Study concludes wildfire smoke causes lower infant birth weight

An economics researcher found that infants’ proximity to smoke pollution while in utero affects birth weight.

Above: Whitetail Fire in South Dakota

(Originally published at 6:17 p.m. MST January 22, 2018)

When researchers seek to determine a single or primary cause for a human health problem, they know they’re battling uphill. Our environments are complex, multifaceted, and permeated by a seemingly infinite number of factors that could shape us. Rare is the circumstance that is so ideal, at least from a researcher’s perspective, that one can sift through the noise and emerge with a definitive root of an issue.

That is, of course, unless nature is on your side — as was the case for UNLV economics professor Shawn McCoy and his University of Pittsburgh economics colleague Xiaoxi Zhao.

It’s hard to imagine anything positive coming out of wildfires. They’ve become six times more likely to occur and four times as large since the 1980s, McCoy said, due to climate and population changes. And yet for his research, which demonstrates that proximity to smoke pollution causes lower infant birthweight, wildfires proved to be a sort of equalizer.

“Wildfires are a meaningful topic to research in and of themselves, but they also help solve this causality problem that is difficult in our studies of pollution,” McCoy said. “Two features make fire pollution different from that of, say, an industrial plant: the random timing of fires and their random location, in that wind patterns on any given day drive the direction and concentration of smoke. This sets up a quasi-experimental research design wherein a fire happens randomly and by chance and randomly and naturally assigns treatment and control groups, because only a certain segment of the population will be exposed to the smoke.”

Several studies have established correlations between pollution sources and negative public health outcomes, McCoy said. However, prior research has faced difficulties demonstrating a direct causal relationship. One reason for this, according to McCoy, is the number of factors that could be involved in past research scenarios.

“Suppose we build an industrial plant,” McCoy said. “Once that plant is built, we need to think about the economics of that problem, which is that people don’t like to live next to plants. Holding everything else constant, home prices will drop in the surrounding area because of that, which could induce geographical sorting, wherein households with lower income might migrate into the areas surrounding the plant and households with higher incomes may leave. When that happens, it becomes harder to determine if changes in health outcomes occurred because of plant pollution, geographical sorting dynamics, or even something else.”

The random timing and location of wildfires mitigate these dynamics, making it ideal for McCoy and Zhao’s research. Wildfire smoke is similar to other sources of ambient air pollution; its particulate matter can be so small that it passes through the heart and lungs, disrupts fetal nutrition, and slows fetal growth. Within this framework, birthweight becomes a useful metric to track because of its link to short-term outcomes, such as one-year mortality rates, as well as long-term outcomes such as educational attainment and earnings, McCoy said.

McCoy and Zhao leveraged geographic information systems (mapping software) to identify ignition sources and smoke paths and plotted the home addresses of infants born during a time that would place them in the smoke’s path while in utero. They then compared the birthweight of those infants to a control group outside of the smoke’s path.

The researchers’ results indicate that wildfire smoke leads to a 4 to 6 percent reduction in birthweight, and these effects are most pronounced among mothers exposed to smoke during the second or the third trimesters of pregnancy. They also found that these effects attenuate (or diminish) with respect to distance to a wildfire, becoming ineffectual three miles and further from the burn source. In contrast, the researchers found that even if infants had been close to a wildfire while in utero, there was no statistically significant effect on their birthweight if they were outside the smoke’s path.

“One really neat thing about this research is that I can do more than tell you what the effect of being exposed to the smoke is or not,” McCoy said. “I can tell you how that effect varies based on where an infant is relative to the source of pollution. Beyond that, we now have the evidence that reinforces earlier findings on the effects of ambient pollution at large and can say that these effects are very likely real, not just loosely correlated or tied up with other economic issues like household migration dynamics.”

McCoy’s hope is that this research will help inform policymakers of the potential economic and health consequences of wildfires, the magnitude of this type of disaster, and the mechanism behind wildfires — all of which enable people to better target the problem.

“There’s a lot of evidence to suggest that homeowners don’t fully acknowledge the risks associated with natural disasters — in particular, the risks associated with wildfire,” McCoy said. “One way to address this problem is to inform the public of risks through information-based regulation, such as posting billboards of people standing on cars during floods to discourage them from attempting to drive through inundated areas in the future. The idea is, if you give people this information, it can affect how they evaluate disaster risks, and it will likely have a spillover effect in terms of how they manage those risks.” That being said, McCoy noted that a one-time exposure to this type of information likely won’t be enough to have a lasting impact, so regulators should share this type of messaging often.

McCoy and Zhao’s research findings have been detailed in their article “Wildfire and Infant Health: A Geo-Spatial Approach to Estimating the Health Impacts of Ambient Air Pollution and In-Utero Stress,” currently under review by a top industry journal.

Source: provided by University of Nevada, Las Vegas (UNLV). Original written by Sara Gorgon. University of Nevada, Las Vegas (UNLV). “Exposure to wildfire smoke in utero lowers birthweight.” ScienceDaily. ScienceDaily, 6 December 2017.

How does tree mortality caused by drought and insects affect forests accustomed to frequent fire?

Research results were published January 17, 2018

This week a group of nine scientists and researchers published the results of their work considering how unusually high tree mortality affects wildfires in California’s Sierra Nevada forests that over thousands of years have adapted to frequent fire. They point out that fire suppression-caused forest densification has increased competition among trees for water and other resources, destabilizing many frequent fire forests by making them prone to mortality from other agents such as bark beetles.


Scott L. Stephens, Brandon M. Collins, Christopher J. Fettig, Mark A. Finney, Chad M. Hoffman, Eric E. Knapp, Malcolm P. North, Hugh Safford, Rebecca B. Wayman

The abstract and conclusions are below. The entire paper can be accessed at BioScience. The illustrations are from the document.


Massive tree mortality has occurred rapidly in frequent-fire-adapted forests of the Sierra Nevada, California. This mortality is a product of acute drought compounded by the long-established removal of a key ecosystem process: frequent, low- to moderate-intensity fire. The recent tree mortality has many implications for the future of these forests and the ecological goods and services they provide to society. Future wildfire hazard following this mortality can be generally characterized by decreased crown fire potential and increased surface fire intensity in the short to intermediate term. The scale of present tree mortality is so large that greater potential for “mass fire” exists in the coming decades, driven by the amount and continuity of dry, combustible, large woody material that could produce large, severe fires. For long-term adaptation to climate change, we highlight the importance of moving beyond triage of dead and dying trees to making “green” (live) forests more resilient.

Fire Responses Post Drought Beetle
A conceptual diagram showing fuel load and expected fire behavior in a mixed-conifer forest prior to and following a major bark-beetle-caused tree-mortality episode, with either (a) no follow-up-fuels treatment or (b) periodic prescribed fire to consume fuels. Surface-fire intensity is expected to roughly follow surface fuel load, whereas crown-fire potential is regulated by the amount of surface fuel (necessary to heat and dry live fuels to the point of combustion), as well as crown bulk density.


Forest Responses Severe Drought
Forest responses following a severe drought (1999–2002) in the Sierra de San Pedro Mártir (SSPM), Baja California, Mexico (a, drought and bark-beetle-caused tree mortality followed by wildfire; b, drought- and bark-beetle-caused tree mortality only) and in the southern California mountains (SCM), California, United States (c, drought- and bark-beetle-caused tree mortality at larger scales; d, drought and bark-beetle-caused tree mortality at stand scale. Note no wildfire in either SCM area). The SSPM and SCM photos were taken in 2004 and 2003, respectively. The SSPM site experienced a wildfire immediately following the multiyear drought (picture from 2003), with the photos capturing effects of both drought- and wildfire-related tree mortality. Pictures (a), (b), and (d) from SLS, (c) from G. Barley.



Unprecedented Sierra Nevada tree mortality has rapidly occurred after a severe drought with effects compounded by forest densification from decades of fire suppression. In the central and southern Sierra Nevada some areas have experienced more than 90% tree mortality, producing extensive landscapes of standing dead trees. This differs from mortality resulting from stand-replacing wildfire because bark beetles do not reduce surface fuels or jumpstart succession of shade-intolerant, fire-resistant pines. Forest managers have been struggling to determine whether these new postmortality conditions will increase wildfire intensity and/or severity, what the near- and long-term effects on forest communities will be, and what the appropriate intervention measures are.

In the first decade, wildfire severity in bark beetle killed frequent fire (FF) forests may be little affected over current conditions. Other than a brief increase during the “red phase” when most dead needles are still on recently killed trees, the reduction in canopy fuels is counterbalanced by an increase in surface fuels (figure 2). However, these are no grounds for complacency because current conditions in the majority of mixed-conifer and yellow pine forests in California already consist of unnaturally high surface fuel loads and corresponding elevated fire hazards (figure 2; Lydersen et al. 2014, Stephens et al. 2015).

The more troubling projection is how extensive loading of large-sized woody fuels in future decades may contribute to dangerous mass fires beyond the predictive capacity of current fire models. These fires can generate their own wind and weather conditions and create extensive spotting, making fire behavior and its impact on structures and public safety difficult to manage and predict. In addition, such intense fires could prevent forests from becoming re-established. Lacking the legacy of live trees that historic FF would have left (Stephens et al. 2008), large unburned areas of dead trees may also produce unusual forest succession patterns. These patterns will likely favor shade-tolerant and hardwood tree regeneration, limited shrub growth, and accumulating large woody fuels that would likely kill regenerating forests when wildfire inevitably occurs. The scale of contiguous tree mortality entrenches the homogeneity produced by fire suppression, reducing the fine-scale heterogeneity of forest conditions that contributes to resilience and biodiversity. Management could enhance adaptation to climate-change-induced stress if it focused more of its resources on creating spatially and temporally variable patterns in green FF forests that are better aligned with local moisture availability and fire patterns (North et al. 2009).

Many of our FF forests have failed to receive the very management that could increase resilience to disturbances exacerbated by climate change, such as the application of prescribed fire and mechanical restoration treatments (Stephens et al. 2016). Recent tree mortality raises serious questions about our willingness to address the underlying causes. If our society doesn’t like the outcomes from recent fires and extensive drought-induced tree mortality in FF forests, then we collectively need to move beyond the status quo. Working to increase the pace and scale of beneficial fire and mechanical treatments rather than focusing on continued fire suppression would be an important step forward.

More evidence of the “fireproofing effect” of insect outbreaks in a forest

Additional research finds evidence of a “fireproofing” effect on host trees from defoliation due to western spruce budworm outbreaks.

Above: Grand fir in Oregon defoliated by western spruce budworms. William M. Ciesla.

There is no dispute that severe outbreaks of western spruce budworm (WSB) and mountain pine beetle (MPB) in a forest have huge visual impacts. Many land managers have worried about more, larger wildfires and politicians have used it as an excuse for more logging.

But the commonly held belief that the effects will lead to higher intensity, more rapidly spreading wildfires has been disproven many times in the last eight years by scientists.

We first wrote about this issue in 2010 (Firefighters should calm down about beetle-killed forests) when some early research started to bring the facts to light.

The WSB and MPB attack trees very differently. The WSB defoliates the tree, consuming the needles and removing fuel from the canopy relatively quickly. The MPB kills the tree from the inside, leaving the dying “red” needles on the tree until they fall off in one to two years. The possibility of crown fires may increase during that red needle period, but it makes sense that fewer fine fuels in the canopy would reduce the fire intensity and make it less prone to transition from a ground fire to a crown fire. Both types of attacks eventually produce more course fuel on the forest floor as the branches break off and  the trees eventually fall over.

Research conducted by Daniel G. Gavin, Aquila Flower, Greg M. Cohn, Russell A. Parsons, and Emily K. Heyerdahl found evidence of a “fire proofing effect” for outbreaks of WSB. It does not address the MPB.

Below is an excerpt from their work:

“Extrapolating Results: Reduced Tree Mortality

“Taken together, the tree-ring and modeling studies suggest a lack of synergism between WSB outbreaks and wildland fires. However, a different kind of synergism may exist: Defoliation might dampen the severity of a subsequent wildfire. To explore this possibility, we used existing empirical equations that show the probability of mortality due to defoliation (fig. 3A) and the probability of mortality due to crown scorch (fig. 3B), combined with the simulated results of canopy consumption at different levels of defoliation (fig. 3C), to extrapolate the summed probability of mortality under a range of surface fire intensities and defoliation levels (fig. 3D). The results suggested a distinct “fireproofing” effect of defoliation: The increased risk of mortality by WSB is more than compensated for by reduced foliage consumption during moderate surface fire intensities. For example, trees with 50-percent defoliation have a distinctly lower probability of mortality when surface fires are less than about 74 kilowatts per square foot (800 kW/m2 ).

“However, we considered only the partial effect of defoliation on fire occurrence; we did not take into account other effects of WSB outbreaks, such as mortality of small trees. Of course, field observations are required to test our prediction. Remotely sensed burn severity maps, in combination with prior surveys of insect effects, could address this issue. One such study of the 2003 B&B Complex Fire in Oregon showed that prior defoliation had a marginal effect on reducing fire severity that was not statistically significant (Crickmore 2011). However, an analysis by Meigs and others (2016) of all post-WSB fires in Washington and Oregon from 1987 to 2011 showed that there is a statistically significant reduction in fire severity that persists for up to 20 years following an outbreak. Thus, the effect of defoliation on crown fire behavior modeled by Cohn and others (2014) appears to be confirmed by the analysis of burn severity data by Meigs and others (2016).

“Fireproofing Effect?

“It may seem reasonable to assume that extensive defoliation, causing sustained low levels of tree mortality in mature trees, should have a measurable effect on wildfire occurrence. However, fire is a highly variable disturbance in itself, and it is highly sensitive to specific climate and winds during the fire event. The scale of fuel changes wrought by WSB may be too small to affect subsequent fire probability in ecosystems where fire is limited by fuel moisture and ignition sources rather than fuel availability. Our data show that these two disturbance types do not share similar histories, despite a common link to drought events.

“Nevertheless, we hypothesize a “fireproofing” effect on host trees from defoliation due to WSB outbreaks. Although such an effect has been detected statistically from recent fire events (Preisler and others 2010; Meigs and others 2016), the inferred processes at play remain to be studied in detail at the site scale.”

UPDATE January 5, 2018: These researchers are not the only ones with similar findings on this subject. If you would like to read more, scroll through the articles on Wildfire Today tagged “beetles”.

Ponderosa pines are not adapted to high-severity fire

And this type of fire is increasingly common in the Southwest

Regenerating Ponderosa Pine
Regenerating ponderosa pine in a high-severity burn patch, 13 years after the 2000 Pumpkin Fire. (From the Fact Sheet)

The increasing size and severity of wildfires in the Western United States may have long term effects on species composition. A Fact Sheet published this month by Northern Arizona University and the U.S. Forest Service looks at ponderosa pine regeneration in patches of high-severity areas of the 2000 Pumpkin and the 2002 Rodeo-Chediski wildfires in the Southwest. Below are excerpts from the document written by Suzanne Owen, PhD student, School of Forestry, Northern Arizona University:


Over the past three decades, wildfires in southwestern U.S. ponderosa pine (Pinus ponderosa) forests have increased in size and severity, leaving large, contiguous patches of tree mortality. Ponderosa pines evolved under fire regimes dominated by low- to moderate-severity wildfires. They are poorly adapted to regenerate in large patches of high-severity fire because they are not a sprouting species and do not have serotinous cones or long-lived soil seedbanks. Consequently, the lack of seed-producing trees in high-severity burn patches may prevent or significantly delay ponderosa pine regeneration. Previous studies have documented low ponderosa pine regeneration densities in large high-severity burn patches, but less is known about the spatial patterns of ponderosa pine regeneration and interactions with sprouting species near residual live forest edges or the interiors of high-severity burn patches.



  • Ponderosa pines were re-establishing in all of our study plots, however regeneration densities were lower farther from forest edges.
  • Ponderosa pines seedlings were found in areas more than 980 feet from potential parent trees on all interior study plots.
  • Regenerating ponderosa pines displayed patterns of small-scale spatial aggregation in all plots, except one edge and one interior plot on the Pumpkin Fire, which displayed random distributions.
  • Dense resprouting trees dominated tree regeneration on the Rodeo-Chediski Fire, but did not influence the spatial location or height of regenerating ponderosa pine.
  • Regenerating ponderosa pine height was positively correlated with neighboring ponderosa pine densities and height.


  • Tree regeneration densities and species composition in high-severity burn patches are highly variable in different geographic locations.
  • Regeneration patterns suggest both short- and long-distance dispersal may play important roles in ponderosa pine regeneration in high-severity burn patches.
  • Ponderosa pine regeneration could be more strongly influenced by intraspecific facilitation than interspecific competition from dense sprouting species.
  • Future forest spatial patterns and composition are still unclear, but at this stage of development, these heterogeneous patches, characterized by drought-tolerant sprouting species or low pine densities, could be more resilient to climate change and severe wildfires than the overly dense ponderosa pine forests that were present before the wildfires.
  • Managers may want to use a “wait and see” approach before replanting in some areas to monitor natural regeneration over time.

Researchers develop model to predict wildfire occurrence

Their model uses temperature and precipitation to determine probability

A team of researchers from the University of Missouri and the U.S. Forest Service are continuing an effort to research how climate influences wildfire frequency. The model focuses on two variables – temperature and precipitation – to understand how climate drives wildfire across the world.

After acquiring historic fire occurrence data from tree ring and other studies they developed a mathematical model using temperature and precipitation as the two variables. In validation runs, the predictions the model generated were close to actual fire patterns.  As they continued to collect additional historic data from locations around the world during the last several years, they refined the model making it more accurate.

research wildfire probability temperature precipitation
A Combustion-Climate diagram (CCd) of climate influences on fire probability. Climate simulated fire probabilities for ‘natural’ ecosystems using mean maximum temperature and annual precipitation in the PC2FM. This rate diagram explains two temporal differences related to the combustion of ecosystems. Temperature and precipitation affect the reaction rate at the time the reaction occurs while the rate of fuel production determines the fuel concentration and its combustion rate. These two timing conditions differentially determine the rates of the two components of the PC2FM model: ARterm and the PTrc3. (From the team’s research)

“You can see patterns in global wildfire frequency that are obviously predictable,” Michael Stambaugh, an associate research professor in forestry, said. “For example, ¹Greenland doesn’t burn. It’s too icy and wet. It’s on one end of the spectrum. The other end of the spectrum is a place like the Sahara Desert, which doesn’t burn either. It’s too dry and there’s not enough fuel. Between those two extremes, we were confident that there was a way to describe the transition.”

The work is being done by Richard Guyette, Michael Stambaugh, Daniel Dey, and Rose-Marie Muzika who developed what they call the “Physical Chemical Fire Frequency Model (PC2FM)”.

More information about their research.


¹Note from Bill: To be clear, Greenland RARELY burns

Researcher looks at the effects of fuels management and previous fire on Rim Fire severity

Rim Fire, August 21, 2013.
Rim Fire, August 21, 2013. Photo by Robert Martinez.

In a November 13 webinar at 1 p.m. MST Jamie Lydersen will present her findings about how the effects of fuels management and previous fire affected the severity of the Rim Fire that started on the Stanislaus National Forest and burned into Yosemite National Park.

It seems intuitive to those who study wildland fire that a reduction in fuels will result in a decreased rate of spread and fire severity for the next wildfire, but it’s always good to have data that can confirm or refute long-held beliefs.

Here is a description of Ms. Lydersen’s research.

The 255,000 acre 2013 Rim Fire created an opportunity to study fuels treatment effects across a large forested landscape in the Sierra Nevada. We assessed the relative influence of previous fuels treatments (including wildfire), fire weather, vegetation and water balance on Rim Fire severity. Both fuels treatments and previous low to moderate severity wildfire reduced the prevalence of high severity fire. Areas without recent fuels treatments and areas that previously burned at high severity tended to have a greater proportion of high severity fire in the Rim Fire. Areas treated with prescribed fire, especially when combined with thinning, had the lowest proportions of high severity.

Jamie Lydersen is an associate specialist in the department of environmental science, policy and management at the University of California, Berkeley and a contractor for the Pacific Southwest Research Station, USDA Forest Service.

Registration is required to view the webinar.