Forest Fires: A Major Source of Dioxins

Introduction

New research suggests that forest fires are a major and natural source of dioxinsⁱ. In fact, in 2002, forest fires probably emitted nearly as much dioxin to the environment as did all other Environmental Protection Agency (EPA)-quantified sources combined. Dioxin emissions from industrial and commercial sources have declined steadily over the past several decades. As emissions from these sources are further curtailed through regulation and technology, forest fires should continue to be viewed as a major source of dioxins to the environment.

A recently published study by Gullett and Touati (2003) demonstrates that dioxin emissions from forest fires originate predominantly from biomassⁱⁱ combustion, and not simply from the vaporization of dioxin compounds bound to vegetation. Additionally, the researchers found that the type of biomass burned has a significant effect on the composition of the resulting dioxin emissions-Oregon forest biomass (average emission factorⁱⁱⁱ of 25 ng-TEQ^iv /kg-biomass burned) produces a different dioxin composition than does North Carolina biomass (average emission factor of 15 ng-TEQ/kg-biomass burned). Based on the new findings, EPA estimates of dioxin emissions from forest fires may be unrealistically low [estimated at 2 ng-TEQ/kg-biomass burned (U.S. EPA, 2000)]. Gullett and Touati are careful to point out that further research is required to elucidate the effect of species type, location and type of fire on emission factors.

Background: Forest Fires and Dioxins

U.S. forest fire statistics are compiled by the National Interagency Fire Center (NIFC). Forest fire occurrence varies from year to year, based on many factors, including climate, weather, topography, soil and vegetation, and proximity to roads and communities. Figure 1 (above) demonstrates the variation in total U.S. acreage affected by forest fires annually since 1995. (The value plotted for 2003 is the number of acres reported burned as of September 23.)

*Calculated using statistics on annual forest acreage burned from the NIFC; an emission factor of 20 ng-TEQ/kg biomass burned from Gullett, and Touati, (2003); and a biomass consumption rate of 9.43 metric tons/acre in areas consumed by wildfires from Ward et al. (1976), as cited in the EPA Draft Dioxin Reassessment (September, 2000).

Figure 2 is a plot of dioxin emissions from annual forest fires since 1995. (The value plotted for 2003 was calculated using the number of acres reported burned as of September 23.) This graph closely mirrors annual acreage burned (Figure 1) because dioxin emissions are directly dependent upon the number of acres affected by forest fire.

Forest Fires: An Increasingly Significant Source of Dioxins

Figure 3 demonstrates the significance of forest fire dioxin emissions (green line) relative to dioxin emissions from all other EPA-quantified dioxin sources for 1987, 1995 and 2002/4 (projected), years for which dioxin emission data are available (blue line). EPA-quantified (non-forest fire) sources declined by 77% between 1987 and 1995 and are estimated by EPA to currently be 92% lower than 1987 levels. It can be seen from the graph that as other sources decline, forest fires have become an increasingly significant source of dioxin to the environment. Simply put, annual forest fire dioxin emissions are becoming comparable in magnitude to combined emissions from all other quantified sources.

Ten Year Statistics

Over the past 10 years, an average of 4.66 million acres of U.S. land were affected by more than 100,000 wildfires annually. In Table 1, dioxin emissions from wildfires over the past decade are compared to current EPA-estimated emissions from quantified sources.

Wildfire dioxin emissions in Table 1 and Figure 3 were calculated using:

• 20 ng-TEQ/kg emission factor (Gullett and Touati (2003), average emission factor)
• 9.43 metric tons/acre biomass consumption rate for wildfires (Ward et al., 1976, as cited and used in the Draft U.S. EPA Dioxin Reassessment)
• Statistical data on wildfires (NIFC, www.nifc.gov).

Table 1: 10-Year Dioxin Emissions (grams-TEQ) Calculated Using Gullett and Touati (2003) Emission Factors

It can be seen from Table 1 that 10-year average annual forest fire dioxin emissions approaches the total combined projected 2002/2004 EPA-quantified emissions. In addition to the 10-year average dioxin forest fire emissions, Table 1 shows potential upper and lower bounds to dioxin emissions over the past decade. Acreage affected during the most destructive forest fire year (2000) is combined with Gullett and Touati's (2003) highest measured emission factor (25 ng-TEQ/kg) to obtain a potential upper bound emission of approximately 2,000g-TEQ. Similarly, acreage affected during the least destructive forest fire year (1993) is combined with the lowest measured emission factor (15 ng-TEQ/kg) to obtain a potential lower bound emission of approximately 300g-TEQ.

EPA-Calculated Forest Fire Dioxin Emissions

In EPA's Draft Dioxin Reassessment, forest fire dioxin emissions are calculated using an emission factor of 2 ng-TEQ/kg. Acreage affected by forest fires was obtained by EPA from the White House Council on Environmental Quality (CEQ) 25th Annual Report (http://ceq.eh.doe.gov/reports/reports.htm). Greater precision can be attained, however, using NIFC statistics. CEQ data were not available for 1995 and a high-range, conservative estimate of 7 million acres of forest fire acreage was used by EPA for that year. This estimate was triple the value reported by NIFC. Additionally, EPA combines estimates of emissions from forest fires with those from prescribed burns. Nevertheless, EPA's method of calculation and the method described here are similar enough so that when estimates of forest fire acreage agree (and only forest fire, not prescribed burn, acreage is considered), calculated emissions differ by one order of magnitude. This difference is attributed to the use of an emission factor of 2ng-TEQ/kg instead of 20 ng-TEQ/kg. For a more detailed analysis of EPA calculations, see the Appendix.

The Bottom Line

Using NIFC wildland fire statistics on forest acreage burned and conclusions regarding dioxin emission factors of Gullett and Touati (2003), dioxin emissions from forest fires are estimated to be comparable to those from combined industrial, commercial and societal sources. As industrial emissions of dioxins are even further reduced through regulation and technology in the years to come, forest fires should continue to be viewed as a major source of dioxins to the environment.

References:

Gullett, B.K. and Touati, A. (2003). PCDD/F emissions from forest fire simulations, Atmospheric Environment 37, p. 803-13.

National Interagency Fire Center. On-line. Available: http://www.nifc.gov. (accessed September 17, 2003).

US Environmental Protection Agency Inventory of Sources of Dioxin-Like Compounds in the United States-1987 and 1995. On-line. Available: http://cfpub.epa.gov/ncea/cfm/dioxindb.cfm?ActType=default (accessed August 11, 2003).

US Environmental Protection Agency (September, 2000). Exposure and human health reassessment of 2,3,7,9-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds (Draft).

Ward et al., 1976 An update on particulate emissions from forest fires. Presented at: 69th Annual Meeting of the Air Pollution Control Association, Portland, OR June 27-July 1, 1976] as cited in the EPA Draft Dioxin Reassessment, September, 2000.

White House Council on Environmental Quality, 25th Anniversary Report (1994-1995), 1993. [On-Line]. Available: http://ceq.eh.doe.gov/reports/reports.htm

ⁱ"Dioxins" here refers to both dioxins and furans.
ⁱⁱPlant material
ⁱⁱⁱA dioxin emission factor for forest fires is the quantity of dioxin generated by the burning of a stated quantity of forest biomass. The unit used here is ng-TEQ/kg-biomass, or nanograms (0.000000001g) of dioxin toxic equivalents generated per kilogram of biomass burned.
^iv"TEQ" denotes "Toxic Equivalents," a quantitative measure of the combined toxicity of a mixture of dioxin-like chemicals, here measured in grams. There are two systems of TEQ, a WHO system and an International system. No effort was made here to distinguish between these two, but it is assumed that interchanging these units would not have a significant effect on the conclusions reached.
^vAll forest fire statistics are from the National Interagency Fire Center (www.nifc.gov).
^viFrom Gullett and Touati (2003).
^viiFrom US Environmental Protection Agency Inventory of Sources of Dioxin-Like Compounds in the United States-1987 and 1995" http://cfpub.epa.gov/ncea/cfm/dioxindb.cfm?ActType=default and based on EPA projections assuming full compliance with regulatory levels by this period and the closure of a copper smelter (personal communication, Dwain Winters, US EPA, 9-9-02).

Calculations:

To obtain dioxin emissions from forest fires:

Dioxin emissions (grams, G) = Acres burned (A) X (9.43 Metric Tons biomass/Acre) X 1,000 kg/Metric Ton) X Emission Factor (ng-TEQ/kg-burned) Dioxin emissions (grams) = Acres burned X 9.43 E3 X Emission Factor X E-9.

Example: To calculate the number of grams of dioxins-TEQ "G," from the burning of "A" acres of forest using an emission factor of 20 ng-TEQ/kg-biomass burned:

G = A X (9.43 Metric Tons/Acre) X (1,000 kg/Metric Ton) X (20 ng-TEQ/kg-burned) X E-9 g/ng) G = A X 0.1886

Appendix: Calculating Dioxin Emissions from Forest Fires Using Two Different Methods

Forest fires represent a potentially large natural source of dioxins. It is instructive to compare total U.S. Environmental Protection Agency (EPA)-quantified dioxin emissions for 1987 and 1995 (http://cfpub.epa.gov/ncea/cfm/dioxindb.cfm?ActType=default) and 2002/2004 EPA-projected emissions with estimates of emissions from forest fires. The blue line in the graphic above represents dioxin emissions from all EPA-quantified sources and demonstrates the impressive reduction brought about by government regulation and industry innovation. The two lower (red and green) lines represent forest fire emissions estimated using two different methods, which are described below. Both methods employ a recently published estimated average emission factor for forest fires of 20 ng-TEQ/kg-biomass burned (Gullett and Touati, 2003)¹.

The figure above permits a closer look at dioxin and furan emissions from forest fires in relation to those from EPA-quantified sources. (The 1987 data point for EPA-quantified sources is not shown here for purposes of magnifying lower emission values.)

Method 1: Forest fire emissions calculated using NIFC statistics and 20 ng-TEQ/kg Emission Factor

Forest fire acreage was obtained from the National Interagency Fire Center website (http://www.nifc.gov/stats/wildlandfirestats.html) and multiplied by a biomass consumption rate of 9.43 metric tons/acre for wildfires (Ward et al., 1976, as cited in the Draft U.S. EPA Dioxin Reassessment) to obtain the number of metric tons of biomass incinerated in annual wildfires. Metric tons of biomass is converted to kilograms by multiplying by 1,000 and then multiplying by the 20 ng-TEQ/kg emission factor of Gullett and Touati (2003) to obtain the number of nanograms of dioxins generated. That number is then converted to grams of dioxins by dividing by 10⁹.

Method 2: Forest fire emissions calculated using EPA's Draft Dioxin Reassessment Method and updated 20 ng-TEQ/kg Emission Factor

EPA calculates the amount of biomass burned in forest fires using the number of acres of forests affected by wildfires as reported in the White House Council on Environmental Quality (CEQ) 25th Annual Report (http://ceq.eh.doe.gov/reports/reports.htm). That report lists 5 million acres in 1987, and a CEQ estimate of 5-7 million acres in 1995 . EPA chose to use 7 million acres in its calculation. Multiplying each of these values by Ward et al.'s biomass consumption rate of 9.43 metric tons/acre for wildfires, the number of metric tons of biomass produced in annual wildfires alone is obtained. (The draft EPA Dioxin Reassessment combines wildfires and prescribed burns; here only wildfires are considered.) Metric tons of biomass is converted to kilograms by multiplying by 1,000 and then multiplied by an updated emission factor of 20 ng-TEQ/kg to obtain the number of nanograms of dioxins generated. That value is then converted to grams of dioxins by dividing by 10⁹.

Table 1 shows the results of the two methods of calculation: Table 1

Except for the large disparity in the number of forest acres affected by fires in 1995, the two methods would produce similar dioxin emission results when the same emission factor is employed. Clearly, greater precision can be achieved using annual NIFC forest fire acreage rather than CEQ estimates.

¹These researchers caution: ".this estimate is only preliminary until additional work can further understand the potential for emission factor differences due to species type, location and type of fire" (p. 811).
²The 1995 figure was shown as "na" in the CEQ report. An asterisk to the bottom of the table indicates fire acreage should be estimated as between 2-7 million. EPA used the high estimate of 7 million. This accounts for the great disparity between the NIFC figure and the CEQ estimate.
³Figures in parentheses are rounded to one significant figure due to the wide margins of error in estimating parameters used to calculate dioxin emissions.