The classes of primary chemical products naturally produced by the
combustion of forest fuels include: carbon dioxide, water, carbon monoxide, particulate
matter, methane and non-methane hydrocarbons, polynuclear aromatic hyrocarbons, nitrogen
and sulfur oxides, aldehydes, free radicals, and inorganic elements. Pesticides are a
class of secondary chemical by-products of fires that have been of some public concern
with the extensive use of herbicides for site preparation and release in some forest
ecosystems and insecticides for insect control in others. Studies conducted on herbicides
and insecticides indicate that hot fires (>500EC)
thermally degrade most pesticides. Smoldering fires (<500EC)
have the potential to volatilize significant amounts of some pesticides. Exposure analyses
indicate that, even under conditions of smoldering fires, no significant human health
risks occur from pesticides incorporated into or on forest fuels. Naturally occurring
chemical by-products of combustion are a far greater risk to human health.
keywords: air quality, herbicides, insecticides, pesticides,
Fire has continued to be a management tool used by public and private
land managers in the southeastern United States to sustain production of natural
resources, preserve and maintain wildlife habitat, and improve grazing conditions. The use
of fire in timber management has raised concerns such as:
1. particulate matter emissions or inorganic and organic gaseous
emissions presenting a threat to public health, along with nuisance concerns,
2. visibility impairment in areas of high humidity, and
3. decrease in recreational aesthetic appeal.
Based on recent health studies, the Environmental Protection Agency
(EPA) proposed air quality standards of particulate matter and ground-level ozone in July
1997. Consequently, land managers must minimize prescribed fire emissions and the adverse
impact of smoke on public health and the environment. In addition, land managers have to
re-evaluate alternatives such as mechanical site preparation, whole tree harvesting, and
yarding of unmerchantable material. Mechanical site preparation in the Piedmont region,
however, often leads to accentuated erosion and soil compaction. Use of herbicides in
combination with other site preparation techniques minimizes soil disturbance. In the
absence of fire, however, herbicides may result in accumulation of dead vegetation, as
well as insect and disease vectors. A movement away from burning as part of site
preparation has the additional disadvantage of on-site slash/fuel buildup and increased
potential for intense wildfire.
Pesticide use patterns in forest
management are governed by a number of factors such as economics, sensitivity of the
ecosystem, potential liability for off-site impacts, etc. Insecticides and fungicides are
extensively used in seed orchards and nurseries, where small acreages are treated to
protect highly valued seed sources and seedlings for out-planting. The use of insecticides
over extensive forested areas, however, is generally restricted to cases where trees with
high commercial or aesthetic value are threatened or where epidemic pest outbreaks occur
(e.g. gypsy moth, tussock moth, spruce budworm, and pine bark beetle). In such cases,
entire watersheds may be treated. In young pine stands, herbicides are used extensively to
assist in site preparation, while insecticides may be applied for insect control. Thus the
incidents when timber stands treated with pesticides might be subject to a fire are
limited to times of site regeneration and cleanup of epidemic pest outbreaks.
Prescribed burns routinely used in forest management include: 1)
slash burns in harvested stands, 2) Abrown and
burn@ for site preparation, 3) under-story burns
for wildlife habitat improvement and weed/brush control, and 4) grassland burns. Site
preparation slash burns and Abrown and burn@ burns are used once in a regeneration cycle (20-80
years) to facilitate planting and early regeneration management practices. The Abrown and burn@
site preparation removes unwanted competing vegetation by burning 30-180 days following
herbicide application. The browning of hardwood foliage and herbaceous plants increases
the fuel source, making late summer burns effective for further reduction of smaller
residual hardwoods that may have been missed by application or were resistant to the
herbicide. Also, burning the area greatly facilitates planting operations by removing
logging debris. Under-story burns and grassland burns can be used every 1-7 years to
control weed competition and improve wildlife habitat, but usually do not consume fuels
treated with pesticides and/or herbicides.
The Abrown and burn treatment@ management practice has raised forest worker and
public concern about possible exposure to herbicide residues in smoke from the fire or
from burning herbicide-treated hardwoods in fireplaces or stoves. The roots of this
concern can be traced back to the warning statement found on herbicide labels, as well as
material data safety sheets. These statements referring to fire hazards and toxic
decomposition products urge the user to Awear a
mask@ or, Aif
burned, stay out of the smoke@. While these
cautions are appropriate in connection with fires near herbicide concentrates and
containers found at mixing and storage sites, they were not intended to apply to the
diluted forms following an application to forested sites. In these cases, on a given acre,
only a few ounces or pounds of herbicide are spread over many thousand pounds of ground
litter and vegetation. The latter material constitutes the predominant fuel in the
prescribed fire and the principal smoke risk factor to the worker or the public.
In a forest fuel combustion study (McMahon et al. 1985), wood treated
with five herbicides and two common insecticides was burned under controlled combustion
conditions (Table 1). Over 95% pesticide decomposition occurred when wood was burned under
conditions of rapid (flaming) combustion, while variable amounts of pesticide residues
were recovered from the smoke stream in the case of smoldering combustion. Relatively
stable compounds such as lindane (insecticide) and dicamba (herbicide), as well as
compounds with significant vapor pres-sures, can be expected to be released under
smoldering or slow heating conditions. For example, the insecticides lindane and dicamba
and the herbicide 2,4-D were extensively recovered intact in the smoke stream (43, 92, and
92%, respectively), while the insecticide chlorpyrifos and the herbicides hexazinone and
picloram were extensively decomposed (>75% decomposition).
In a second study (McMahon and Bush 1992), 14 prescribed burning
operations (Abrown and burn@) were monitored to determine possible worker
exposure. Field worker breathing zone concentrations of smoke suspended particulate matter
(SPM), herbicide residues, and carbon monoxide (CO) were monitored on sites treated with
labeled rates of forestry herbicides containing the active ingredients imazapyr,
triclopyr, hexazinone, and picloram. The sites were burned 30-169 days after herbicide
application. No herbicide residues (sensitivity 0.1-4 Fgm/m3)
were detected in 140 smoke samples from the 14 fires. These detection levels are several
hundred to several thousand times less than any occupational exposure limit for these
The SPM and CO levels monitored on these fires were highly variable,
depending on fire conditions, size of tract, and worker assignment. The toxicology of
combustion products (polynuclear aromatic hydrocarbons, SPM, CO, etc.) and the larger
issue of the EPA Air Quality Standards proposed in 1997 are beyond the intended scope of
In follow-up laboratory studies (McMahon and Bush 1986), herbicide
recoveries in the smoke stream were compared. As expected, the upslope fires where
herbicide distillation is likely resulted in low combustion efficiency (high smoke
production) and recovery of 2,4-D and picloram (5% and 0.04%, respectively). Herbicide
recoveries from downslope fires were <0.02% and 0.08% for picloram and 2,4-D,
respectively. Thus, fire intensity directly impacts the extent of herbicide combustion and
Bark beetles, especially the southern pine beetle (Dendroctonus
frontalis), are a serious threat to forest stands and individual high value trees
throughout the South. Insecticidal control of bark beetle infestations is effective but
disposal of insecticide-treated trees could present a problem. In a fourth study (Bush et
al. 1987b), wood samples collected 4 month post-treatment for pine bark beetle control
were found to contain lindane and chlorpyrifos residues ranging from 0.32 to 35.8 mg/kg
for lindane and <0.1 to 76.1 mg/kg for chlorpyrifos. Combustion of these samples under
smoldering fire conditions resulted in 43% and 28% recovery of lindane and chlorpyrifos,
respectively, in the smoke stream. With rapid combustion in a well developed fire, all
lindane and chlorpyrifos residues were thermally degraded.
WORKER AND PUBLIC EXPOSURE ASSESSMENT
Worker exposure to herbicide residues released from burning treated
vegetation was estimated in the U.S. Department of Agriculture Forest Service Southern
Region Environmental Impact Statement (Weeks et al. 1988). This analysis assumed that: 1)
3.0x107m3/ha smoke is produced, 2) herbicides are applied at maximum
labeled application rates, 3) herbicides degrade with time at published dissipation rates,
and 4) no thermal decomposition of the parent compound occurs in the burning process.
Margins of safety (MOS=s) were estimated for all
registered herbicides, comparing predicted smoke residue levels to threshold limit values.
All MOS=s were found to be >150 except for
triclopyr ester, which had a MOS of 84. For the scenario where wildfire occurs on the day
of application, the MOS=s were all >50 except
for imazapyr applied by the aerial foliar method, which was 46. The estimated MOS=s were undoubtedly higher than those likely to occur
in an actual fire, where a large fraction of the herbicide residues would be destroyed
during combustion (McMahon et al. 1985, Bush et al. 1987a). Herbicide concentrations in
the air dissipate with distance from the burn site; thus the public would be expected to
have lower exposures than on-site workers.
Forestry-use herbicides have been detected in the air at short ranges
(<1 km) after aerial applications (spray drift) but generally not after prescribed
fires in herbicide-treated stands. Forestry herbicides also have not been detected in
regional air mass samples or rainfall during nationwide air quality studies (Majewski and
Cadel 1995). However, agricultural herbicides have been detected in these studies.
Risk analysis concerning the use of herbicide-treated wood in home
fireplaces conducted in the Southern United States (Bush et al. 1987a) clearly
demonstrated that under assumptions that unrealistically produced complete volatilization
of pesticide residues, exposures resulting from burning herbicide-treated wood in a
fireplace resulted in household air concentrations >100 times lower than the acceptable
daily intake. Thus, the safety factor is high and the exposure risk from burning
herbicide-treated wood in fireplaces is very low.
Worker exposure assessments and field studies have shown that risk from
herbicide exposure to forest workers under Abrown
and burn@ conditions is small (MOS >50), even
if the fire occurs immediately after herbicide application, as might occur in a wildfire.
Thus, use of herbicides in combination with fire in site preparation, under-story
vegetation management, or creating wildlife habitat/openings does not increase human
exposure over risks associated with fire alone. Likewise, human exposure to insecticides
from wildfires in recently-treated stands is not likely due to the rapid, flaming
combustion associated with these fires.
This work was supported by the National Agricultural Pesticide Impact
Assessment Program, U.S. Department of Agriculture (USDA) and administered by Forest
Health Protection, USDA-Forest Service, R8, Atlanta, Georgia.
Bush, P.B., D.G. Neary, C.K. McMahon, and J.W. Taylor. 1987a. Suitability
of hardwoods treated with phenoxy and pyridine herbicides for use as firewood. Archives of
Environmental Contamination and Toxicology 16:333-341.
Bush, P.B., J.W. Taylor, C.K. McMahon, and D.G. Neary. 1987b. Residues of
lindane and chlorpyrifos in firewood and woodsmoke. Journal of Entomological Sciences
Majewski, M.S. and P.D. Cadel. 1995. Pesticides in the atmosphere:
distribution, trends, and governing factors. Ann Arbor Press, Chelsea, MI.
McMahon, C.K. and P.B. Bush. 1986. Emission from burning herbicide treated
forest fuels - a laboratory approach. 79th Annual Meeting of the Air Pollution
Control Association, Minneapolis, MN.
McMahon, C.K. and P.B. Bush. 1992. Forest worker exposure to airborne
herbicide residues in smoke from prescribed fires in the Southern United States. American
Industrial Hygiene Association Journal 53:265-272.
McMahon, C.K., H.B. Clements, P.B. Bush, D.G. Neary, and J.W. Taylor.
1985. Pesticides released from burning treated wood. 8th National Conference on
Fire and Forest Meteorology, Detroit, MI.
Weeks, J.A., S.B. Donahoe, G.H. Drendel, R.S. Jagan, T.E. McManus, and
P.J. Sczerzenie. 1988. Appendix A: Risk assessment for the use of herbicides in the
Southern Region, USDA Forest Service. Section 4: Human exposure analysis. Pages 61-63;
Section 5. Human health risk analysis. Pages 31-34 in U.S. Department of
Agriculture, Forest Service. Environmental impact statement: vegetation management in the
Coastal Plain/Piedmont appendices, Volume II. Management Bulletin R8-MB 15. U.S.
Department of Agriculture, Forest Service, Southern Region, Atlanta, GA.
Table I. Parent pesticide (%) and particulate emissions (%) recovered
from burning herbicide-treated (Study 1) and insecticide-treated (Study 3) wood under slow
and rapid burning conditions (adapted from McMahon et al. 1985)
Pesticide recovered %
||Pesticide recovered %
a % of dry fuel
b 98 and 64%, respectively, was recovered as picloram decomposition product 2,
3, 5 trichloro-4-amino-pyridine.
c High recovery reflects an enhanced instrument response in the presence of