Informing public health protection under new patterns of wildfire smoke

For years within the United States, wildfire was considered a relatively rare event that would produce smoke and impact communities in close proximity to the fire for rather short durations, often ranging from a few days up to a few weeks. These shorter duration smoke exposures were reflected in some of the initial epidemiologic studies examining the relationship between wildfire smoke and health which focused on assessing whether differences existed between time periods before, during, and after the wildfire event (1). While the number of wildfires occurring on a yearly basis has remained relatively constant, there has been a dramatic increase in acres burned over the last few decades (2). The amount of smoke emitted has correspondingly increased substantially, leading to dramatic impacts on air quality. For example, wildfire is now one of the main sources of primary emissions of fine particulate matter (PM_2.5, particles with an aerodynamic diameter less than or equal to 2.5 µm) and recent evidence demonstrates that the increase in smoke emitted is starting to reverse decadal trends in reductions in PM_2.5 in many locations in the United States (3, 4).

This increase in smoke is resulting in new and unique patterns of smoke exposure across many communities in the United States. Not only are communities being subjected to the traditional, short-term, daily smoke exposures often associated with wildfires, but many are experiencing more sustained exposures occurring over many months, multiple times during a year and/or over multiple wildfire seasons. In this issue of PNAS, Ma et al. (5) provide initial evidence on the relationship between longer duration smoke exposure and health, with a focus on mortality, that informs the continued effort to refine risk communication and public health actions for wildfire smoke.

PM_2.5 is a main component of smoke, and there are well documented health effects associated with ambient PM_2.5 exposure (6, 7). Therefore, consistent with many epidemiologic studies focusing on wildfire smoke exposure and health, Ma et al. (5) use wildfire-PM_2.5 as the exposure indicator. Focusing on all 3,108 counties in the contiguous United States from 2007 to 2020, the authors define long-term exposure as the 12-mo moving average of wildfire-PM_2.5 concentrations. Using a unique statistical model for air pollution epidemiology, a panel fixed effects model, and controlling for nonwildfire-PM_2.5 to estimate mortality attributed specifically to wildfire-PM_2.5, Ma et al. (5) report evidence of an increase in monthly mortality rates for months in the same county when the 12-mo wildfire-PM_2.5 concentration was above 0.1 µg/m³. To assess potential nonlinear effects, monthly mortality rates were examined across nine bins of wildfire-PM_2.5 concentrations spanning 0.1 µg/m³ increments from 0.1 to 0.5, and then from 0.5 to 0.7, 0.7 to 1, 1 to 5, and 5+ µg/m³. Over 75% of county-months had concentrations ranging from 0.1 to 0.7 µg/m³. Across each of the bins, the nonaccidental monthly mortality rate increased by 0.16 to 0.63 deaths per 100,000 people per month for concentrations ranging from 0.1 to 5 µg/m³, with a larger increase of 2.11 deaths at concentrations above 5 μg/m³, though only 0.1% of all county-months had concentrations this high. The observed increase in mortality attributed to long-term wildfire-PM_2.5 exposure was estimated at over 10,000 deaths annually. The combination of information presented in Ma et al. (5), conveying both the health risks and broader public health impacts of wildfire smoke, is instrumental in shedding light on this growing public health issue that now encompasses extended exposure durations, as well as the continued need for community smoke preparedness efforts (i.e., smoke-ready communities).

While Ma et al. (5) represents one of a small number of epidemiologic studies that has examined the relationship between longer duration smoke exposure and health, the analysis highlights uncertainties around the examination of these new and unique smoke exposure durations that justifies additional discussion and exploration. A challenge in embarking on examining these longer duration smoke exposures is the fact that they are highly dynamic and vary in intensity, frequency, and duration within and across years. These exposure patterns are fundamentally different from the types of exposures we are accustomed to in the context of ambient PM_2.5 (8). Smoke exposures are highly variable, both in space and time, and wildfire-PM_2.5 is not present in ambient air every day within a year. Smoke is often limited to specific months of the year and can vary substantially over years (Fig. 1). Patterns of smoke exposure are markedly different from ambient PM_2.5, where the exposure is constant and ubiquitous, and remains relatively stable over time; there are no days during a year when the ambient PM_2.5 concentration is zero (9). In addition, the patterns of smoke exposure are anticipated to change over time in response to a changing climate and the projected lengthening of the wildfire season, further complicating the ability to identify appropriate exposure metrics for wildfire-PM_2.5 exposure.

Fig. 1.

Year-to-year variability in PM_2.5 concentrations impacted by wildfire smoke in Seeley Lake, MT from 2017 to 2019. Color coding corresponds to U.S. EPA Air Quality Index (AQI) categories. The graphics were accessed from the U.S. Forest Service's AirFire Tools website, which can be found at https://tools.airfire.org/historical. The underlying data were obtained from the U.S. Environmental Protection Agency's Air Quality System Data Mart, an internet database available at http://www.epa.gov/ttn/airs/aqsdatamart. The data were obtained on January 3, 2023, and the graphics were accessed on September 18, 2024.

The changing patterns of wildfire-PM_2.5 exposure complicate the use of traditional long-term air pollution exposure study designs that often rely on comparisons across space or between cities. In the context of long-term (i.e., annual average) ambient PM_2.5 exposures, the rank order of locations by annual PM_2.5 concentrations typically remains the same even as concentrations decline over time, allowing for an understanding of the health implications of populations exposed to higher versus lower concentrations (10). With respect to wildfire smoke, there can be a large degree of variability not only within a year, but across years, such that wildfire-PM_2.5 concentrations could vary substantially by location on a year-to-year basis and within a year between locations that may not be far apart. Thus, averaging the exposure to wildfire-PM_2.5 over a year may complicate the interpretation of results. For example, is the health of a population affected by a few days of extreme smoke in the same way as by many days of moderate smoke? Both scenarios could produce the same annual average exposure. With this in mind, as the broader research community embarks on further elucidating the relationship between longer duration smoke exposures and health, it is worthwhile to assess the transferability of exposure metrics (e.g., annual average exposures) used for ambient PM_2.5 to studies of wildfire-PM_2.5.

Currently, it remains unclear whether the traditional approaches used to develop exposure metrics in studies of ambient PM_2.5 can capture the dynamic exposure patterns of wildfire-PM_2.5. Ma et al. along with other recent studies, such as Zhang et al. (11), focus on one approach that relies on a traditional method used in studies of ambient PM_2.5 to develop the exposure metric—the annual average method. However, because the evidence base for health risks of longer duration smoke exposures remains limited, studies such as Ma et al. are needed in combination with additional studies that use innovative approaches to capture aspects of the intensity, frequency, and duration of smoke exposure within and across years to establish a collective body of evidence that allows for a more complete assessment of the exposure–health relationship.

A few recent studies spanning both wildfire-PM_2.5 and ambient PM_2.5 apply different and unique approaches to developing the exposure metric, which could contribute to providing the evidence base needed for assessing longer duration smoke exposures. In a study of wildfire smoke, instead of averaging concentrations over a defined time period, Gao et al. (12) used the 3-y cumulative concentration of wildfire-PM_2.5 to assess the totality of smoke exposure over time. In a study of ambient PM_2.5, Smolker et al. (13) developed multiple exposure metrics including annual average, number of days above a defined concentration cutoff, and maximum daily concentrations, which collectively informed the intensity, frequency, and duration of exposure. Smolker et al. (13) potentially provides a model for how to assess longer duration smoke exposures moving forward, where instead of relying on one exposure metric, studies apply multiple exposure metrics to assess specific aspects of the highly dynamic exposure over time. Regardless of the approaches employed in individual studies, it is the combination of evidence spanning studies using different exposure metrics that will allow for a broader exploration of longer duration smoke exposures and subsequently the information necessary to support efforts to develop more refined risk communication and public health actions. For example, such information on longer duration smoke exposures could improve risk communication regarding the longer-term health implications of smoke exposure overall and specifically on children, as well as aid in the protection of outdoor workers that repeatedly experience smoke exposure.

"In PNAS, Ma et al. provides initial evidence on the relationship between longer duration smoke exposure and health, with a focus on mortality, that informs the continued effort to refine risk communication and public health actions for wildfire smoke."

Although the focus of Ma et al. (5) is on wildfire smoke, as the United States continues to grapple with the wildfire crisis, there is an additional complicating factor that should be noted—prescribed fire. Currently, there is an expansive and coordinated effort across federal land management agencies to increase the scale of prescribed fire to reduce available fuels and try to reduce the risk of large, catastrophic wildfires (14). The substantial increase in prescribed fire that is anticipated will lead to additional smoke emissions. The smoke emitted from prescribed fire, although often at lower concentrations than wildfire, will result in an additional layer of complexity in terms of the spatial and temporal patterns of exposure and the assessment of the longer duration smoke exposures communities will experience (15).

Wildfire smoke is and will continue to be a public health threat in the years and decades to come. In order to effectively protect public health, there is a continued need to ensure the best available science exists to support ongoing efforts to communicate the health risks of smoke and ensure that the most effective public health strategies are developed and implemented. As the protection of public health from wildfire smoke primarily revolves around exposure reduction, continuing to develop an understanding of the health implications of different durations of smoke exposure are paramount especially in the context of ensuring healthcare professionals have the necessary information at their disposal to inform and prepare their patients most at-risk (e.g., people with preexisting heart and lung disease, children, outdoor workers) of potentially experiencing health effects in response to smoke exposure. In this new reality of more persistent smoke exposures, the public continues to ask questions about the potential health risks of smoke and what they can do to protect their health. Additional exploration of the health effects, including mortality, associated with longer duration smoke exposures will allow for such information to be obtained and support more refined efforts to reduce smoke exposure and protect public health.