This summer, once again, fires burned across Colorado and our air has been further polluted by smoke that wandered into Colorado from California and Canada. There have been evacuations along the Front Range, and we have all become more sensitized to the real possibility of disaster since the 2021 Marshall Fire. Wildfire season continues through September, so this is a good moment to discuss what we know about wildfires and the health risks they pose. 

Why have wildfires become more severe? 

When I started writing this commentary on July 28, the National Interagency Fire Center was reporting 103 active fires across the US that had burned more than 2 million acres. The notorious Park Fire near Chico, California—started by an arsonist—had already burned over 170,000 acres. There was not an active fire in Colorado at that moment, but the Front Range was blanketed by visible smoke that traveled from faraway fires. By August 13, the Front Range had experienced four fires, now controlled, and the Park Fire had consumed more than 400,00 acres. Looking back, the 20 largest fires in Colorado’s history burned in the 20th century with 2020 having the three largest: the Cameron Peak, East Troublesome, and Pine Gulch fires. The 2021 Marshall Fire was the costliest ever for Colorado, destroying more than 1,000 residences and other structures in Boulder County.   

Surprisingly, the area burned by wildfires over the last 100 years has diminished in the United States. However, the impact on communities has increased, the fire season has lengthened, and the severity of the fires has worsened. Climate change, bringing higher temperatures and long-lasting droughts, is one increasingly powerful driver. Forest management practices such as fire suppression tactics over the last century have also enhanced risk, creating conditions that foster the eruption of large wildfires. Plus, as Coloradans know too well, increasing development at the wildland-urban interface has heightened risk for explosive fires, like the Marshall Fire, that destroy structures and take lives.  

Wildfire Smoke 

We have learned much about the composition of wildfire smoke by collecting data by flying heavily instrumented airplanes through fresh plumes and by collecting the particles and gases in smoke after it has traveled to distant locations. Analogous to tobacco smoke, burning organic material at high temperatures generates numerous compounds, including carcinogens and toxins. Many of these compounds are classified as hazardous air pollutants by the Environmental Protection Agency (EPA). Metals are present as well. Wildfire smoke loads the atmosphere with carbon particles in the small size range referred to as PM 2.5, the very small particles regulated by the EPA that penetrate deeply into the lung. Visibility is diminished by their presence, a phenomenon often noticed in Colorado. Wildfire smoke also contains precursors for ozone production, worsening our Front Range ozone problem. When fires at the wildland-urban interface burn buildings and their contents, the chemistry of the resulting smoke differs from that of wildfire smoke depending on what burns. The National Academies of Sciences, Engineering, and Medicine has proposed a research agenda to better understand the composition and health risks of the unique fires at the wildland-urban interface. 

We know when wildfires are burning. Using satellite imaging and air pollution monitoring data, the presence of fires and the spread of smoke plumes can be tracked, keeping downwind communities informed and supplying data that can be used for research. We know that wildfire smoke can travel hundreds and even thousands of miles. Coloradans will remember polluted days from the long-range transport of smoke from California and Canadian wildfires. The AirNow Fire and Smoke map is a valuable resource for tracking fires and smoke. 

Figure 1. EPA’s Fifth National Assessment, Impact of Wildfires  

Health Risks 

The figure above from EPA’s Fifth National Assessment provides a broad overview of wildfires, covering their origins and consequences. While I focus on human health, the figure captures the sweeping consequences of wildfires, including degradation of water, damaged ecosystems, loss of valued recreational and cultural areas, and destruction of and damage to structures. The human health concerns related to inhalation of wildfire smoke mirror those for air pollution generally and include irritation from inhaling smoke, worsening of asthma and chronic obstructive pulmonary disease, and possibly increased risk for respiratory infection (see the table). An analysis of health data for the western United States showed that wildfire smoke enhanced the overall risk of airborne particles creating risk for COVID-19 cases and deaths. For the longer term, extending the general findings on the risks of airborne particles to wildfire smoke specifically implies that smoke particles could have diverse adverse effects on health and even shorten lifespan. I participated in a multi-country study to estimate the burden of premature mortality from wildfire smoke. Overall, about 0.6% of deaths in the general population were attributed to wildfire smoke.   

Table 1. Health effects of wildfire particulate matter 

When it comes to wildland firefighters, they have far higher exposures to wildfire smoke than the general population, especially when combatting fires at the wildland-urban interface. The potential risks, however, are not well studied. Generally, the concerns reflect those around exposure to airborne particles and toxic gases. Recently, one definitive determination has been made on cancer. In 2022, the World Health Organization’s International Agency for Research on Cancer found the evidence sufficient to classify occupational exposure to wildfire smoke as a definite human carcinogen.  

Wildfires and Mental Health and Wellbeing   

We know that disasters have consequences for mental health and well-being. For the short-term, post-traumatic stress disorder (PTSD), anxiety, and depression may occur, stemming from the multiple stressors of experiencing a wildfire: a threat to one’s life and a loss of property and possessions for some, and the fearsome presence of fires and pollution for many. For example, people exposed to the 2018 Camp Fire in California were at increased risk for PTSD, generalized anxiety disorder, and major depressive disorder six months after the fire. A national study links wildfire smoke to suicide in the United States, finding increased risk for rural residents on days with wildfire smoke exposure. In addition, the mental health concerns around wildfire smoke reflect those linked more broadly to climate change. 

Led by Colorado School of Public Health faculty member Katherine Dickinson, a collaborative program has been launched to track the mental health of those affected by the Marshall Fire and to gauge whether preparedness policies made a difference. The first wave of data was collected six months after the fire from a mix of non-evacuees, evacuees, and people who lost their homes. It showed that 25% were still experiencing high levels of distress at the time. At wave 3 of data collection in the Spring of 2024 over two years after the fire, distress continued for about the same proportion of respondents, but many expressed rising resiliency. Unfortunately, the devastating experience of the Marshall Fire has been repeated elsewhere, e.g., Lahaina.  The findings of the Marshall Fire study will be informative on how well preparedness strategies worked with this disaster and what needs to be done to support victims of disasters as they rebuild their homes and lives.  

Dealing with Wildfire Smoke 

The recommendations for reducing the risk from exposure to wildfire smoke follow those for air pollution more generally. Many channels alert us to the presence of wildfire smoke; beyond media channels, we often detect the presence of wildfire smoke because of its effect on visibility and its smell. The particles in wildfire smoke may also drive up the EPA’s Air Quality Index (AQI) to higher risk levels.   

There are steps that can be taken to lessen exposure both outdoors and indoors. Similar to air pollution generally, exposure can be lowered by remaining indoors if wildfire smoke levels are high and avoiding outdoor exercise, which increases the amount of pollution reaching the lungs. Closing windows decreases the amount of pollution entering buildings. If the building or home has a heating, ventilating, and air conditioning (HVAC) system, it should be operated without drawing in outdoor air. Air cleaning can also be effective at removing particles and gases both by a high-efficiency HVAC filter or a room air cleaner that can remove small particles and gases.   

One overarching, but unanswered, question is whether the health risks of wildfire smoke are greater than those of airborne particles generally. Research on this question is facilitated by models that estimate the concentration of wildfire smoke but are complicated by the difficulty of separating the risks of wildfire smoke from the risks of the air pollution mixture generally. More research is in progress to address the public's concerns and fully capture the health costs of climate change more broadly.  

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Learn More: Book Recommendations 

To go deeper on this subject, I recommend two books. Stephen J. Pyne’s The Pyrocene explores humankind’s relationship with the environment and how fire has changed across the millennia creating conditions for massive conflagrations. Per Pyne, we are in a deepening fire age because we have created conditions for unconstrained fire that he terms “third fire.” Reading this book and remembering the spate of fires in Colorado over the past few years, The Pyrocene made me pessimistic, but Pyne does outline solutions while addressing the root causes of climate change as a central one. 

Fire Weather by John Vaillant is the story of Fire 0009, which burned much of Fort McMurray in the Province of Alberta, Canada in 2016. Fort McMurray is a major site for the Canadian oil shale industry, which involves the extraction of petroleum precursors from the sands. Vaillant tells the story of the horrendous course of Fire 0009, day-by-day and neighborhood-by-neighborhood. Flawed disaster management and poor communications led to a harrowing evacuation of Fort McMurray. He emphasizes how Fort McMurray’s fossil fuel industry contributes to climate change and why its location at the wildland-urban interface with a large arboreal forest created risk for uncontrollable fires. Like Pyne, he reminds us that we have tamed fire, but not wildfires 

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This article is part of a monthly column, The Jon Samet Report, on the biggest issues facing us today in public health, written by the former dean of the Colorado School of Public Health, Jon Samet, a pulmonary physician and epidemiologist, and Professor of Epidemiology and Occupational and Environmental Health. Dr. Samet is a global health leader, shaping the science and conversation on issues ranging from tobacco control to air pollution to chronic disease prevention and more. Each month he shares expert insights on public health issues from local to global.