Wildfires have ravaged California again this year, destroying hundreds of homes and killing six firefighters, as well as several residents.
This year also produced the largest single fire in California’s history, the Ranch Fire, which had burned more than 400,000 acres and spread across four counties near the Mendocino National Forest as of late August.
Smoke up and down the state, for days at a time, has blanketed metro areas miles from the fires, raising worries about air quality and pollution.
The wildfires have led some, including House Majority Leader Kevin McCarthy, to claim the blazes are spewing out more pollution than the state’s millions of vehicles.
"I have a real concern about air quality because of where I come from," McCarthy, a Republican who represents the southern San Joaquin Valley, including Bakersfield, said during a speech in Sacramento on Aug. 15, 2018. "(We’re at the) bottom of all the mountains and we collect (air pollution) all the way from San Francisco."
"What I have found, because of these fires, there are studies that are showing they are producing more emissions than all of our cars are doing," he added.
SOURCE: Public Policy Institute of California
Was McCarthy right?
We decided to fact-check his wildfires-versus-cars comparison.
Sean Raffuse, an analyst at the UC Davis Air Quality Research Center, told us McCarthy’s statement is correct, but for only for one category of emissions — particulate matter.
The claim is not true, however, for climate-warming greenhouse gas emissions, he said. Cars still produce far more of those.
Particulate matter, or PM, is a mixture of microscopic particles and liquid droplets that, when inhaled, can affect the heart and lungs and cause serious health problems.
For months, officials have issued health warnings that particulate matter from the wildfires could trigger asthma attacks or aggravate chronic heart and lung diseases.
Specifically, Raffuse said, the fires produce more PM 2.5 than all cars. PM 2.5 refers to particles two and one half microns or less in width, or about thirty times smaller than that of a human hair.
"PM 2.5 emissions are greater than what we see from cars, and that’s typical, not just of California during the wildfire season," he said, "but even if you look overall at the nation for an entire year, there will be more emissions from particles from wildfires than there will be from motor vehicles."
Clean combustion regulations, he said, are one reason vehicles produce fewer particulate matter than fires.
Earlier this month, the Trump administration proposed rules that would ease vehicle emissions standards nationwide. The proposal would also challenge the right of states, California in particular, to set their own, more stringent tailpipe pollution standards.
Greenhouse gas emissions
Bill Stewart, a researcher at the UC Berkeley Center for Forestry, said there’s a key point missing from McCarthy’s statement.
While fires emit more particulate matter, cars produce far more greenhouse gas emissions, particularly carbon dioxide, he said.
"Those are two important but different issues," Stewart said.
Mike Kleeman, a UC Davis professor of environmental engineering, added that wildfires also emit some greenhouse gases.
"But burned areas also regrow during which time they act as a net carbon sink. Assuming a burned area regrows completely over a time of decades, the net CO2 emissions should be low," Kleeman wrote in an email.
"Wildfires are a problem," he added, "and we should be taking steps to reduce their frequency and intensity. But it isn't an ‘apples to apples’ comparison to an every-day source like cars."
Asked for evidence supporting the claim, McCarthy’s spokesman pointed to an October 2017 NBC News story that examined the emissions produced by that year’s deadly Wine Country fires.
The story had the misleading headline: "California Wildfires Emitted a Year's Worth of Car Pollution in Less Than a Week."
It quoted Raffuse of UC Davis, who told NBC the fires had released the same amount of particulate matter as come from California vehicles in one year. He didn’t claim the fires had produced more of all types of emissions than cars.
In the article, the researcher "pointed out cars pollute in other ways, too."
Raffuse summed up the congressman’s statement saying it’s "partially correct but omits a key distinction."
McCarthy’s spokesman did not cite any specific studies to support the claim. We reviewed several that described wildfire emissions as a growing concern, but did not say they produce more pollution overall compared with cars.
One such report by the Sierra Nevada Conservancy in 2014 found that by the year 2085, emissions from wildfires are projected to be equal to the annual emissions of about one fifth of all of the cars currently registered in California.
House Majority Leader Kevin McCarthy recently claimed "because of these fires, there are studies that are showing they are producing more emissions than all of our cars are doing."
Wildfires produce more of one key pollutant, particulate matter, than cars, both in California and nationwide. But that’s all.
McCarthy cherry-picked this one category and ignored the fact that cars produce far more greenhouse gas emissions, such as carbon dioxide, than fires.
We rate McCarthy’s claim Half True.
HALF TRUE – The statement is partially accurate but leaves out important details or takes things out of context.
Click here for more on the six PolitiFact ratings and how we select facts to check.
CORRECTION: This article has been corrected to show Mike Kleeman is a professor at UC Davis, not at UC Berkeley.