Shifting the Conversation on Wildland Fire Smoke Exposures: More Smoke Within and Across Years Requires a New Approach to Inform Public Health Action

Abstract

With the increase in acres burned from wildfire over the last few decades, wildfire smoke is an increasing global public health threat. To date, wildfire smoke research, risk communication, and public health action has focused on short-term (or daily) smoke exposures. However, the patterns of wildfire smoke exposure are transitioning to include longer duration and repeated exposures occurring within and across years. Epidemiologic and experimental studies represent important lines of evidence that have informed risk communication and public health actions for short-term smoke exposures; however, they have yet to provide the science needed to refine public health approaches to include other dynamic exposure durations such as repeated, episodic, or cumulative. This commentary provides an overview of methodological approaches used and recent findings from epidemiologic and experimental studies that examined longer duration, repeated smoke exposures. Based on the current science, we recommend that future epidemiologic and experimental studies of wildfire smoke examine multiple exposure metrics to capture the duration, frequency, and intensity of exposures. Such studies would improve the science produced to best support the needs of the public as we strive to further protect public health in a world projected to have more smoke.

Graphical Abstract

Introduction

As wildfire risk has increased globally, it has corresponded with an increase in area burned ^{1, 2}. The smoke emitted represents a complex mixture of pollutants, including water vapor, gases (e.g., carbon dioxide, carbon monoxide), volatile organic compounds (VOCs), and particulate matter (PM) ³. PM, specifically fine particulate matter (PM_2.5; particles with an aerodynamic diameter generally less than or equal to 2.5 μm), has been the primary focus of research efforts on wildfire smoke and health ⁴. This focus can be attributed to PM_2.5 being one of the biggest contributors to smoke ⁵ and the extensive scientific evidence supporting a relationship between short-term (often referred to as daily) ambient PM_2.5 exposures (i.e., exposures experienced on a typical day from anthropogenic sources such as industrial facilities and automobiles, not wildfire) and respiratory- and cardiovascular-related health outcomes and premature mortality ⁶.

In the US, the increase in wildfire smoke is leading to a unique situation. Air quality has improved continuously in response to regulatory actions under the Clean Air Act that have led to reductions in air pollutant concentrations including PM_2.5, but this trend has started to change in the West where wildfire smoke has become the dominant source of PM_2.5^{7, 8}. Wildfire represents a growing threat to public health worldwide, and in the US this is evident by the substantial increase in acres burned over the last three decades ^{9, 10}. While fires themselves can be directly devastating to individuals and communities ¹¹, the smoke they emit can impact orders of magnitude more people and is a larger threat to public health ^{12, 13}.

The decades of studies examining the health effects of short-term ambient PM_2.5 exposures in combination with more recent studies focusing specifically on wildfire smoke, provide an extensive evidence base that has been used to support public health actions around smoke¹⁴. However, within the US, the frequency of substantial smoke events has increased and resulted in a major shift in the dynamics of smoke exposure. This shifting pattern of smoke exposure is not limited to the US, but provides an example depicting the complexity of the exposures being experienced. While people are still experiencing short-term smoke exposure from singular wildfire events, which has well-document health risks ^{15, 16}, in many places smoke exposures are lasting longer (e.g., weeks) and in some cases occurring each year and/or multiple times during a year. As a result, what once were isolated, singular short-term smoke events where the exposure-health relationship was well understood, have turned into unique exposures. These exposures are highly temporally variable within and across years and much different than the long-term exposures (often referred to as annual or chronic) assessed in studies of ambient PM_2.5.

Adding another layer of complexity to wildfire smoke exposures is the expansion of prescribed fire, particularly in the US and Australia. Prescribed fire is one of the key approaches being implemented to combat the increased occurrence of large and catastrophic wildfires by reducing the amount of fuels available, but it too emits smoke, albeit likely at lower concentrations than wildfires ^{17, 18}. Within the US, it is currently estimated that wildland fire (i.e., wildfire and prescribed fire) represents the biggest contributor to primary PM_2.5 emissions with wildfire representing approximately 29% and prescribed fire approximately 13% ¹⁹. At the scale of prescribed fire being proposed by US federal agencies, upwards of 50 million acres treated over the next ten years, prescribed fire will also result in temporally variable smoke exposures within and across years for some communities at levels they have not commonly seen to this point ²⁰. Therefore, the increase in prescribed fire projected into the future, in combination with the projected increase in wildfires anticipated due to climate change, will result in more smoke being emitted into the air on a yearly basis ²¹.

The combination of prescribed fire and wildfire yields a scenario where communities not only experience short-term smoke exposures but also exposures within and across years that are more variable compared to the patterns of ambient PM_2.5. The unique nature of wildland fire smoke exposure requires a shift in the conversation to (1) improve the lexicon used to discuss how humans experience smoke, (2) design relevant epidemiologic and experimental studies, and (3) encourage consistent terminology when examining these different exposure paradigms. This shift is imperative to provide appropriate scientific data to further inform risk communication and public health actions that are responsive to community concerns around smoke impacts in a new wildland fire landscape.

While short-term exposures to both ambient PM_2.5 and wildland fire smoke may be analogous in research studies, repeated wildland fire smoke exposures within and across years are not easily captured through traditional long-term exposure metrics (e.g., annual average concentrations) used in epidemiologic and experimental studies assessing health effects, as they do not capture the unique frequency, duration, and intensity of wildland fire smoke exposure ²². Thus, we seek to distinguish long-term ambient PM_2.5 exposures, which are often referred to as “chronic” or “annual” and are characteristically low level and relatively consistent over time, from the dynamic wildland fire smoke PM_2.5 exposures, which are much more variable. As such, throughout this commentary wildland fire smoke exposures will often be referred to as repeated, episodic, or cumulative. ^23–26.

Addressing the most pressing scientific questions around wildland fire smoke exposure occurring within and across years relies on scientific evidence spanning different scientific disciplines, including epidemiology and toxicology which play key roles. This commentary highlights research conducted to date to inform the broader understanding of longer duration (i.e., repeated, episodic, and cumulative) wildland fire smoke exposures; current limitations in the approaches used; and future research directions that could better aid in refining risk communication and preparing individuals and communities for this growing public health threat.

Using Epidemiologic Studies to Inform Public Health Action on Wildfire Smoke

Epidemiologic studies have been instrumental in informing our understanding of the health implications of wildfire smoke and subsequently providing the information to support risk communication and public health action. Initially, as wildfires were once considered relatively rare events, epidemiologic studies of wildfire smoke were quite challenging to conduct due to both the short duration of exposures and the small sample size of people impacted, resulting in epidemiologic studies comparing health effects associations before, during, and after the smoke event or conducting surveys ^{27, 28}. As a result, up until around 2010, relatively few epidemiologic studies had been conducted focusing on wildfire smoke. This contributed to much of the initial understanding of the relationship between wildfire smoke exposure and health relying on the extensive evidence base from ambient PM_2.5, which demonstrates a range of health effects (e.g., respiratory and cardiovascular-related outcomes and mortality) in response to short-term exposures using a 24-hour average exposure metric ^{6, 29}.

More recently, as the number of substantial wildfire smoke episodes have increased, there has been a marked increase in epidemiologic studies conducted, but still with an emphasis on examining short-term smoke exposures and health. While these studies have used different exposure indicators (e.g., wildfire-specific PM_2.5, smoke density, total PM_2.5 with an indicator variable for smoke day) there is consistent evidence for impacts on respiratory, specifically asthma-related, outcomes, with inconsistent findings for cardiovascular-related outcomes, and fewer studies examining mortality and other health outcomes (e.g., birth outcomes) ^{15, 17, 30}.

Epidemiologic Studies that Examined Smoke Exposures Within and Across Years

In assessing epidemiologic studies that have attempted to account for smoke exposure within and across years, the following section focuses on those studies that used quantitative methods to develop an exposure metric to account for these longer duration repeated exposures. Studies that have used such quantitative methods primarily focus on examining associations between wildfire smoke exposure and physical health outcomes (e.g., respiratory- and cardiovascular-related outcomes, mortality), which is the focus of this section, versus mental health outcomes.

To date, a few epidemiologic studies have been conducted that capture some aspects of the frequency, duration, and intensity of wildfire fire smoke exposure. One such approach that has been employed is the metric of the “smoke wave” (defined as multiple days with PM_2.5 concentrations above a specific threshold), but these studies have focused on short-term exposures rather than capturing other durations of smoke exposure ^{31, 32}. Epidemiologic studies broadly have yet to identify an exposure metric (or metrics) that would aid in differentiating health effects between long-term ambient PM_2.5 and repeated, episodic, or cumulative wildfire smoke exposure. As a result, the annual average approach often used for ambient PM_2.5 is also being used for wildfire smoke. For example, Zhang et al. (2023) averaged smoke-specific PM_2.5 concentrations over a defined time-period (i.e., 10-years) prior to follow-up in examining incident dementia in the US ³³. In addition, Ma et al. (2024) used a 12-month moving average exposure metric to estimate the increase in monthly mortality attributed to wildfire-PM_2.5 across all US Counties, ³⁴. The exposure metrics used in both studies, however, are unable to capture the dynamism in smoke exposure people may experience over time. In addition, such a metric equally distributes smoke exposure over a year (or years) and is unable to represent how intense smoke from one wildfire may affect health differently than smoke from one or more longer but less intense wildfires. In contrast, a few studies used a cumulative exposure metric in examining the relationship between annual wildfire-related PM_2.5 exposure and cause-specific cancer mortality as well as other mortality outcomes within the UK Biobank cohort ^{35, 36}. The authors estimated smoke exposure within and across years as the 3-year cumulative concentration of wildfire-specific PM_2.5 prior to death or end of follow-up. Although this approach doesn’t explicitly account for the dynamic nature of exposures over time, by focusing on the cumulative exposure it may better account for the variability in smoke concentration experienced during a year and across years than an annual mean concentration.

While there is a growing body of literature on the impacts of wildfire smoke on birth outcomes, these studies often employ traditional approaches used for ambient PM_2.5 exposure by averaging concentrations over months, trimesters, or the full pregnancy, and do not attempt to disentangle frequency, intensity, and duration of exposure in ways that could be informative for protecting the health of birthing parents and infants ³⁷. However, some studies are trying to incorporate such approaches by examining number of days of wildfire-specific PM_2.5 above concentration cut points (e.g., 5 μg/m³) during each trimester in addition to average wildfire-specific PM_2.5 during each trimester ^{38, 39}, which could inform understanding of the frequency of intense smoke exposures on birth outcomes.

Additional Epidemiologic Studies that Inform Smoke-related Health Effects

While relatively few studies have attempted to examine repeated, episodic, or cumulative wildland fire smoke exposure and health, some studies have examined exposures longer than a few days (e.g., weeks, months) and health effects at some time in the future, sometimes referred to as delayed effects. While these health effects cannot be conclusively labeled as “delayed”, since the etiology of these outcomes has not been elucidated, it is an effort to convey the chronological separation of exposure and effect. A recent review assessed studies that examined weekly or monthly smoke exposures and health effects at least one year post exposure⁴⁰. In this review, Gao et al. found very few studies that focused on these delayed health effects of acute smoke exposures with most focusing on mental health in response to experiencing a fire and not smoke, yet a few studies reviewed demonstrated associations with future physical health outcomes.

Orr et al. examined lung function changes in subsequent years after the 2017 Rice Ridge Fire in Montana that lasted almost 6 weeks and had average daily PM_2.5 concentrations around 220 μg/m^{3 41}. In the two years following the wildfire in which no substantial additional wildfire smoke exposure occurred, participants’ lung function was significantly reduced and that reduction persisted for a minimum of two years. In another study conducted in Montana from 2009 – 2018, Landguth et al. reported an increased risk of influenza in the winter season following intense wildfire smoke exposures during the summer ⁴². Lastly, Xue et al. found evidence of an association between monthly landscape fire PM_2.5 and child mortality in children under the age of 18, but not specifically in those less than age 5 years of age⁴³.

Final Thoughts on Epidemiologic Studies of Wildfire Smoke Exposure

The combination of studies focusing on alternative exposure durations (e.g., annual, cumulative, and count of days above a threshold concentrations) as well as studies assessing the health implications of weekly and monthly exposures collectively provide initial evidence of the health ramifications of wildland fire smoke outside the traditional focus on short-term exposures. Relatively few epidemiologic studies, however, have been conducted to date examining alternative wildfire smoke exposure durations and health that incorporate measures of duration or frequency, in addition to intensity, of exposure. Additionally, there have been no epidemiologic studies examining patterns of exposure for prescribed fire smoke which will likely have its own unique exposure patterns and add to those smoke exposures already being experienced from wildfire. The contrast in the patterns of exposure between ambient PM_2.5, wildfire smoke, and prescribed fire smoke complicates the ability to use studies of long-term ambient PM_2.5 exposure to further inform public health messaging to encompass longer duration smoke exposures. While the total evidence base is small, it does indicate the need for more research that attempts to understand the health impacts of longer duration and episodic exposures to wildfire smoke to refine and improve public health messaging to protect people from these exposures.

Examining Wildland Fire Smoke Exposures and Health in Experimental Studies

While epidemiologic studies can capture the health implications of smoke exposure at the population-level under real-world conditions, experimental studies play an important complimentary role in furthering our understanding of the relationship between different durations of wildfire smoke exposure and health. Specifically, experimental studies can elucidate the biological mechanisms by which exposure to wildfire smoke can elicit health effects and can do so under controlled conditions, which can address research questions that cannot be broached in epidemiologic studies.

As the types of smoke exposures change due to the overall increase in wildland fire smoke that is anticipated, animal models and the corresponding exposure characteristics also must change to mimic the human experience more closely. Such a change would take into consideration the route of exposure, as well as its duration, frequency, and intensity. In addition, experimental studies, compared to epidemiologic studies, provide a unique opportunity to explore and characterize variability in the chemistry of smoke exposures. This variability could occur under different real-world situations and potentially lead to differential health effects.

Potentially the greatest hurdle to examining the health effects of wildland fire smoke exposure in the laboratory is the development of an appropriate model that represents the human experience during wildland fire smoke episodes. In Figure 1, we demonstrate the complexity of modeling exposures that represent the human experience, as patterns of PM_2.5 exposure can vary substantially between ambient (i.e., anthropogenic sources) or wildland fire exposure scenarios. As shown in Figure 1, the dynamism of the wildland fire smoke exposure scenario highlights the need to consider duration, frequency, intensity (PM_2.5 concentration) and the potential for highly temporally variable exposures within and across years. Additionally, Figure 1 raises questions such as:

• What are the health effects and do the health effects differ for smoke exposures over different concentration thresholds such as 100, 400 or 600 ug/m³ (Figure 1A)?

• Is the health effect transient and does it subside after a defined period of recovery time? For example, there were approximately 600 days between major events in Chico compared to approximately 1,000 days for Boise (Figure 1).

• What is the best way to summarize an exposure (e.g., annual average, event average, quartiles, number of days above a certain concentration threshold, etc.)?, Figure 1B highlights the variability in the number of days and magnitude of PM_2.5 concentrations between different locations impacted by smoke.

Figure 1. Comparison of PM_2.5 concentrations between different locations.

The graphs in Figure 1A show PM_2.5 concentrations that occurred between 2017 and 2020 in four different community settings to demonstrate the dynamism of wildland fire smoke in contrast to more traditional ambient PM_2.5 exposures from anthropogenic sources such as industrial and automobile emissions. The light orange bars represent a typical wildfire season in the US and the additional graphs for Chico and Seeley Lake are included to present the full peaks in PM_2.5 concentrations. A summary of exposure characteristics are shown in Figure 1B including the number of days where air quality at regulatory monitors exceeded thresholds that coincide with air quality index levels of moderate (> 9.0 μg/m³), unhealthy for sensitive groups (35.4 μg /m³), unhealthy (55.5 μg /m³), and very unhealthy (125.4 μg /m³). The three Western US communities referenced in Figure 1C are commonly impacted by wildfire smoke exposures that vary in duration, frequency, and intensity compared to the ambient PM_2.5 concentrations experience by those in the representative community of New Albany, OH. Underlying data used to produce graphs available at https://www.epa.gov/outdoor-air-quality-data/download-daily-data.

How these exposures are translated into the laboratory setting is vital for research groups to compare results and make efficient advances in the field to best inform and refine public health action to encompass all exposure durations.

Laboratory modeling of human exposures

In addition to variation in duration and dose from single exposures or episodic (days or weeks) exposures from multiple events within and across years ⁴⁴, exposures from wildland fire smoke PM_2.5 also have the potential for wildly variable chemistries based on multiple factors, including fuel source, flaming/smoldering, and age of smoke. All these unique aspects of smoke exposures illustrate the difficulties in assessing and defining community and individual exposures, as well as modeling in the laboratory setting, especially in the context of longer duration exposures.

Smoke Composition

Animal studies have established that toxicity is dependent on individual fuel source (e.g., peat, eucalyptus, and oak) for a single exposure model⁴⁵. In addition, whether an individual is proximal or distal to the fire source impacts the composition of the smoke, in particular the particulate to gas ratios. Communities or outdoor workers near the fire may be exposed to high concentrations of gases (e.g., CO) and PM directly emitted, whereas PM but not gases tend to be of greater concern for those living farther away^{3, 46}. This juxtaposition is important in the context of designing experimental studies and the questions they can inform. For example, Hargrove et al. exposed mice to eucalyptus smoke for 1hr/day for 3 days using a nose-only system under flaming conditions with and without a high efficiency particulate air (HEPA) filter and found that removing the particles partially, but not fully, ameliorated the negative pulmonary effects seen in the total smoke exposure group ²⁴. Another consideration is “smoke aging” which occurs as particles travel and interact with the atmosphere and sunlight which may result in increased reactivity ^{23, 47}, and the production of ground-level ozone, which can be formed downwind of a fire. These contributors to toxicity are important to describe in laboratory settings as they could inform on potential future observations of health outcomes that differ between similar exposure concentrations at the population-level. This approach would then allow for comparisons of wildfire events from distinct locations, such as the Quebec wildfires of 2023 in local towns versus those exposed on the eastern coast of the US.

Another concern is the issue of human-made materials and structures affecting the chemical composition of smoke in the case of wildfires that enter the wildland-urban interface, as noted above when comparing a Montana wildfire with one in California. For example, there are distinct differences in smoke from a fire that consumes human-made materials and structures, such as the Camp Fire in California with over 19,000 buildings burned, compared to an almost exclusive vegetation fueled fire (a single structure was consumed) in the Rice Ridge Fire in Montana^{23–25, 47–49}.

Additionally, burn conditions such as flaming versus smoldering have also been shown to impact the chemical profile of smoke and subsequently observed effects in laboratory studies ^{25, 48, 49}. This variability in chemistry can complicate comparisons of potential health effects between exposures.

Smoke Generation and Route of Exposure

Challenges in laboratory-based exposure paradigms exist as they relate to the method of smoke generation and route of exposure. Smoke exposure can be direct or indirect. Indirect exposure includes collecting particles (most often the gases are not preserved) on filter material, in liquid suspension or by sedimentation and then after some storage period, delivering the particles to the animal or cell/tissue. Indirect exposure delivery can vary by instillation technique each with its own benefits and limitations ^50–52. Direct exposure is often accomplished using some version of a biomass burning system which produces smoke (both particles and gases) and then immediately delivers the smoke to the animals at a target concentration ^{53, 54}. Some research teams concentrate the wildfire smoke collected during an event and deliver the concentrated PM directly to the animals ⁵⁵. Routes of exposure for direct exposures also vary across laboratories to include whole body chambers or nose only. A benefit to laboratory smoke generation is the ability to tightly control the fuel, burn condition, and delivered concentration. In this setting, research teams can also routinely observe the chemical profile and PM size distribution of the smoke, which is useful in understanding potential differential toxicity or differential health effects. These indirect and direct exposures do not uniformly model a specific human experience because some studies select for PM only while others include a mixture of gas and PM and some instillation techniques bypass natural defense mechanisms (e.g., intratracheal instillation) whereas others are more like what a person experiences when outside living and working in an area affected by wildland fire smoke.

Dosing Regimen

There is currently little consensus in the field for selecting an appropriate dosing regimen or timeline when attempting to recapitulate longer duration exposures. We find that some laboratory models used the PM_2.5 concentrations experienced by humans, some attempted to translate an exposure to animals by calculating relative particle deposition based on either species’ lung function or surface area, and others based exposures on gas (e.g., CO, CO2) concentrations and combustion efficiency ^{24, 53, 55–57}. In addition, there are different approaches for modeling cumulative exposures. For example, there are studies that model a single prolonged community wildfire smoke event or an entire career of a firefighter by calculating a mouse-specific deposition amount and dividing this over shorter and more feasible exposure windows ^{53, 56}. The lifespan of the model organism in relation to the exposure duration and expected deposition is also important to consider. Cumulative exposure paradigms could include animals across the lifespan to appropriately capture risk of health effects due to smoke exposures within and across years within a community following exposure from gestation through geriatric periods.

In summary, as illustrated in Figure 1, the general types of exposures people may experience in response to wildland fire smoke will differ. All experimental methods should consider what human experience is being modeled and pragmatically balance this with feasibility. Relevant smoke exposure details and limitations should be clearly discussed such that the reader can weigh and appropriately interpret the data and make reasonable comparisons between laboratories.

Discussion

Wildfire smoke is an increasing source of PM_2.5 ⁷ and the increasing length of the wildfire season is changing the pattern of smoke exposure ⁵⁸. Such changes are resulting in the need to more clearly communicate the health risks of smoke to the public in the face of these new wildland fire smoke exposure durations to ensure public health protection while also balancing the need to maintain healthy lifestyles (e.g., outdoor exercise, school activities, and sporting events).. Unfortunately, the traditional scientific approaches used to understand the health risks from long-term ambient PM_2.5 exposure are not sufficient in the context of wildland fire smoke. This is because smoke exposures are more dynamic over longer durations, vary substantially within and across years, and the approaches to protecting public health from wildland fire smoke are rooted in individual- or community-level behavioral change resulting in exposure reduction versus regulatory actions (e.g., instituting engineering controls to reduce PM_2.5 emissions from point and mobile sources). While a well-documented and expansive evidence base has existed to adequately inform the health risks and public health actions around short-term wildland fire smoke exposure, a different evidence base is needed to further refine risk communication and support public health protection during longer duration exposures that are repeated and episodic¹⁴. Layered on top of these dynamic exposure patterns is the fact that smoke exposures overall are projected to increase as a result of not only the continued impact of climate change on wildfire, but also an increase in prescribed fire that is occurring in some countries (e.g., US and Australia) ^{59, 60}. The combination of wildfire and prescribed fire will lead to highly temporally variable smoke exposures that will vary across populations, and in the case of prescribed fire depend on proximity to the fire ^{18, 61}. The collective increase in smoke from wildland fire will ultimately change the smoke exposure patterns we have been accustomed to and the corresponding public health implications. Unlike wildfire where public health action around smoke has often been reactionary, with prescribed fire, due to its planned nature, opportunities exist to inform and prepare communities to reduce smoke exposures⁶². Yet even with an expansion in public health action around prescribed fire, it is inevitable that wildland fire smoke exposure patterns and durations will change.

As we embark on the scientific exploration of the health implications of the additional wildland fire smoke exposure durations of concern discussed in this commentary, an agreed upon terminology would benefit the field in understanding and categorizing the resulting health effects of such exposures. A common vernacular would also help with literature searches, translation, and interpretation across studies. However, we note this may ultimately prove difficult as our team, which represents different disciplines, had trouble finding consensus when writing this commentary. Ultimately, this led to terms such as dynamic, repeated, episodic, and cumulative being used throughout as our team’s attempt to contrast wildland fire smoke exposures with the more consistent, low-level nature of ambient PM_2.5. If consensus language is not achievable then the next prudent option is for research teams to clearly articulate how they are evaluating smoke exposure in their study, taking into consideration dose, intensity, duration, and frequency. To provide the most influential science to support the public health needs of those communities most impacted by smoke in this new reality of repeated smoke exposures within and across years, the research community needs to grapple with the fact that the traditional study design approaches used for ambient air pollution may not be adequate due to the variability in the frequency, intensity, and duration of smoke exposure ²².

As a first step, we need to ensure that future studies inform the most pressing questions being asked by the public such as “what are the health implications of being exposed to smoke every year?” and “will the smoke my children are exposed to from this wildfire season have long-lasting health effects?”. Designing studies to address such questions requires the recognition that protecting public health from smoke is a public health action approach that relies on behavior change and not a regulatory one. For example, in the context of setting U.S. National Ambient Air Quality Standards (NAAQS), it is necessary to understand all aspects of the air pollution source-to-health effect relationship, including detailed evidence of biological mechanisms, to support a scientifically defensible decision on the level (i.e., concentration) of the standard that can be defended in the court of law. In contrast, for wildfire smoke, the regulatory tools used for other air pollution sources, such as engineering controls, will not work and thus the research focus should be on providing evidence to support risk communication and the protection of public health through actions. Therefore, a complete understanding of all facets of the smoke to health relationship is less needed and more research is needed into studies that can further inform risk communication to support public health protection. Such an approach requires the delineation of scientific questions between those that can inform a basic science understanding of smoke and health, and may be interesting scientifically, from those questions where the results can directly support risk communication and public health action. This is a new way of approaching an air pollution question, but a shift that is warranted as many people across the country are impacted by smoke and looking for answers.

In addition, while not a new concept, we need to continue to strive to improve linkages between epidemiologists and toxicologists. This interaction is imperative as we grapple with not only terminology around these alternative wildland fire smoke exposure durations, but also to ensure the studies conducted are providing the science that can inform these new needs around wildland fire smoke risk communication and public health protection. Collectively ensuring that the exposures being examined, specifically in the context of experimental studies, capture the human experience to wildland fire smoke exposures can help produce the science needed to address public health concerns. One such example of examining exposures relevant to the human experience was the study by Black et al. (2017) that examined the longer-term health implications of early life smoke exposure on a cohort of rhesus macaque monkeys that were unexpectedly exposed to wildfire smoke⁶³. Experimental studies such as this, while challenging to design, that can utilize “real-world” exposures can help bridge the gap between epidemiologic and experimental evidence.

Lastly, the limitations around current epidemiologic and experimental studies with respect to their ability to inform our understanding of longer duration smoke exposures reflects current exposure measurement and modeling tools ⁶⁴. As noted throughout this commentary, wildland fire smoke exposures are more dynamic than ambient PM_2.5 exposures and as such require new and innovative approaches to account for the repeated, episodic, and cumulative exposures people may experience. Ultimately, the ability to address the questions needed to further refine risk communication and public health action to also include these exposures will be predicated on the ability to develop new exposure measurement and modeling tools that are able to account for the dynamism of the exposure. From an epidemiologic study perspective, one such approach could be similar to the one employed by Smolker et al. (2024) in a study of ambient PM_2.5 and adolescent behavior. In this study, instead of using one exposure metric (e.g., annual average concentrations), multiple exposure metrics (including days above 35 μg/m³) were used to capture the dynamism of the PM_2.5 exposure ⁶⁴.

In conclusion, as we are faced with new air quality challenges due to wildland fire smoke and new patterns of exposure, scientific questions are being posed that require new and innovative approaches. By keeping public health action as our ultimate goal, we, as a research community, can provide the science to further inform and refine risk communication and exposure reduction measures around all wildland fire smoke exposure durations to protect public health in the face of this growing challenge.

Synopsis.

Assessing the health implications of longer duration wildland fire smoke exposures requires that epidemiologic and experimental studies embark on developing and testing new approaches to account for these dynamic exposures.