USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
United States Department of Agriculture Forest Service. USDA logo which links to the department's national site. Forest Service logo which links to the agency's national site.

Research Topics Fire Science


Smoke obscures the sun across a mountainous landscape with vegetation  shadowed in the foreground.
Drifting smoke from a prescribed fire on the Stanislaus National Forest. Photo by Jay Power.

Fire has been frequent across California's landscape over the last 10,000 years or more, providing an ecological influence vital to species distribution and forest health.

Historically, smoke was widespread throughout the fire season in California. Smoke levels prior to European settlement may have been comparable to those of present "extreme" fire seasons because of the similarity of areas burned across the landscape. However, modern wildfires can release more smoke per acre in areas that have more accumulated fuel and haven’t burned in a long time.

Smoke from large, high intensity fires affects large expanses that often encompass urban areas, resulting in nuisance complaints, adverse impacts to human health, and loss of visibility. Much of California currently does not meet federal and state air quality standards for ozone and particulate matter due to anthropogenic (human-caused) emissions from a variety of sources. Additional emissions from smoke are difficult for air regulators to accommodate. This results in a major challenge for land managers seeking to use fire as a management tool at a landscape level.

Considerations for managing the impacts of smoke

Emissions from any fire can affect human health, but proper forest management at the landscape scale, including the use of fire for ecological benefit, can potentially reduce fuel loads while also minimizing the negative long-term impacts of smoke. Land managers have several options for addressing the impacts of smoke.

Using prescribed fire treatments or allowing naturally ignited wildfires to burn more frequently can reduce smoke emissions from a single fire event. Additionally, prescribed fires are often ignited when meteorological conditions are conducive to good smoke dispersal, and with frequent prescribed burning, total emissions are spread out to lessen smoke's impacts on human health.

Reducing fire size and intensity can result in less extensive smoke impacts. However, this may also increase the duration of fire as it burns through the season, subjecting local populations to prolonged smoke exposure and diminished visibility. Some areas directly affected by smoke plumes from large fires, like those from the Aspen or Rim Fires, could experience periods of severely impaired air quality.

Research needs and collaboration

State of the art measurements and modeling of smoke emissions from chaparral is also an active area of research involving the Forest Service, other federal and state agencies and university cooperators.  The information collected ranges from detailed laboratory experiments to measurement of field scale prescribed burns and wildfires.  Related recent work suggests that the use of polyethylene plastic to cover forest debris piles does not contribute additional emissions when the piles are burned.

On the western slopes of the Sierra Nevada (particularly from Yosemite National Park south to Sequoia National Forest), researchers have closely monitored smoke impacts since around 2006. Effective collaboration between air and fire managers, including evaluation of smoke impacts following controlled burning, has allowed for an improved understanding of impacts of individual fires and development of long-term goals for improving forest health and air quality. Additional research is needed to understand how prescribed fire intensity, fire suppression, and the complex interactions between various management measures affect smoke emissions, transport and distribution, and, ultimately, human and ecosystem health.

A need also exists to review and develop a strategy for social science research on the use of fire as a management tool in California. The general public and public health advocates may assume that fire suppression leads to the best air quality since smoke emissions are avoided in the near term. This assumption can be a constraint for proactive management techniques until the smoke's impacts on human health are thoroughly documented and understood.

Fire scientists are also working on understanding the influence of smoke induced inversions in the Klamath-Siskiyou ecoregion. Smoke inversions have been found to influence fire behavior and subsequent fire severity patterns. Additionally, exploratory research with federal and tribal partners is examining the influence of smoke inversion on cooling rivers and how fish respond. The larger landscape influence of wildfire smoke inversions on fire behavior, resulting vegetation patterns, how aquatic species, and water quality are affected is being addressed with new research approaches.

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