Wildfires leave behind more than scorched earth and destroyed homes: Rising smoke plumes can contain chemicals that disperse not only into the air but in soil, water, indoor dust, and even wildlife.
Polycyclic aromatic hydrocarbons (PAHs), a class of more than 100 chemicals that can cause cancer and other ailments, is one of those ingredients. As the West continues to suffer more intense and destructive wildfires, the smoke from those fires needs to get a closer look, including how PAHs factor into the load, air experts said.
“It’s a situation where our governments are going to have to get used to dealing with that reality,” said Bill Magavern, policy director at the Coalition for Clean Air. “As we learn more about wildfire smoke, it’s important that people know there are these constituents that cause cancer, that cause cardiovascular disease.”
But research on wildfire-generated PAHs is relatively limited, with scientists instead focusing on calculating particulate matter and not the chemicals found in those tiny particles that are smaller than the width of a human hair. Analyzing for PAHs is more sophisticated and requires chemists, which increases costs. Exposure to many PAHs, rather than a single one, is also common.
“It is an absolute gap,” said Kim Anderson, a professor of environmental and molecular toxicology at University of Oregon. “To me, it’s just missing too big of a piece of the puzzle.”
Anderson studied indoor air after wildfires in Oregon in 2018 and found concentrations of PAHs indoors were higher than in outside air, signifying that once the chemicals get inside, they recirculate and don’t readily dissipate.
PAHs, the product of combustion, are everywhere. Grilling a burger, smoking a cigarette, or lighting a candle can produce them. So can diesel exhaust and burning coal in addition to wildfires.
Broad regulations for PAHs, as a class in air and water, are rare, according to the Agency for Toxic Substances and Disease Registry. The Occupational Safety and Health Administration has set a permissible exposure level of 0.2 milligram/cubic meter in workplace air.
The Environmental Protection Agency and state of California have a drinking water standard of 0.2 parts per billion for benzo(a)pyrene, the PAH that poses the most severe cancer risk. EPA has also set drinking water standards for five other carcinogenic PAHs.
In the 1970s, the agency named 16 PAHs—including naphthalene, fluorene, and pyrene—as high-priority pollutants, and those have primarily gotten the attention over the years.
One issue with calculating the risk of PAHs is that EPA and other governments tend to regulate and set a threshold for one compound, rather than several, said Susan Tilton, an associate professor in environmental and molecular toxicology at Oregon State University who studies the effects of carcinogenic PAHs on genes.
“That’s not the reality of how individuals are exposed to them,” Tilton said. “We certainly have an interest in looking at a mixture of PAHs in wildfire smoke in order to detect how toxic it may be to individuals.”
The results of Tilton’s work, published recently in the journal Toxicology in Vitro, could help model the effects of other PAHs that haven’t been studied but are in the environment. It also could help regulators key in on the dangers in wildfire emissions. And using genes means scientists can assess how PAHs may affect people with asthma or other preexisting conditions to determine susceptibility.
EPA didn’t respond to questions about PAHs, if it planned to regulate the chemicals as a class, or add more to the priority list.
Elsewhere around the world, the chemicals are under more scrutiny. In 2015, the European Commission amended regulations setting PAH maximum concentrations in food products to add supplements, herbs, and other edibles.
Some PAHs are carcinogenic and others can cause skin irritation, and renal and gastrointestinal damage, but more study is needed to determine the effects of chronic exposure, according to a case study from the federal Agency for Toxic Substances and Disease Registry.
So far this year, wildfires in California alone have burned more than 4 million acres, with the smoke spreading hundred of miles.
People are exposed to PAHs every day and it would be wise to reduce additional exposures from wildfires, said Gina Solomon, a clinical professor in the Division of Occupational and Environmental Medicine at the University of California, San Francisco.
“They damage our genes and thereby cause mutations, which can cause cancer,” she said of PAHs. “The very young and the very old often lack the full capability to repair genetic damage. Wildfires are an additional burden.”
Solomon sampled tap water in homes that didn’t burn during 2018’s Camp Fire, the state’s most deadly and destructive fire. Many of the area’s water pipes were depressurized, and the going theory is that smoke was sucked into plumbing pipes, contaminating service lines.
Preliminary screening at University of California, Davis, detected several PAHs in the tap water, but more work needs to be done to verify the constituents, said Solomon, who did her research with the Oakland-based Public Health Institute.
The State Water Resources Control Board plans to study the issue more and is talking with Oregon officials soon to discuss that and other topics, said Dan Newton, an assistant deputy director in the division of drinking water.
Ingredients of Smoke
The composition of smoke depends on where a wildfire burns, if just wooded areas are affected or homes, cars, and other structures.
The California Air Resources Board has studied particulate matter and PAH exposure in firefighters, but now is looking more deeply at smoke, said Bonnie Holmes-Gen, the board’s health and exposure assessment branch chief.
“The more fires that we have and the more acreage that’s burned, the more we have these mixtures of urban and rural fires, so we’re definitely concerned with looking at the mixtures of chemicals that can occur when we do have buildings, and cars, and this huge mix of urban sources,” Holmes-Gen said.
The agency has commissioned a study with University of California researchers at the Berkeley and Riverside campuses to capture smoke and measure the composition of PM 2.5, the microscopic fine particles emitted by wildfire. The study method can detect more than 400 chemicals.
PAHs are among them, but aren’t the only focus.
“Wildfire smoke from wood in a forest contains thousands of individual components,” said Nehzat Motallebi, an air pollution specialist at the Air Board’s Air Quality and Climate Science Section. “It’s a very complex state of the science. PAHs is one of the players.”
Anderson is also broadening her Oregon study examining indoor air after wildfires to include California, Idaho, and Washington state.
Water, Soils, Wildlife
But air isn’t the only place to look.
“If the concentration of PAHs in the air is really high, it will deposit into the soil,” Anderson said. “They can actually revolatize into the air some weeks or months later or they can also be part of runoff.”
Wildfires can destroy root systems when trees and shrubs burn and those soils can be washed out during later storms, spreading contamination.
Researchers from University of California, Northridge, sampled Malibu Creek and tributaries in late 2018 after the Woolsey Fire near Los Angeles. Water and soil samples collected during storm events showed an increase in PAHs in the watershed during and right after storm events. Those concentrations elevated again a year later, according to a 2020 paper.
“The post-fire increase in erosion mobilizes particle-bound contaminants, including PAHs, into the watershed, potentially affecting wildlife and human health,” the paper said.