Are Controlled Burn Emissions Overestimated?

Public discourse and policy often overestimate the air pollution contributions of controlled burns, sometimes misinterpreting their role in air quality discussions. This perception can lead to policies that overly restrict prescribed burns, potentially resulting in worse outcomes later.

Misconceptions and Concerns

Public discussions and policymaking frequently focus on concerns such as local air quality impacts and visibility reduction from prescribed fires. There is a prevalent notion that "any fire is bad" from a pollution standpoint. These concerns, if not properly contextualized, can hinder effective land management strategies.

Regulatory Perceptions and Their Irony

Prescribed fires are typically subject to strict air-quality regulations, often requiring permits and specific conditions (e.g., burning only when downwind air quality remains safe).

Ironically, these well-intentioned regulations can make it difficult to conduct enough controlled burns, leading to the accumulation of fuel that could feed larger, more destructive wildfires.

Policymakers and community members may compare the visible smoke from a prescribed fire to a normal, smoke-free day, concluding that the burns are a major pollution source. This is a short-term view.

Studies demonstrate that not burning now can lead to far worse smoke later. Expanding prescribed burning can lead to net reductions in regional smoke emissions by preventing severe fires.

Regions where prescribed burns have been limited due to smoke regulations, liability, or public opposition, have experienced larger and more destructive wildfires, ultimately releasing much greater pollution.

Western North America is cited as a prime example where decades of fire suppression have contributed to the current era of megafires and heavy smoke summers. Agencies are now acknowledging that a bit of controlled smoke now can avert an "inferno of smoke later".

Debunking the Myth: "Controlled Burning Causes More Pollution than Wildfires"

The assertion that controlled burns contribute significantly more to air pollution than wildfires is largely a myth, contradicted by scientific evidence when total emissions and impacts are properly compared.

Wildfires overwhelmingly emit more PM₂.₅, CO, and other pollutants in total. Empirical data from the western U.S. revealed that even with comparable areas treated, wildfire emissions were approximately an order of magnitude higher than those from prescribed fires.

One study found that a prescribed fire produced only about 17% of the PM₂.₅ smoke that a wildfire would have emitted over an equivalent area.

Prescribed fire treatments have been shown to reduce subsequent wildfire severity by about 16% and cut resulting smoke production by roughly 14% on average.

Public discourse sometimes fixates on the visible, local smoke from a managed burn, but this should be weighed against the much larger, longer-lasting regional smoke clouds produced by uncontrolled fires.

A research economist noted that while people think of wildfires in terms of flames, "the smoke is a silent and far-reaching hazard," and prescribed fire can be a tool to reduce total smoke exposure over time.

Communication Challenges and Solutions

Social media and local news can amplify incidents where a prescribed burn "goes wrong," such as escaping control or causing unexpected smoke in a town, which can distort public perception.

Despite these rare incidents, most prescribed burns are conducted safely with minimal smoke impacts. However, such events have led agencies to pause programs, invariably increasing the risk of larger wildfires.

Fire scientists warn that overestimating the emissions or risks of prescribed burns leads to underutilization, exacerbating public and policymaker misconceptions and ultimately worsening wildfires and pollution.

Encouragingly, there is increasing outreach and education efforts explaining that "some smoke now prevents massive smoke later," alongside technological improvements in smoke forecasting and air quality monitoring to address public concerns.

The scientific consensus is that strategic prescribed burning is a net positive for air quality in the long term, as it prevents the catastrophic emissions of uncontrolled wildfires.

Pollutant Emissions

While both types of fires emit pollutants like fine particulate matter (PM₂.₅/PM₁₀), carbon monoxide (CO), volatile organic compounds (VOCs), and nitrogen oxides (NOₓ), wildfires produce more smoke and pollution per unit of area or fuel.

Wildfires can produce roughly twice as much PM₂.₅ per ton of fuel consumed compared to prescribed burns due to drier, more erratic conditions favouring incomplete combustion. They also consume heavier fuel loads, emitting greater total particulate mass.

Wildfires generally emit more CO, methane, and other incomplete-combustion gases because the burning is less efficient, with more smouldering and oxygen-starved combustion in dense fuels.

Total NOₓ output, black carbon, and hazardous air pollutants (like formaldehyde, acrolein, and benzene) tend to be higher in large wildfires due to their size, duration, and the larger volumes of biomass burned with incomplete combustion.

Fire Duration, Intensity, and Scale of Emissions

Size and Area Burned: Prescribed burns are typically limited in extent (a few hectares to hundreds of acres), whereas wildfires can engulf tens or hundreds of thousands of acres, leading to a far larger total emissions footprint. In the western U.S. in 2017, prescribed fires accounted for only about one-tenth of the PM₂.₅ emitted by wildfires.

Fire Intensity and Combustion Efficiency: Wildfires often burn under hot, dry, windy conditions, leading to high-intensity fire behavior and less efficient overall combustion, resulting in large amounts of partially burned material and prolonged smoldering.

Prescribed burns, by design, are low to moderate intensity, promoting more complete combustion of fine fuels to CO₂. Wildfires thus produce more smoke per acre because they consume more fuel (including coarse fuels and sometimes soil/peat) and have higher emission factors for smoke pollutants.

Duration of Burning and Smoke Exposure: Prescribed burns are short-lived events, often burning out in a day or two. In contrast, wildfires can rage for weeks or months, continuously producing smoke, leading to prolonged exposure for downwind populations.

Wildfires often occur during peak fire season when atmospheric conditions exacerbate pollution buildup, while prescribed burns are conducted in cooler, wetter seasons when smoke disperses more readily.

Smoke Plume Behavior: Fire managers can predict and manage the smoke trajectory from controlled burns, often avoiding populated areas. Wildfires offer no such control, emitting massive smoke plumes that can travel hundreds of kilometres and blanket cities.

Health and Environmental Impacts

Human Health: Wildfire smoke affects far more people and at higher pollutant concentrations, causing widespread hazardous air. A single extended wildfire event (e.g., Saddleworth Moor fire) can expose millions to PM₂.₅ levels exceeding WHO guidelines and lead to premature deaths.

Prescribed burns rarely cause such broad health crises; their impacts are local and short-lived, with a much lower scale of exposure. Crucially, by preventing extreme wildfires, prescribed burns may protect public health in the long run, with modeling studies showing substantial reductions in PM₂.₅ emissions and smoke-related deaths when areas burn under prescribed conditions instead of wildfire conditions.

Biodiversity and Ecosystems: Controlled burns are low-intensity and part of planned management that can benefit fire-adapted ecosystems. Uncontrolled wildfires, especially severe ones, cause extensive habitat loss, promote invasive species, and lead to longer-term ecological damage, sometimes even sterilizing soils or consuming peat. A severe peat wildfire is far more damaging than many small heather burns.

Climate Impact (Carbon and Emissions): While both release greenhouse gases, prescribed burns primarily release carbon from recent growth, which can regrow. Severe wildfires release huge amounts of carbon stored for decades or centuries, especially when they burn into long-unburned thick forests or carbon-rich peat, leading to effectively irreversible emissions. Preventing intense wildfires via controlled burns likely reduces overall carbon emissions.

Takeaway

The scientific evidence overwhelmingly indicates that uncontrolled wildfires are vastly more damaging in terms of total emissions, pollutant concentrations, exposure duration, and broader health and climate impacts. While prescribed burns do produce smoke, they are managed to limit pollution and are typically much smaller and shorter events.

The strategic use of controlled burning can reduce the overall pollution burden by preventing the most extreme fires. Public policy is gradually shifting to reflect this understanding, recognizing the need to balance short-term, manageable emissions from controlled burns against the uncertain but potentially catastrophic emissions from wildfires if fuels are left unmanaged.

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