Forest Fires: A Major Source
July 8, 2005
New research suggests that forest fires are a major and natural
source of dioxinsi. In fact, in the past few years,
forest fires probably emitted nearly as much dioxin to the
environment as did all Environmental Protection Agency (EPA)-quantified
sources combined (see Figure 1). Dioxin emissions from
industrial 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.
*With the exception of forest fire data, dioxin emissions
source data are based on EPA projections for 2002/4, assuming
full compliance with regulatory levels and the closure of
a copper smelter (personal communication, Dwain Winters, US
#The dioxin contribution from forest fires was calculated
using National Interagency Fire Center acreage burned in year
2004 wildland fires; an emission factor of 20 ng-TEQ/kg biomass
burned [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).
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-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)]. Nevertheless,
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.
|*2004 statistics do not include state lands in North
**2005 includes acres burned up to June 15, 2005.
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 2 demonstrates the variation in total U.S. acreage
affected by forest fires annually since 1995. (The value plotted
for 2005 is the number of acres reported burned as of June
|*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 3 is a plot of estimated dioxin emissions from annual
forest fires since 1995. (The value plotted for 2005 was calculated
using the number of acres reported burned as of June 15.)
This graph closely mirrors annual acreage burned (Figure 2)
because dioxin emissions are directly dependent upon the number
of acres affected by forest fire.
Forest Fires: An Increasingly Significant Source of Dioxins
Figure 4 demonstrates the significance of forest fire dioxin
emissions (green line) relative to dioxin emissions from all
other EPA-quantified dioxin sources for 1987, 1995, 2000 (value
for 2000 represents draft data released March, 2005) and 2002-4
(EPA-projected), years for which dioxin emission data are
available (blue line). EPA-quantified (non-forest fire) sources
declined by 89 percent between 1987 and 2000 and are estimated
by EPA to currently be 92 percent lower than 1987 levels.
It is evident from Figure 4 that, as other sources decline,
forest fires have become an increasingly significant source
of dioxin to the environment and that 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 5.60 million acres
of U.S. land were affected by almost 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
Figures 3 and 4 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: A Comparison of 10-Year Forest
Fire Dioxin Emissions with EPA-Quantified Non-Forest Fire
Number of Forest Fires
Millions of Acres Affected
Dioxin Emission Factoriv
Forest Fire Dioxin Emissions (g-TEQ)1
EPA-Quantified, Dioxin Emissions,
g-TEQvii (EPA-Projected, 2002/2004)(non-forest
15 (N. Carolina)
| 1Figures in parentheses
are rounded to one significant figure due to the wide
margins of error in estimating parameters used to calculate
It is evident from Table 1 that 10-year average annual forest
fire dioxin emissions are very close to 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 2,000
g-TEQ. Similarly, acreage affected during the least destructive
forest fire year (2004) is combined with the lowest measured
emission factor (15 ng-TEQ/kg) to obtain a potential lower
bound emission of approximately 300 g-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 2 ng-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
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,
National Interagency Fire Center. On-line. Available: http://www.nifc.gov.
(accessed June 15-16, 2005).
U.S. Environmental Protection Agency, The Inventory of Sources
and Environmental Releases of Dioxin-Like Compounds in the
United States: The Year 2000 Update (External Review Draft).
On-line. Available: http://www.epa.gov/ncea/pdfs/dioxin/2k-update/,
U.S. 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).
U.S. 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 World Health Organization 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 0.0001886
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.