Forest Fires: A Major Source
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.
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-TEQiv /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
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
- 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
Number of Forest Firesv
Millions of Acres Affected
Dioxin Emission Factorvi
Forest Fire Dioxin Emissions (g-TEQ)1
Non-Forest Fire Dioxin Emissions,
g-TEQvii (EPA-Projected, 2002/2004)
15 (North Carolina)
| 1Figures in parentheses are
rounded to one significant figure due to the wide margins
of error in estimating parameters used to calculate dioxin
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
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.
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,
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.
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).
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,
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
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:
Millions of Acres
of Forest Affected
|Method 1: Using NIFC Statistics
|Method 2: Using CEQ Statistics Employed
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.