from any flames, a haze has overspread the sky and cast an eerie tinge to the sunlight.

Where the smoke is now

Satellite imagery showed the smoke concentrated in a band over central California on Thursday afternoon, stretching northeast through the Great Basin of Nevada and into eastern Oregon, Idaho, most of Montana and parts of the High Plains. Much of the smoke was riding north around the periphery of high pressure, which was parked over the southern Rockies.

More fires were spreading smoke across portions of Arizona, Utah, Colorado, New Mexico and even west Texas.

Weather models even suggest some of the diffuse high-altitude smoke could reach parts of the Upper Midwest and Great Lakes in the coming days. The smoke could be visible in skies across the entire western half of the Lower 48 by this weekend.

Smoke degrades air quality

Where the smoke is densest at the lowest levels, air-quality indexes have plummeted. In fact, California’s air quality was regionally among the worst in the world on Thursday, with only a couple of monitoring stations in India and Mexico reporting dirtier air during the early afternoon.

At the San Lorenzo Valley Middle School, in Santa Cruz, Calif., the air-quality index spiked to 351 on Thursday afternoon. According to the Environmental Protection Agency, values below 100 are considered “acceptable.” Air-quality indexes exceeding 300 are in the worst category and are shaded in maroon, corresponding to a “health warning of emergency conditions” when “everyone is more likely to be affected” whether or not they have underlying conditions.

Areas around Sacramento reported particularly bad air-quality indexes, between 150 and 300, in the “unhealthy” and “very unhealthy” categories. The main contributor was elevated levels of fine particulate matter originating from the fires.

Where health risks are possible

Robert Gould is the president of San Francisco Bay Physicians for Social Responsibility, a nonprofit education and advocacy organization. He says that the combination of wildfire smoke along with a continued rise in reported covid-19 cases could prove overwhelming on top of an already stressed health-care system.

“We’re very much concerned about respiratory diseases, the exacerbation of chronic conditions such as asthma, and cardiovascular conditions,” Gould said.

He cited several academic studies that have found a “direct relationship between chronic exposure to particulate matter … and what we’re seeing as a toll of [covid-19]."

“They weren’t saying such an exposure caused the death to [covid-19],” Gould said, “but for people who have a lot of other stressors, you worsen it. The wildfires represent the acute exacerbation of this.”

Gould noted that vulnerable populations were especially at risk and likely to have frequent exposure to particulate matter because of California’s pollution problems even in the absence of wildfires. He said communities of color, for example, tend to be more significantly affected by proximity to pollution sources.

“On top of that, we’re already aware that covid itself has had much more of a toll on communities of color, poorer communities, African American, Latinx communities, and that’s been, in my view, evident in what’s been reported on the increased morbidity and mortality of such communities.”

Risk for children

The wildfire smoke and steep drop in air quality can be particularly problematic for children and infants, whose lungs are still developing.

“This tiny particulate matter, PM2.5, is around all the time,” said Lisa Patel, a pediatrics professor at Stanford University. “But it is a toxin to the body. In an individual that is struggling with a respiratory condition, it certainly will not help.”

Children breathe faster, Patel said, and their maturing lungs are more susceptible to air pollution.

“We always tell parents to run an air purifier, keep the kids inside, close windows …” Patel said. “But the problem is that we’re going to be seeing these wildfires every year now because of climate change. One wildfire event may not affect their whole life, [but] now we’re talking kids exposed to this high level of pollution year after year after year. We don’t have the data to know what that’s going to mean.”

By altering the timing of spring snowmelt, speeding up summer drying and raising the odds of extreme heat events, climate change is contributing to larger wildfires across much of the West. One study, published Thursday in Environmental Research Letters, found that climate change has already led to a significant increase in the number of fall days that have extreme fire weather conditions in parts of California.

This trend is expected to continue unless human-caused emissions of greenhouse gases are curtailed in the coming decades, the study found.