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Smoke from wildland fires and public health

Firefighters, forest managements, and residents are preparing for another fire season in the western part of the United States. Wildfires burn large expanses of forested lands in California, but it’s not just rural Californians who need to worry about effects of such fires. Residents in urban areas and neighboring states experience the through smoke from hundreds of miles away.

A recent case study analysis in the Journal of Forestry examined how smoke impacts from wildland fires that burn under hazardous conditions compared to fires that are purposefully managed to achieve more beneficial outcomes (described as “resource objective wildfires”). Within California and especially within the Sierra Nevada mountains, forests have evolved to burn frequently, but suppression activities have funneled much of the burning into large wildfires that are a major source of fine particulate matter (Fig. 1) and cause serious health threats to downwind communities.

Figure 1: Fine particulate matter from wildfires, prescribed fires, and other sources in California.  Figure prepared by Jonathan Long using data from the USEPA’s triennial National Emissions Inventory. Image used with author permission.

To learn more about this impact, the scientists compared the number of people likely exposed over time to increases in fine particulate pollution from smoke plumes (measured from satellite imagery) during representative wildland fires that burned in the same vicinity but under favorable or unfavorable conditions. Using fire under favorable weather and fuel conditions can facilitate restoration of large areas of forest while minimizing impacts to people downwind.  Under such conditions, fires often spread moderately, emit less pollution per day and produce fewer pollutants per unit area by avoiding the increased consumption of logs and tree crowns that occurs during severe fires. Once a fire is underway under suitable conditions, managers can also accelerate (“push”) fire spread when weather lofts smoke away from communities or slow spread (“pull”) when dispersion is less favorable. Burning under favorable conditions allows for more advance warning of potential smoke impacts and helps members of the public to reduce their exposure.to potential harm. Over time, managers can prioritize prescribed fires, appropriate use of wildfire, and other fuel treatments in places and times to create landscape-scale “anchors” that can constrain both the extent and rate of spread of future wildfires.

Figure 2: Smoke plume from a single day of the Rim Fire, when high density smoke overlay more than 2 million people in California and Nevada. Created by Jonathan Long with smoke plume data from the National Oceanic and Atmospheric Administration. Image used with author permission.

In contrast, millions of people in California and Nevada were exposed to enormous plumes of smoke from the 104,000 hectare Rim Fire in 2013 (Fig. 2). This extremely large wildfire had much higher daily emissions than wildfires that were managed to benefit forests in the same area around Yosemite National Park between 2002 and 2013 (Fig. 3). Even after considering the impact on an acre-by-acre basis, the smoke impacts were much greater during the Rim Fire than the fires managed for such resource objectives. The differences reflect not only greater consumption of forest fuels but also the concentration of that burning into extended periods of elevated emissions that reach large populations residing in metropolitan areas, many of which are already vulnerable to elevated levels of pollution.

Figure 3: Average daily emissions (metric tons of PM2.5/day) by fire type in our analysis of 10 years of wildland fires in the Yosemite National Park area, created by Leland Tarnay using data from the National Park Service. Image used with author permission.

The results were drawn from the Central Sierra Nevada, but the framework for evaluating smoke impacts is applicable to other regions. The framework can be applied to fires that have already happened, as in this study, but researchers are now applying the framework prospectively by using models to predict smoke plumes and pollution levels in downwind communities under alternative fire scenarios. That research will help to evaluate the air quality implications of restoring a more frequent fire regime by managing fires under favorable conditions, rather than shunting burning into large conflagrations under conditions that are difficult to control and often expose downwind populations to heavy, prolonged smoke. By applying fuel reduction treatments and encouraging more frequent fires under appropriate weather and fuel conditions, fires in ecosystems that are prone to burning are likely to spread more moderately, emit less pollution per day, and minimize impacts to people.

Featured image credit: CC0 via Unsplash.

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