“The more clearly we can focus our attention on the wonders and realities of the universe about us, the less taste we shall have for destruction.” ― Rachel Carson

Introduction

Wildfires have dramatically increased in number, size, and duration over the past several decades. Defined as unplanned and uncontrolled fires, wildfires are spreading to every corner of the globe. From the Siberian taiga forest to the Australian bush, wildfires have affected 17 million people since 2010. Impacting agriculture, transportation, power and gas services, water supply, and the health of humans, animals, and ecosystems, wildfires affect every aspect of life.

The intersection between wilderness and developed land, which is referred to as the wildland-urban interface (WUI) is particularly vulnerable to catastrophic wildfires. In the United States (U.S.), over 3,000 structures per year are lost to wildfires in the WUI. Population growth and the corresponding expansion of the built environment are rapidly increasing the WUI worldwide. For example, in the U.S. between 1990 and 2010, the total area of WUI increased by 33%, while the number of homes in the WUI increased by 41%. This expansion of the WUI is increasing the risk of wildfires, which are most often ignited by human activity.

Another important factor contributing to the growing risk of wildfire is climate change. Warmer temperatures lead to drier conditions, which make fires easier to spread and more difficult to put out. If climate change is not addressed, wildfires are predicted to burn more often, for longer periods of time, and in places previously untouched over the coming years. Below, we review the role of climate change in fanning wildfires, the chemistry of wildfires, the negative impacts of wildfires on human health, wildfire mitigation, and critical research needed to better protect humans and our environment from the devastating effects of wildfires.

Climate change is worsening wildfires

There is now compelling evidence that climate change is a major factor driving the increased incidence and severity of wildfires. Energy from the sun heats surface waters and the land, while gases in the atmosphere, including carbon dioxide, act as an insulator to prevent heat from escaping. While natural changes in energy emission from the sun and the earth’s rotation contribute to climate change, the rapid changes in climate since the industrial revolution are largely due to human activities, including deforestation, expansion of the WUI, and emission of greenhouse gases from urban transportation, electricity generation, and industrial pollution. These activities increase the levels of carbon dioxide in the atmosphere, trapping heat and resulting in increased average global temperatures, rising sea levels, and altered ocean currents, which in turn regulate weather patterns, atmospheric vapor pressure – i.e., the partial pressure, or relative amount, of water vapor in the atmosphere – and rainfall on land masses.

Global shifts in atmospheric vapor pressure and water distribution are tightly linked to wildfire activity. Between 2000 and 2015, there was a yearly increase in the atmospheric vapor pressure deficit in the western U.S. As the vapor deficit increases, so does fuel aridity, a measure of dryness and flammability. The increasing deficit in vapor pressure in the western U.S. coincided with a 4.2 million hectare increase in burned area and a 54% increase in wildfire season length. Wildfires are also promoted by increasing deficits in soil moisture (agricultural drought) and rain (meteorological drought). Just as dry matches ignite more readily, these drought mechanisms raise fire risk by increasing fuel availability and flammability. Droughts associated with warming temperatures are positively correlated with total burn area in Australian bushfires and a 30% increase in fire risk from 1997 to 2018. Strikingly, even regions with historically low fuel availability and low flammability, such as the tundra and boreal forest regions in Alaska and Siberia, are predicted to experience a 33-50% increase in wildfire risk because of warming temperatures.

Wildfires increase human exposure to toxic components

Wildfire smoke is composed of particulate matter (PM) and gases produced by incomplete combustion. The specific chemicals in wildfire smoke vary between burn sites, type(s) of fuel burned, combustion phase (flaming versus smoldering), and weather conditions.

PM is a complex mixture of microscopic droplets of liquids and solids of diverse chemical composition. PM is further classified by size, or particle diameter (Dp), and includes coarse (PM10: 2.5 < Dp < 10 µm), fine (PM2.5: Dp < 2.5 µm), and ultrafine particles (UFP: Dp < 0.1 µm). To put these sizes in perspective, a human hair, which measures 50-70 µm in diameter, has a cross-section 400 times larger than that of even the largest PM2.5 particles. PM2.5 particles are similar in size to bacteria, while UFPs are similar in size to viruses. PM2.5 and UFPs are a major health concern because they can penetrate deep into the lung, cross into the circulatory system, and even translocate to the brain.

Combustion gases can also be hazardous to human health and the environment. These are comprised predominantly of carbon dioxide, carbon monoxide, water vapor, and trace species like acid gases (e.g., oxides of nitrogen) and volatile organic compounds (e.g., polycyclic aromatic hydrocarbons; PAHs). Combustion gases can undergo further reactions in the atmosphere driven by solar radiation to form secondary organic aerosols and ozone, the latter representing a major component of haze or smog.

When wildfires breach the WUI, the fuel burned includes not only biomass (vegetation), but also the built environment, such as construction materials, consumer products, vehicles, commercial facilities, plastics, waste products, infrastructure, and more. While toxic emissions from biomass combustion are well characterized, less is known about the emissions from synthetic fuels. However, recent research suggests PM2.5 from burning plastic may be more reactive than PM2.5 from burning biomass. Synthetic fuels also contain toxic metals like lead, chromium, arsenic, and mercury, and chemical additives like plasticizers, flame retardants, and halogenated polycyclic hydrocarbons, including biphenyls, diphenyl ethers, dioxins, and furans. When they burn, these toxic components can be directly released into the smoke as PM or gases, or altered to form other combustion products, which are poorly characterized but measurable in the smoke.

Wildfires produce other potentially toxic hazards such as ash and soot. Ash is the residual powder created from the combustion of vegetation that coats the post-fire environment. A mixture of inorganic and organic matter left over from the combustion process, ash includes toxic organic compounds like PAHs at concentrations potentially harmful to humans, animals, and ecosystems. Ash generated by burning of a structure may also contain hazardous metals. A key difference between smoke, which dissipates as the fire is extinguished, and ash is that ash accumulates and persists in the environment, constituting a long-term health effect. During active burning, ash settles on soils and on surface waters. In the aftermath, vegetation can take up toxic chemicals from ash deposited on soils, and rainstorms can transfer ash from soils into downstream reservoirs, which can alter the potability and treatability of drinking water supplies, trigger algal blooms, and threaten aquatic species. Soot, also referred to as black carbon, is the byproduct of incomplete combustion of carbon-containing materials and can be found in both ash and PM, most notably PM2.5 and UFPs. There is limited research on wildfire soot, but a large body of research on soot from other sources like vehicular emissions link it to increased incidence of cancer, as well as cardiovascular and respiratory disease.

Acute and chronic human health impacts of wildfires

Most of the research to date on human health effects of wildfires have focused on wildfire smoke, which predominantly enters the human body via inhalation, but can damage multiple organ systems.

While wildfire smoke contains both PM and gases, the adverse effects of PM2.5 have been most extensively studied. Studies done in mice, and in human lung cells, show that wildfire PM is more toxic to the lung than equal amounts of ambient air PM. Inhaling wildfire smoke can cause immediate symptoms in healthy individuals, such as coughing, difficulty breathing, irritation of the throat, nose and eyes, chest tightness, and wheezing. These effects are exacerbated in individuals with pre-existing respiratory disease. The acute respiratory effects typically subside when the wildfire-associated air pollution subsides. Less is known about the long-term or chronic effects of wildfire smoke inhalation on healthy individuals. However, researchers at the University of Montana found that adults exposed to a wildfire event had significant decreases in lung function that persisted even two years after the wildfire. The mechanisms by which wildfire smoke causes these prolonged health effects are poorly characterized. Addressing this data gap is increasingly important as wildfire incidence and duration increases.  

The lung is not the only organ adversely affected by wildfire smoke. Chemicals and PM that deposit in the lung because of wildfire smoke inhalation can transit to the blood and be delivered directly to other organ systems via systemic circulation. Additionally, smoke-induced lung inflammation can trigger systemic inflammation that exacerbates disease processes in other organ systems. For example, in humans with pre-existing cardiovascular disease, exposure to wildfire smoke increased the risk of heart attack and stroke. A 2020 study of California wildfires from 2015-2017 found that cardiac arrest risk increased significantly during wildfire events. Wildfire smoke exposure has also been shown to decrease heart rate variability in human subjects, which is correlated with compromised heart health. The PM in wildfire can induce cardiac arrhythmias, exacerbate heart failure, alter platelet function to increase risk of blood clots, and trigger ischemic cardiovascular complications, while the UFPs increased cardiovascular disease risk in a mouse model. 

Wildfire smoke increases inflammation throughout the body via effects on multiple components of the immune system.

Studies of firefighters before and after a shift show increased levels of inflammatory markers in the blood and sputum, and experimental exposure to wildfire PM increased levels of pro-inflammatory cytokines in cultured human airway epithelial cells. While studies investigating the effects of wildfire smoke exposure on susceptibility to viral infection have yielded mixed results, a recent epidemiologic study reported a positive correlation between exposure to wildfire PM2.5 and increased COVID-19 caseload. In other studies, exposure to bioaerosols in wildfire smoke, which may contain infectious bacteria and fungi that are lofted during combustion process, appear to impede wound healing. Such observations have raised concerns about increased vulnerability to the adverse health effects of wildfire smoke amongst populations with compromised immune systems, such as individuals with allergic asthma, lupus, or type 1 diabetes. This concern is amplified by studies showing that exposure to PM2.5 exacerbates disease conditions in animal models of lupus and type 1 diabetes. There is also concern that exposure to wildfire smoke may promote the development of autoimmune disorders based on epidemiological evidence linking PM to increased incidence of these diseases.

The brain is emerging as another vulnerable target of wildfire smoke. A recent review of the research on mental health impacts of wildfires concluded that individuals affected by wildfires experienced increased rates of post-traumatic stress disorder (PTSD), depression, and generalized anxiety disorder. This review included individuals from multiple countries and measured outcomes using both clinical evaluations and self-reported symptoms. The rates of these psychiatric illnesses were shown to increase both shortly after and up to several years following the wildfire event. Women, especially widowed women, and individuals of lower socioeconomic status or non-Caucasian ethnicity had significantly greater risk of developing PTSD. Rates of depression and anxiety were increased in adults and adolescents, and depression symptoms persisted in adults for up to 10 years after the wildfire event.

How wildfires impair mental health remains unclear. Psychological and physical stress associated with experiencing a wildfire event are likely important factors. However, experimental studies of Australian bushfire smoke suggest that PM2.5 in wildfire smoke may directly impact brain health as well. For example, systemic inflammation caused by inhalation of wildfire smoke may increase the permeability of the blood brain barrier, increasing the likelihood that PM2.5 transferred from the lungs into the systemic circulation can access brain tissue. Alternatively, experimental models show that PM2.5 in the nasal passages can translocate to the brain via olfactory nerves. Once in the brain, wildfire PM can cause oxidative stress, microglial activation, and neuronal cell death. Additionally, many of the components found in both the PM and gases of wildfire smoke are known to be neurotoxic, including metals, plasticizers, and PAHs.

Factors that increase vulnerability to adverse health effects of wildfire smoke

Factors such as pre-existing medical conditions, age, pregnancy, occupation, and social vulnerability can increase susceptibility to the negative effects of wildfire smoke. As discussed earlier, pre-existing respiratory and cardiovascular diseases are associated with increased risk of cardiac arrest, heart arrhythmias, and decreased lung function following exposure to wildfire smoke. Age is another important factor, with the very young and the elderly being most vulnerable. Compared to adults, infants and children breathe more smoke relative to their body weight during a wildfire event. This higher level of exposure makes children with asthma particularly susceptible to acute effects, leading to higher rates of hospitalization during wildfire events. Because organs like the lung, brain and immune system continue to develop throughout childhood, wildfire exposure during early life development may be associated with long-term effects. For example, a recent study in children found that immune function was negatively affected 90 days after a wildfire event, and in nonhuman primates, early life exposures to wildfire smoke affected immune function later in life. Older individuals' immune systems may also be negatively affected because immune function decreases as individuals age, a possibility supported by studies indicating that the elderly are particularly susceptible to adverse health outcomes during wildfire events. 

Pregnant women are also highly susceptible to the negative effects of wildfire events. Breathing rates increase during pregnancy, leading to greater inhalation of polluted air during a smoke event. This heightened exposure to PM2.5 is associated with increased risk of maternal high blood pressure and diabetes, as well as increased chance of miscarriage. Recent evidence shows that PM2.5 can cross the placenta, which is significant because fetal PM2.5 exposure is associated with low birthweights and preterm birth, both of which are risk factors for metabolic diseases later in life. Lastly, negative effects of wildfire events – including evacuation, loss of loved ones, loss of property, and displacement – can increase maternal stress, further enhancing risk of adverse birth outcomes.   

Occupation influences exposure to wildfires. Especially vulnerable are outdoor workers, such as landscapers, construction workers, and agricultural workers, not only because of increased exposure to wildfire smoke, ash, and soot, but also because their jobs often involve physical activity. Recent regulations in California for agricultural workers have created guidelines based on the air quality index (AQI), which employers must follow if they anticipate wildfire smoke exposure. These regulations define an AQI value of greater than 150 as unhealthy for all individuals, and employers must implement a plan to reduce harm to the workers. In addition to administrative controls such as shifting work schedules, employers must also provide personal protective equipment such as masks and adequate training on using the equipment effectively. Further research and outreach is needed to guarantee worker safety from these additional hazards and to inform workers of protection strategies.

Households and communities with high social vulnerability may also be disproportionately affected by wildfires. Of the 29 million Americans who live with significant wildfire risk, 41% are socially vulnerable. The capacity to adapt to wildfires is driven by socioeconomic status, language and education, housing and transportation, health, and demographics. These factors influence an individual’s ability to afford fire mitigation strategies such as tree trimming to protect their homes, air purifiers and face masks to mitigate health effects from poor air quality, especially if a home is not well insulated, and insurance coverage to rebuild following a fire. Resources to protect or recover from wildfire events may not be accessible to individuals in rural areas, low-income neighborhoods, and immigrant communities. For example, federal assistance that is available to homeowners following a wildfire is often not available to renters. Lastly, socially vulnerable communities may not be given a voice in policy-making decisions relevant to wildfire risk in their area, further contributing to environmental injustice.  

Approaches for mitigating wildfire risks and impacts

One hundred years of fire suppression and logging in the western U.S. and elsewhere has resulted in dense, uniform forests that are more prone to drought, disease, and severe fires. Thus, one approach to mitigate wildfires is to bolster forest resilience via ecological forestry, which aims to remove fuels while protecting the oldest trees and minimizing soil disturbance. This involves strategic forest thinning to remove fuels mechanically or by prescribed grazing in combination with prescribed fire (controlled, intentional burning) and managed fire (allowing natural fires to burn in safe areas). However, the pace and scale of these efforts is impeded by lack of funding, unpredictable weather conditions due to climate change, environmental regulations, and public perception of risk.

There are a number of preventive measures that individuals can implement to mitigate the risk and impacts of wildfires on one’s home and personal health. The former involves managing landscaping to reduce fuel sources, proper storage of combustibles, and use of non-combustible or fire-resistant materials in building construction. Another critical strategy for mitigating risk is to create an evacuation plan and prepare an emergency bag of personal items that is easily accessible in the event of rapid evacuation. During a wildfire event, exposure can be minimized by keeping windows and doors shut, using high-efficiency particulate air (HEPA) filters on air handling systems, and wearing a properly fitted N-95 mask. The effective management of wildfires will require the creation of fire-adapted communities that work collectively to mitigate wildfire risks in the WUI through policies and regulations.

In addition to managing forest, watershed, WUI, and residential environments, advanced technologies will be crucial for effective wildfire mitigation. These technologies include computational fire spread modeling, distributed networks of remote monitoring devices like high-definition cameras and drones, and development of efficient fire-retardant transporters. For example, the California firefighting agency employed computer programming and artificial intelligence to predict fire patterns that facilitated effective containment of 96.5% of wildfires under 4 hectares in 2021. High-definition cameras capable of discerning wildfire smoke and monitoring wind are being placed over rural areas in California, Oregon, and Nevada. These cameras allow fire managers to remotely monitor local environmental conditions and fire status in real-time. Together, such technologies enabled firefighters to respond to fire incidence more quickly and efficiently, preventing and reducing fire burn areas and duration. Technological advancements like these rely heavily on research and development from private companies, industries, academic institutions, and non-profit organizations. Therefore, it is vital to provide sufficient funding to these institutions to advance firefighting technologies. Government agencies also play a critical role in wildfire prevention by implementing regulations, laws, and even capital punishments for practices that promote wildfires.

Take home messages and the path forward

In recent years, wildfires have become a global concern, threatening our safety, livelihoods, and well-being. The toxic components in wildfire smoke drift across borders and oceans, while ash and soot deposit toxic chemicals in soil and water, posing serious threats to human health. Adverse health effects may be immediate or long-lasting, physical or mental, and are more severe in various vulnerable populations.

As global temperatures continue to rise, driven by increased atmospheric concentrations of greenhouse gases, wildfires will worsen. Warmer and drier conditions lead to longer fire seasons with larger, faster-moving, and more severe wildfires. In turn, wildfires themselves exacerbate global climate change, releasing more greenhouse gases and depositing soot on faraway snow and ice to accelerate their melting.  

We must break this vicious cycle. In addition to mitigating individual exposure to wildfire smoke, we must take measures to prevent and reduce the risk of wildfires, such as forest management and implementation of policies to mitigate wildfire risk and impacts in high risk areas like the WUI. However, these measures are not enough. In order to truly mitigate the threats posed by wildfires, we must invest resources to address critical gaps in our understanding of wildfires, and we must urgently address the drivers of global climate change. With a global, policy-led effort to quickly and dramatically curtail the warming of the planet, we will be able to diminish the wildfire peril that currently looms large.

“I will not be silenced while the world is on fire – will you?” -Greta Thunberg.