More toxicological and epidemiological studies are needed to understand human health risks
A review crafted by UC Davis graduate students reveals that research gaps remain in the science of human health toxicity of California wildfire smoke mixed with pesticides and fire suppression chemicals.
Millions of pounds of pesticides are applied to agricultural regions, especially in the Central Valley, to foster plant growth and combat pests. Firefighters use millions of gallons of fire suppression chemicals, such as gels and foams, to snuff out fires which threaten residential structures. Such chemicals can be broken down by the heat of wildfires and inhaled by humans living near urban interfaces, places where wilderness and urban areas meet.
“The literature out there on what these chemicals can do is very limited, especially these unique exposure situations,” said Michelle Kossack, a Ph.D. candidate in pharmacology and toxicology at UC Davis and one of the first authors of the paper. “You might be studying the effects of this chemical through direct exposure, but what happens when that chemical is transformed through heat of a wildfire?”
Wildfire smoke is known to cause health problems in eyes and airways exposed to the acrid air. Less is known about how burned pesticides and fire suppression chemicals can affect humans, animals, and the environment. When fires burn through agricultural areas, such as during the Napa, Sonoma and Solano County fires of October 2017, pesticides and fungicides can be involved in combustion.
“We wanted to learn more about wildfires and wildfire smoke because it’s a big problem in California,” said Cameron Flayer, a Ph.D. candidate in pharmacology and toxicology at UC Davis and another of the paper’s authors. “We have mega wildfires now, where a ton of smoke is polluting the atmosphere, and then we’re breathing it in. We were curious about the constituents of the smoke.”
The major components of wildfire smoke are particulate matter, which are the charred remains of trees and brush, and chemicals such as ozone and carbon dioxide. Although burned pesticides and fire suppression chemicals serve as minor components, they could pose serious health effects of their own to humans and animals.
“The particulate matter does cause a threat, but how is that compounded by other chemicals in the mixture?” Kossack said. “It’s no longer a virgin forest burning. Many of these forests have pesticides applied to them. Even national forests do it to control invasive populations of species.”
Toxicological and epidemiological studies often compare how individual substances can be harmful when exposed to animal models, often mice and rats. One of the research gaps identified in the review is the abundance of new chemicals created in the heat of a blaze, often structurally different from the parent compound they are derived from, which have not been extensively studied.
“It’s important to start looking at larger trends and patterns — classes of chemicals and groups of chemicals that can interact with each other,” said Sarah Carratt, a postdoctoral scholar at Oregon Health and Science University and one of the authors of the paper. Carratt previously worked as a Ph.D. candidate in pharmacology and toxicology at UC Davis before graduating in September 2017. “Even a few pilot studies to get a good idea of what we’re looking at in specific regions. If somebody were to look at emissions from the Napa Valley fires that are ongoing right now, they’d be able to figure out what’s in the air when these regions where they are growing agricultural crops burn.”
When researchers study smoke exposure and its health impacts, the tendency is to lump distinct types of wildfires together for simplicity and control, but forests burning in Yosemite National Park may have different chemical smoke mixtures than fires burning in the Central Valley or near Los Angeles.
“We were just trying to bring to light the fact that, when wildfires burn pesticide-treated land, or when we use these suppression chemicals to fight back against wildfires, there may be consequences for human health,” Flayer said.
One of the research gaps present is a lack of smoke mixtures to compare against one another. Collecting smoke from different sites can be a challenge to scientists in the field.
“We need a state agency or academic researchers who can respond to situations like the current wildfires here in Napa, Sonoma, and Mendocino Counties by setting up sampling equipment during the fire episodes with this specific goal in mind,” said Jerold Last, a distinguished professor in pulmonary, critical care and sleep medicine at UC Davis and the senior author of the paper.
As pesticides and fire suppression chemicals burn, the chemicals can break down into different compounds and structures, which may not be properly regulated by state and federal agencies.
“We really need to understand what the derivatives are of the parent compound and if they are found in the smoke, because all the regulations are just for the parent compound and not for any of the derivatives,” Flayer said.
The heat from wildfires can warp parent compounds of pesticides and fire suppression chemicals into new forms. While some remain close to the ground or soil, others can volatilize into the air and travel far from the flames.
“Once you have a very basic structure, and you expose it to heat, lots of things can adapt on to it,” Kossack said. “It can change very minorly, but that can have a huge effect on toxicity. One chemical can become hundreds. It is almost impossible to do all of these studies on each individual compound.”
Pesticides and fire suppression chemicals are not the only mixture public health officials need to be concerned about. Hundreds of structures have been incinerated by the North Bay fires in October 2017, creating smoke from construction materials which can be inhaled by fleeing residents, rescue workers and firefighters.
“It’s all the buildings that are burning, too,” Carratt said. “Think about all the chemicals that are added into the buildings. It’s not just fire suppression chemicals in terms of what they’re using to put out the fires. But most furniture and buildings have some kind of built-in fire retardant. Those burn as well, and those are other chemicals that you don’t think of being released into the atmosphere. But those things have the potential to transform when they are burned, as well.”
Studies considering not only multiple types of chemical mixtures but different routes of exposures can be challenging and expensive for researchers, but that could be one of the next steps to better understand more of the possible health effects of wildfire smoke.
“We would love to see other studies do more complex, more real-life relevant exposures, including what happens when pesticides burn or other chemicals,” Carratt said.
Firefighters battling the flames are the most vulnerable to harmful fumes. Migrant farmers who gather crops and tend to the land may also be vulnerable if they remain outside during smoky days. Children, the elderly, the immunocompromised, and those with existing respiratory challenges may also suffer disproportionately from effects of wildfire smoke.
“During wildfires, when the fire is burning, and it’s a high smoke day, where there’s a lot of particulate matter or ozone, EPA already recommends that you don’t go outside and participate in physical activity,” Flayer said.
While fires can occur in many places around the U.S., California faces unique challenges in managing the flames.
“Most of the western states have wildfires and relatively large usage of pesticides and fire suppression chemicals in agriculture and forest maintenance,” Last said. “What makes California unique is the size and scale of California agriculture and the proximity of wildfires in rural areas to large population centers — what we call the urban-rural interface. Think about the impact of fires in rural forests and brushy areas in Napa and Sonoma where large numbers of people live and farm grapes and other crops nearby.”
Wildfire smoke causes known health effects in humans, animals and the environment, but the flames can also introduce pesticide and fire suppression chemicals into the air far from their sources. More data is being collected to better understand the health impacts of these smoke constituents, since current evidence is lacking. Better study designs, improved collection techniques, and renewed funding streams can help protect some of the vulnerable populations in California.
“I think people are really interested in public health, especially in California, where everyone is very aware of how much smoke is in the air,” Kossack said.
Written by: George Ugartemendia — science@theaggie.org
