CSIRO PUBLISHING
www.publish.csiro.au/journals/ijwf
International Journal of Wildland Fire, 2004, 13, 1–15
Long-term forest fire retardants: a review of quality, effectiveness,
application and environmental considerations
Anna GiménezA,B , Elsa Pastor A , Luis ZárateA , Eulàlia PlanasA and
Josep ArnaldosA
A CERTEC:
Centre d’Estudis del Risc Tecnològic (Centre for Studies on Technological Risk)
Department of Chemical Engineering, Universitat Politècnica de Catalunya, Av. Diagonal,
647, E-08028 Barcelona, Catalonia (Spain).
B Corresponding author. Telephone: +34 93 4016675; fax: +34 93 4017150;
email: anna.gimenez@upc.es
Abstract. Since the beginning of the 1930s research has been directed towards improving the effectiveness of
water as a forest fire extinguishing agent. Throughout this time various chemical substances have been added to
the water, and this is still the case today. Among these substances are the various types of long-term forest fire
retardant, which maintain their ability to alter combustion when the water has been removed by evaporation. In
order to provide an account of the current state of development of studies on long-term forest fire retardants,
we carried out a bibliographic analysis with special attention to work done after 1976 on the different aspects
that influence the final effectiveness of forest fire retardant: quality (programs and evaluation), effectiveness,
application and environmental impact on streams and aquatic organisms, vegetation and humans. The scope of this
work covers the wide subject of fire retardants and it introduces the significant works related to all the aspects of fire
retardant use.
Introduction
Long-term fire retardants have been used extensively for
several years in different countries. Since the first studies (Barret 1931; Truax 1939; Tyner 1941), research has
been carried out on the large number of parameters that
influence their effectiveness and use. As different mixtures
of fire retardant were tried, new aspects arose in relation to their operational use. For example, the corrosive
effects and environmental impact of the retardants were
observed during Operation Firestop (1955a, 1955b). Nevertheless, these aspects were not studied more extensively
until after the works that focused on how to evaluate the
effectiveness of mixed products and on the use of thickening agents were carried out (Davis et al. 1962; Johansen and
Shimmel 1963).
Hardy et al. (1962), as a result of a study to test new mixtures of retardants, showed that ammonium sulfate (AS) and
diammonium phosphate (DAP) reduced the rate of spread and
the radiant energy flux most effectively. From that moment,
research was dedicated to comparing the effectiveness of
sulfates and phosphates, studying their effects on the combustion, pyrolysis and flammability of cellulose. George and
Blakely (1970) studied the rate of weight loss, which is related
to the energy release rate (Er ), the rate of flame spread and
© IAWF 2004
the mass of residue in fuel beds treated with AS and DAP.
The fuel treated with DAP gave a significant reduction in
Er , and a greater amount of char was observed. A further
study by George and Blakely (1972) showed that DAP was
more effective than AS in reducing the parameters studied as
regards flammability. George and Sussot (1971) studied the
thermal behavior of ammonium sulfate and phosphate and
treated fuels in order to identify their effects on combustion
and pyrolysis. They found that the thermal behavior of these
two retardants was different, which may explain why phosphates have a greater impact on glowing combustion than
sulfates and the differences observed by George and Blakely
(1970, 1972).
While the effectiveness studies were developed, the
study of the aerial application of fire retardant began. The
experimental works consisted of drop tests in which a
fire chemical was delivered aerially over a cup-and-grid
system. This was to determine the coverage level produced for each drop type (Storey et al. 1959; Davis 1960;
Grigel 1970, 1971, 1972; Newstead 1973). The data provided by various drop tests allowed modeling of the aerial
delivery in order to estimate the ground pattern of fire
retardant in relation to the interaction of fuels (Swanson
and Helvig 1973, 1974; Anderson 1974) and to the
10.1071/WF03001
1049-8001/04/010001
2
A. Giménez et al.
Table 1. Time-line chart of reviewed research
Studies
1977–1979
1980–1989
1990–1999
2000–2002
Quality
studies
George et al. (1977)
Gehring (1978)
Gehring (1980)
USDA Forest Service (1982)
Van Meter et al. (1985)
George and Johnson (1986)
Gehring and George (1986)
USDA Forest Service (1986)
Johnson and George (1990)
USDA Forest Service (2000)
NWCG (2000)
Blakely (1983)
Blakely (1985)
Blakely (1988)
Blakely (1990)
Effectiveness
studies
Application
studies
Rawson (1977)
Andersen and
Wong (1978)
Van Meter and George (1981)
George (1982)
Blakely et al. (1982)
Rees (1983)
Newstead and
Lieskovsky (1985)
Loane and Gould (1986)
Northeast Forest Fires
Supervisors (1987)
George and Johnson (1990)
Vandersall (1994)
Robertson et al. (1997a, 1997b)
Vandersall (1998)
Johnson and Jordan (2000)
Lovellette (2000)
Calogine et al. (2000)
Xanthopoulos and
Noussia (2000)
Environmental
considerations
studies
Johnson and
Sanders (1977)
Norris et al. (1978)
Larson and Duncan (1982)
Bradstock et al. (1987)
Norris and Webb (1989)
Labat Anderson Inc. (1994)
Hamilton et al. (1996)
Gaikowski et al. (1996)
McDonald et al. (1997)
Buhl and Hamilton (1998)
Adams and Simmons (1999)
Larson et al. (1999)
Kalabokidis (2000)
Buhl and Hamilton (2000)
Little and Calfee (2000)
Little and Calfee (2002a,
2002b, 2002c)
Little et al. (2002)
rheological1 properties (Andersen et al. 1976). In other drop
tests (George and Blakely 1973; George 1975) the authors
observed differences in drop characteristics and in the ground
pattern between retardants that contained gum or clay as
a thickening agent and unthickened retardants. Generally,
gum retardants exhibited more cohesiveness under high shear
conditions and allowed obtaining effective retardant coverage >2 gpc2 (0.8 L m−2 ) with higher and safer drops. Using
the data obtained in these two drop tests, Swanson et al.
(1975) developed the Pattern Simulation Model (PATSIM)
to obtain the first guidelines for aerial retardant drop. This
program was revised 2 years later (Swanson et al. 1977),
and validated experimentally by Swanson et al. (1978). This
work showed the effects of the airtank, gate systems and air
drop performance on the ground distribution pattern. During the extinction operations, at a technical level, it was
significant to know the amount of retardant mixture necessary to attack a given type of wildland fire. Rothermel
and Philpot (1975) developed a model, which was based on
the model of Rothermel (1972), to estimate the maximum
useful concentration to reduce fire spread for different fuel
types. The values varied from 0.67 gpc (0.28 L m−2 ) to 13 gpc
(5.34 L m−2 ) and were determined for each fuel type defined
1 Those
by the National Fire Danger Rating System (NFDRS) of the
Forest Service (USA).
Sometimes, after extinction operations next to streams,
high fish mortality was observed which led to the study of the
acute toxicity of fire retardants to aquatic organisms when it
is applied directly in streams (Davis et al. 1961; Dodge 1970;
Blahm et al. 1974). George (1970) reviewed studies showing
the effects of retardants on aquatic organisms and concluded
that the toxicity effects of retardants came from the amount
of available ammonia.
The performance of fire retardants during extinction operations is influenced by several parameters which are very
different from each other. Since the early stages of research
on fire retardants, these parameters have been studied separately. In the review of George et al. (1976), the parameters
were divided into six categories: product formulation and
evaluation; base requirements; retardant cloud characteristics; delivery characteristics; fire situation effectiveness; and
environmental impact. Later, Hardy (1977) included, more
extensively, all of these parameters in a general work.
The present work reviews the studies (Table 1) carried out
since the article of George et al. (1976), and it focuses on
the analysis of the most studied parameters that affect the use
properties, including viscosity and elasticity, affecting the flow characteristics of a fluid. These properties affect the behavior of the retardant as it is
dropped from an airtanker.
2 gpc: gallons per 100 square feet.
Long-term forest fire retardants
and application of long-term forest fire retardants: quality,
effectiveness, application systems and environmental impact.
Quality of fire retardants: programs and evaluation of
forest fire retardants
As the different types of forest fire retardant were approved,
various methodologies were introduced to determine the
chemical properties of the mixtures and concentrates; and
this allowed the evaluation of their applicability and use in
wildland fires.
Standards for the evaluation of commercially available
long-term forest fire retardant products are to be found
in USDA Forest Service (2000). This document defines
the essential terms employed in relation to the use of fire
retardants and describes the test procedures followed to analyse their constituents to evaluate the fire retardant, in the
laboratory or in the field: tests of mammalian toxicity and
irritation, combustion retarding effectiveness, determination of optimum mixing and physical properties (active salt
content, viscosity, density, pH, product stability, corrosion,
pumpability, abrasion, air drop characteristics, field visibility
and operation field evaluation). This publication sets limits required for each of the measured characteristics that
commercial fire retardants must meet.
The chemical analysis procedures for monitoring the
variety of commercial fire retardant products are based on
methodologies used to test other chemical substances, but
specific conditions, operations, and procedures have been
developed to yield optimum results with normal mixtures of
the ingredients present in fire retardants (Van Meter et al.
1985). The procedures in the field have to be fast: some
of them, for example the amount of retardant salts, require
calibration curves or tables relating to other properties.
Laboratory procedures, on the other hand, have to be exact
in order to improve on the indirect measures in the field
and detect chemicals that are partly absorbed into or react
with others and are therefore not detected. Van Meter et al.
(1985) gave a detailed description of the chemical analysis
procedures in the laboratory to determine the main chemical and physical properties of forest fire retardant mixtures
[ammonium polyphosphate (APP), DAP, monoammonium
phosphate (MAP), and AS].
The amount of active retardant salts is one of the most
important parameters, as it is these substances that alter the
process of combustion in a forest fire; this parameter and the
viscosity are the characteristics analysed for the evaluation of
fire retardants in the field. The sampling and analysis procedures are described in George and Johnson (1986) and later
in NWCG (2000). According to the value obtained, George
and Johnson gave the corrective actions required in order to
bring the properties within the range of these two parameters, while another report showed the values for some of
3 The
3
the parameters for the retardants currently used by the Forest
Service (USDA). The viscosity and the amount of active salts
provide information on the quality but not the effectiveness
of fire retardants. Van Meter et al. (1985) described a method
for measuring the rate of coverage to determine the amount of
mixed retardant applied. This was the only aspect associated
with the effectiveness of the retardants that was evaluated in
the field, under operational conditions.
The most extensive study of retardant evaluation was carried out by George et al. (1977) in order to evaluate a new
type of retardant: the ammonium polyphosphate (APP) liquid
mixtures (LC), and more specifically Fire-Trol 9313 -L, N, P
and Fire Trol 931-L with and without gum or inverter emulsifier thickened. The polyphosphate mixtures are considered
to be the most effective (George et al. 1977), as well as the
following mixtures: DAP, liquid type ammonium pyrophosphate (Pyro) and AS. The evaluation of the APP mixtures
was based on the study of the following parameters: storage, specific weight, viscosity, corrosion, pH values, abrasion
and erosion, combustion retarding effectiveness (superiority
factor), color, health and safety, mixing, salt content, separation, pumpability and air drop characteristics. All of the
methodologies used to analyse the different parameters of
those products followed strictly the applicable regulations,
and the corrosion, stability and air drop characteristics were
studied in depth.
The discussion about the corrosion caused by the APP
formulations was due to the lack of studies and specifications in this field. Fire retardants cause several types of
corrosion and effects of different kinds of the equipment
alloys; the corrosion studies developed later were centered
to cover some of these aspects. These studies were significant because corrosion is one of the characteristics that
has an affect on safety during extinction operations, particularly if we consider the potential hazards and damage
that may be associated with the aerial release of fire retardants. The types of corrosion detected in the alloys of the
aerial release and ground equipment and the mix and storage tanks are: uniform corrosion, intergranular corrosion,
pitting corrosion, galvanic corrosion, fatigue corrosion and
stress corrosion cracking. The maximum allowable corrosion rates because of uniform corrosion are published in
the specification cited above, USDA Forest Service (2000),
which specifies only the execution of corrosion tests for uniform corrosion and intergranular corrosion for four types
of alloys.
Gehring (1978, 1980) carried out laboratory studies to
determine which alloys are most affected by corrosion,
specifically uniform corrosion. Together with previous work
of Gehring (1974), this formed the basis for determining and checking the approved corrosion values caused by
commercialized retardant mixtures.
use of trade firm names in this publication is for reader information and does not imply endorsement by the Centre for Studies on Technological Risk.
4
A. Giménez et al.
The results of the works analysing retardant-induced corrosion are used both to determine whether the mixtures with
corrosion-inhibiting agents fall within the limits set, and
also to ascertain which alloys are most affected by a given
retardant mixture. Gehring and George (1986) developed
guidelines for preventing and reducing the corrosion caused
by fire retardants in airtankers and ground support equipment
(storage and mixing tanks).
Johnson and George (1990) evaluated the corrosivity of
the most frequently used long-term fire retardants (nine
types), short-term fire retardants and foams under different
temperature conditions. This study was performed both with
fresh products and with products that had been stored for
some time. The authors studied the damage caused to the
alloys by uniform and intergranular corrosion. The alloys
used in this study were representative of those exposed in
airtankers and helicopters and in use at retardant storage facilities: 2024 T3-aluminium, AISI 4130 steel, yellow brass and
Az-31-B magnesium. The methods were executed according to the appropriate Forest Service regulations for each
case (USDA Forest Service 1969, 1970, 1975a, 1975b, 1982,
1986). The study was conducted to compare the corrosiveness of the different products that were being used at the
time. The results showed that all the retardants tested were
within the corrosiveness limits for uniform and intergranular corrosion. The significance of this factor does not relate
only to safety; a knowledge of the uniform corrosion caused
by the fire retardants also allowed technicians to choose the
most appropriate retardant in relation to the type of equipment and the specific conditions of the determined base. With
reference to intergranular corrosion, although the alloys did
not exhibit this type of corrosion, the tests had to be carried
out. This was because, according to the results and other data
obtained to date (1990), there was no way of predicting which
salt–inhibitor combinations would cause intergranular corrosion (Johnson and George 1990); just the specification for
long-term retardant USDA Forest Service (2000) determines
within the tests of physical properties.
Effectiveness of fire retardants
The increasing use of the long-term fire retardants proposed
by Hardy et al. (1962) has been accompanied by a steady rise
in the cost of this use, due in large part to the increasing prices
of the chemical products that constitute the active ingredients
of these fire retardants. This increasing price has prompted
further studies aimed at determining which fire retardant is
the most effective. The effectiveness of a fire retardant, as has
been discussed above, is a concept that should include other
more general parameters involved in the use of fire retardants,
but to determine its effectiveness experimentally the parameters to study are the rate of weight loss, the energy release
rate and the amount of residue after burning fuel beds in the
4A
laboratory. The retardant effectiveness cannot be accurately
predicted by analysis of total concentration of active salts, but
must be quantified by burning fuels treated with the chemical
formulations (Blakely 1988).
Blakely (1983) corroborated the conclusions of the studies of George and Sussot (1971), where it was determined
that the effectiveness of fire retardants associated with the
reduction in flammability and combustion was related to the
available P2 O5 in the retardant mixture. Blakely compared
the effectiveness of several retardant products containing
MAP obtained by means of different manufacturing processes and from different manufacturers with the effectiveness of DAP, which has been the standard for comparison
since 1970 (George and Blakely 1972), in an attempt to determine whether the manufacturing process or manufacturer had
an influence on the effectiveness of fire retardants. The rate
of spread and the rate of weight loss were studied statistically
and the authors found that the MAP mixtures tested on an
equal P2 O5 equivalent basis were as effective as DAP and
therefore fire retarding effectiveness of each MAP and DAP,
in a pure form, can be equated on the P or P2 O5 content.
The effectiveness of fire retardant is based on the chemical ability to hinder or reduce the combustion and it can be
evaluated by the superiority factor (SF), the value of which
must be greater than 0.6 for the retardant to be approved by
the Forest Service. Blakely (1988) determined the SF for four
forest fire retardants and compared the effectiveness of the
active salts contained in the long term retardant mixtures. The
salts studied were: MAP, DAP, AS and an MAP–AS mixture
(the samples had no additives or impurities). For the comparison between the fire retardants the amount of active salts
was calculated in DAP equivalents. The MAP–AS mixture
had more effectiveness than DAP or AS when used alone.
These studies about the effectiveness did not determine for
how long retardant agents maintained their ability to reduce
flaming combustion, when they were insufficient to stop all
combustion and then flaming recurred (combustion recovery). Different retardants may have similar capabilities for
reducing flaming combustion but be very different in their
ability to maintain this effect (Blakely 1985). With this idea in
mind, Blakely (1985) conducted experimental tests to validate
a new method for determining the effectiveness of retardants
according to their ability to extinguish combustion recovery.
The assessment was carried out by the study of the effects of
different concentrations of retardants and thickening agents
when the combustion was already under way. The water, fire
retardant and thickening agents were not applied before burning the fuel but when the maximum energy release occurred4 ,
thus allowing the effect of the fire retardant on combustion
recovery to be studied.
Early tests showed that water was the primary agent contributing to flame extinction; therefore, to compare the results
directional radiometer (Gier-Dunkle) was used to measure the maximum value of energy released.
13
12
11
10
9
8
7
6
5
4
3
2
1
100
Water ⫹ 24 g MAP
Water
150
200
250 300 350
Mass of water
400
450
Fig. 1. Time to start of combustion recovery for water, and for water
plus MAP. (Blakely 1990)
100
80
Reduction Er (%)
obtained in the experimentation, the amount of water was
maintained constant in all the trials, in order to determine the
effectiveness of long-term forest fire retardants and thickening agents independently. To test the fire retardants, the
amounts of water and thickening agents were held constant
and different weights of chemical treatments were added as
a variable and, to determine the effectiveness of the thickeners, the amount of the thickening agent was changed while
the amount of water and fire retardant remained constant.
This study determined the value and duration of the rate of
energy release, Er (or the rate of weight loss). By monitoring
the evolution of the variables the author was able to designate eight segments in the resulting weight–time curve. Two
steady states of Er were detected in the tests on treated fuel.
The duration and the value of the steady states of Er were used
to evaluate the effectiveness of the different mixtures applied.
The number of tests carried out by Blakely was insufficient
to evaluate the data obtained statistically, but it was shown that
Blakely’s method was valid for evaluating the extinguishing
ability of different forest fire retardants.
Blakely (1990) used this methodology with different
amounts of MAP and water. This study included a new
parameter, known as ‘knock-down’, which occurs when the
treatment has just been applied. The ‘knock-down’ is a period
of ∼15 s, during which the greatest effect of the fire retardants
and water on flaming combustion is noted. Tests were performed with plain water (144, 216, 288, 360 and 432 g) and
with the addition of 24 g of MAP into the 144, 288 and 432 g
of water to obtain solution percentages of 14.3, 7.7 and 5.3%,
respectively (by weight). The values of Er and the duration of
the steady states obtained were compared for the untreated
and treated fuels with plain water and water plus fire retardants. The study monitored the behavior over time and the
value of energy release during the steady-state periods and
at specific points. It analysed the duration of the buildup to
maximum energy release during the first steady-state period
(time until start), the total time during which the burning fuel
was held at a low Er by the addition of MAP before building
up to its maximum (time to maximum), and the magnitude
of the maximum Er (maximum magnitude).
The objective of the study by Blakely (1990) was to determine the extinguishing ability of plain water and whether
adding small amounts of chemical substances is enough to
increase the period of its effect. The study showed that the
fire retardant used improved the effectiveness of plain water,
reducing the rate of energy release and extending the duration of its effects on flaming combustion. Therefore, chemical
retardant can be a definite advantage during direct attack in
fire suppression efforts (Blakely 1990). When chemical products were added to the water, the time until start (during
the first steady-state) was prolonged, in comparison with
plain water (Fig. 1). By increasing the water treatment from
144 to 432 g the delay increased about 6-fold, while 24 g of
MAP increased this period of time by 273% when added to
5
Time (min)
Long-term forest fire retardants
60
40
Water ⫹ 24 g MAP
Water
20
100
150
200
250 300 350
Mass of water
400
450
Fig. 2. Combustion recovery energy release rate for water and for
water plus MAP. (Blakely 1990)
the 144 g water treatment, and by 56% when added to the
432 g water treatment. The same trend was followed by all
the parameters; there was a significant difference between
the plain water treatment and the treatment with added MAP
when little water was used, and a smaller difference between
a large amount of water and this weight of water plus chemical. This was most notable in the value obtained for maximum
magnitude (Fig. 2), which was reduced by 62% when 144 g
of water was applied and by 80.3% when 24 g of MAP was
added. In contrast, the difference between the treatment with
the largest amount of plain water and the same amount of
water plus MAP was only 2.8%.
Although chemicals reduced the fire intensity and
extended the time until combustion recovery, water proved
to be the agent with greatest capability to reduce flaming to
smoldering combustion. These improvements in the use of
fire retardant on combustion recovery indicated not only the
advantages of their use, but allowed better understanding of
when and how they retarded combustion.
6
A. Giménez et al.
Table 2. Aerial drop tests of long-term forest fire retardant
Agency and country
Reference
Conditions, observations
and factors studied
Results
USDA Forest Service,
United States
Johnson and
Jordan (2000)
Load size
Drop height
Drop speed
Three types of fire retardant: water,
foam and gum thickened
Development of aerial delivery guidelines of fire
retardants for several airtankers. (Wildland Fire
Chemical Systems)
Victorian Forests
Commission, Australia
Rawson (1977)
Havilland Beaver plane
Air speed constant at 148 km h−1
Constant drop heights
Distribution of aerially applied was
studied in Pinus radiata aged
8, 12 and 15 years
Ground distribution patterns from effective ground
level concentration (>0.61 or >0.25 L m−2 )
for studying the influence of canopy
Victorian Forests
Commission, Australia
Rees (1983)
Six types of plane
Air speed between 176 and 215 km h−1
Drop heights were observed
Effective length and width of firebreak (>0.61 gpc
or >0.25 L m−2 )
National Bushfire
Unit (CSIRO)
Loane and
Gould (1986)
Project Aquarius
The firebreaks built were not effective (0.24–5.4
or 0.1–2.2 L m−2 ) for intensity of forest fire
>2000 kW m−1 , in Eucalyptus forest
Canadian Forest
Service, Canada
Newstead and
Lieskovsky (1985)
Four types of airtanker
Load size
Drop height
Drop speed
T ª, wind speed and direction
and the relative humidity
Viscosity
Demonstration and discussion about how physical
retardant properties, environmental conditions and
the type of the tank effect on drop pattern
Fire retardant application
Fire retardant is usually applied from aerial delivery systems, ground application representing less than 1% of the
total fire retardant used each year (Blakely 1990). Within
the aerial delivery it has been estimated that 30% is carried out in direct attack (Gale and Mauk 1983). The ground
application of fire retardant has some disadvantages against
aerial delivery and requires some planning and specific equipment (Northeast Forest Fires Supervisors 1987); however, the
ground application gives better control of the fuel covered.
Aerial delivery is much more frequent than ground application since the latter presents difficulties in the transport of the
chemical products and the problems involved with the use of
this application system.
Aerial delivery and therefore the final ground distribution
are affected by many parameters, each of which influences
retardant drop at different moments. The type of fire retardant, its rheological properties, the type of airtanker and the
drop height are some of the factors that have an impact on
the ground distribution patterns (length, width, and coverage
level). Studies are usually inconclusive because of the large
number of variables, many of them interacting, the magnitude
of the effect of these variables, and the difficulty in quantifying the overall effectiveness of the retardant application
(George 1982). For this reason there has been little agreement on how to evaluate the most significant parameters
(drop height, airspeed, flow rate, load size and elasticity
of retardant) in order to assess the effectiveness of aerial
retardant delivery.
According to Robertson et al. (1997a), these parameters
can be divided into factors that affect the ground pattern,
the actual firebreak characteristics and the critical firebreak
characteristics. The main parameters determining the ground
pattern are the flight altitude and airspeed, the wind direction and speed, the tank geometry and the physical properties
of the additives. The actual effectiveness of the firebreak is
influenced by the vegetation height, the coverage level and
the thickening agents. Finally, the behavior of the forest fire
and the characteristics of the firebreak (width, length and
ground pattern) are the parameters that influence the critical
firebreak effectiveness.
Experimental drop tests have been carried out in the
field to determine the effect of each parameter considered.
Drop tests are usually conducted under a variety of conditions (flight altitude, airspeed and type of fire retardant),
above a grid of cups. The ground distribution pattern, length,
width, and covered area were calculated from the amount of
fire retardant in the cups. Robertson et al. (1997b) gave a
detailed explanation of the drop test methodology used in
New Zealand.
Several drop trials have been carried out by forest agencies in different countries: the United States, New Zealand,
Canada and Australia (Table 2). The trials performed in the
United States by the Forest Service (Johnson and Jordan
Long-term forest fire retardants
2000) have yielded operational results and these tests have
been used to develop aerial drop guidelines. These guidelines associate the ground pattern of the fire retardant with
the airspeed and flight altitude and the coverage level for different types of airtankers and water-like fire retardants or gum
thickened. The other studies summarized in Table 2 evaluate
the effect of some parameters and characteristics of the drop
test on the coated vegetation. Table 2 shows that the drop
height and the drop speed were factors included in all the
studies as factors that influence the pattern of ground distribution. Each study included other factors, depending on their
ability to be measured and the specific aim of the study. The
parameters affecting drop performance and ground distribution are known, but it is still difficult to determine the ground
distribution from specific known parameters.
The rheological properties of the fire retardants are considered to exert a strong influence on the ground distribution
pattern (Van Meter and George 1981; Vandersall 1994, 1998).
It is known that the viscous nature of the solution during
periods of extreme and relaxed shear and its elasticity are
important rheological properties (Vandersall 1994).
Viscosity is not directly associated with the characteristics
and behavior of fire retardant during aerial delivery; however,
its quantification may make it possible to relate these properties more appropriately to the retardant’s full-scale field
performance (Van Meter and George 1981). The retardant
cloud produced just after discharge is torn and rendered into
small droplets because of the interaction of external forces
(shear stress, wind and gravity). Droplet size will depend on
the rheological properties of the retardant mixture. Larger
droplets will be affected by external forces to a lesser extent
than smaller, lighter ones (Vandersall 1994, 1998), and will
result in less dispersion on the ground. Fire retardant mixtures
that contain clay thickening agents (water-like retardant; no
elasticity) exhibit droplet diameters of 2–3 mm, whereas the
individual droplets of guar gum thickened retardant solutions
(gum thickened; elasticity) varied from ∼3.5 up to about
5 mm, depending on the amount of gum contained in the
formulation (Andersen and Wong 1978).
Van Meter and George (1981) carried out laboratory
tests to determine the impact of rheological properties on
aerial delivery effectiveness.The trials employed a 17.8 m s−1
(maximum speed) wind tunnel air stream; there was a special mechanism to apply fire retardant mixture from different
heights (maximum ∼0.762 m) above an array of cups (103
cups) to simulate aerial delivery of retardant. Specific weight,
viscosity, shear stress, surface tension and the droplet size
distribution were quantified for three thickening agents and
for three fire retardant solutions. By correlating the experimental data together with other experimental data from
previous tests, an expression was obtained for the dispersal area pattern. The correlation is valid statistically,
but the different scale in field experiments must be taken
into account.
7
The computer program developed by Swanson et al.
(1975) (Pattern Simulation Model), which did not work for
constant flow tank systems, was used to devise the performance guidelines for airtankers by George and Johnson
(1990). The model requires one main input: the flow rates for
each compartment. The methodology to quantify the flow
rate was described by Blakely et al. (1982), and is executed
by static testing. The other inputs of the program are related
to airtanker characteristics and drop performance, and the
outputs consist of a detailed pattern plot, contour interval
summaries and a scaled pattern plot. If it is necessary to perform consecutive releases from a single tank to reach the
desired ground pattern, the program will need more input
data and a subroutine (PATADD) is used. The guidelines were
developed for 35 different types of airtankers (representing
around 90% of fire retardant aerial delivery tankers in 1990
and now are not 100% applicable). The guidelines provide
information about the characteristics of the tank, the coverage
level, strategic charts from the PATADD option, and finally,
the safe drop height. For the fixed wing airtankers, this factor is the distance below the airtanker at which the retardant
begins to fall vertically, and it has lost its forward momentum.
This correlation, obtained by filming drop tests, depends on
the load size and the peak flow rate, and is expressed as:
S = 151 + 0.112L + 0.0202P,
(1)
where S = Safe drop height (feet), L = Load size (gal), and
P = Peak flow rate (gal s−1 ).
The above equation to estimate the safe drop height is
expressed as follows using the metric system:
S = 46.025 + 0.9018L + 1.6265P.
(2)
In this case, the units of parameters L and P are m3 and
m3 s−1 , respectively.
For the constant flow tank and single airtankers, equations
(1) or (2) do not provide an accurate representation. The safe
drop heights for most of the fixed wing tankers have been
calculated by Lovellette (2000), who determined this factor
for full and partial drops.
Other programs have been started up by various research
centers. The most notable programs have been the ACRE
project, a recent European project coordinated by CEREN
(Centre d’Essais et de Recherche de l’Entente) and the program that have been carried out by USDA Forest Service over
several years (since the 1960s). Since 1998, Wildland Fire
Chemical Systems took care of most of the factors related
to the use of fire retardants, although Aerial Delivery Systems (ADS) was recently created for this purpose. This latter
group has developed drop guidelines for fixed and rotary
wing tankers on the basis of the drop tests summarized in
Table 2 (Johnson and Jordan 2000). ADS has studied the tank
and gating systems, flow rates, drop heights and airspeeds.
These characteristics significantly affect the aerial delivery
8
A. Giménez et al.
of fire retardants. The drop guide developed gave practical
and operational instructions for the most effective coverage
in every fire situation, and the characteristics of the tank.
These guidelines considered fixed-wing tanks’ ability to control flow rates, in order to improve retardant delivery, so as to
obtain a great line length for each coverage level. Although
this type of airtanker provides an effective coverage, not all
fixed-wing aircraft are equipped with it.
The main objective of the ACRE project was to provide
greater insight into the effectiveness and the methodology
for applying fire-retarding chemical additives (Giraud and
Picard 2000). Several studies were carried out to obtain a
model for aerial drop performance in various aerial delivery
systems. The ACRE project began in July 1998 and was completed in October 2000. One of the studies included in this
project was the mathematical modeling of aerial retardant
drop (Calogine et al. 2000). To test this model a simulation was run which provided the results of the length and
the amount of retardant on the ground. These results are
yet to be validated with experimental tests. The model has
several disadvantages, as it does not show the ground distribution and is applicable only to Newtonian fluids. More
work will have to be done to improve and extend the model to
non-Newtonian fluids (such as fire retardants). Nevertheless,
this was the first attempt to solve fluid mechanics equations
more precisely than the rough approximation of the Bernoulli
equation used in PATSIM on modern computers. This
work focused on modeling the rheological behavior of fire
retardants.
Another phase of the ACRE project was centered on smallscale study of the effective firebreak width to reduce and stop
fire spread in wildland fires. A new wind tunnel was used to
carry out two different tests: in the first one retardant-treated
fuel was located after the firebreak, and in the second the
treated fuel was located before the firebreak. The firebreak
was simulated by leaving a gap of varying width (10–50 cm)
in the fuel bed (needles of Pinus halepensis). The retardant
product used was a 20% (4 : 1) solution of Fire-Trol LC (Liquid Concentrate), which was applied in two coverage levels:
1.47 gpc (0.6 L m−2 ) and 2.21 gpc (0.9 L m−2 ). In total, 52
burnings (with the treated fuel after the firebreak) were performed under different wind speed conditions (0.9 m s−1 ,
0.4 m s−1 and without wind) and different load and depth.
There were three thermocouples along the fuel bed. The temperatures recorded and the visual evaluation of the reach of
the fuel after the firebreak by the flame was studied. The
results showed a reduction in the minimum required width of
the firebreak when the fuel was treated. The results obtained
when the fuel was treated before the firebreak also showed
a reduction in the width of the firebreak; however the number of burnings (seven) was not enough to lead to consistent
conclusions.
5 The
Heat received by the fuel on the opposite side of the gap
is the factor most heavily affecting whether a fire will cross
the fuel break and ignite this fuel (Xanthopoulos and Noussia
2000), and the parameter to evaluate this is the first temperature peak; the lowest value recorded for which the flames
crossed the firebreak was 481K (208◦ C).
Environmental impact of fire retardants
Fire retardant delivery into the environment could have toxicity effects on organisms. Initially it was thought that fire
retardants would have no adverse effect on the environment,
as their main active ingredients are agricultural fertilizers;
however, even materials of inherent low toxicity can cause
adverse environmental effects when the rate or intensity of
use is sufficiently great (Norris et al. 1978).
Of the different substances that are present in fire retardant mixtures, various corrosion inhibitors and ammonia are
toxic components. Ammonia comes from the dissociation
of ammonium salts, which are present in the majority of
long-term retardant products.
The toxicity studies have been classified by: (a) the effect
on water quality and consequently on aquatic organisms;
(b) toxicity effects on vegetation; and (c) toxicity effects on
humans. There are three works that review the various studies
of the impact of fire retardant use on people and ecosystems,
by Labat Anderson Inc. (1994), Adams and Simmons (1999)
and Kalabokidis (2000). Kalabokidis’ work has many references to studies on the effects of fire retardants on people,
whereas the work of Adams and Simmons (1999) presents a
review of the environmental impact of long-term fire retardants and foams, with less emphasis on their effects on
people. Labat Anderson Inc. (1994) deals with the ecological
and human risks of the use of fire retardants.
For each long-term fire retardant mixture, the suppliers
publish the safety information in relation to its use (MSDS,
Material Safety Data Sheet); primarily human health hazards
and environmental regulation data are specified along with
toxicity tests5, chemical and physical properties of the fire
retardant, storage and handling conditions and reactivity.
Generally, environmental impact on surface fresh water
caused by retardant delivery is the topic that has been studied
in most detail, whereas fewer studies have been conducted
to determine the effects on vegetation and the reduction of
species diversity caused by added nitrogen and phosphorus
in soils (Adams and Simmons 1999; Larson et al. 1999).
Toxicity effects on water and aquatic organisms
Evidence shows that the main impact that fire suppression
chemicals have on the environment may be through adverse
effects on water quality and subsequently on freshwater fish
and other stream biota (Kalabokidis 2000).
toxicity tests are conducted by the manufacturers themselves or by a contracted laboratory.
Long-term forest fire retardants
As mentioned above, ammonium salts are one of the toxic
components of some long-term fire retardants for aquatic
organisms when it is dissociated by water to ammonia. On
the other hand the oxidation of ammonia to nitrate or nitrite
is very low under test conditions, and these are considered
low toxicity agents (Buhl and Hamilton 2000). The amount
of ammonia in water does not only depend on the concentration of ammonium sulfate or phosphate delivered, but
also on the pH and the temperature of the water, as these
are parameters that influence the equilibrium reaction of
ammonium-ammonia.
Toxicity studies on aquatic organisms relate the results
obtained in the laboratory to the determined amount of fire
retardant and ammonia. The data show that ammonia is the
component which has most impact on these organisms under
laboratory conditions (Johnson and Sanders 1977; Hamilton
et al. 1996; McDonald et al. 1997; Buhl and Hamilton 1998,
2000). The objective of these studies was to establish dose–
response relationships to evaluate the effects of given levels
of fire retardant on specific fishes.
The acute toxicity tests performed in these works compare
the toxicity levels of both long-term fire retardants and foams.
Generally, the results show that foams are more toxic for
the species tested. The methodology and the conditions of
the trials followed the guidelines provided by ASTM (1989,
1998), and the groups of organisms studied were freshwater
fishes, algae and scuds. The tests were carried out in hard and
soft water. Table 3 describes the toxicity tests conducted by
some of these authors.
Many of these tests used freshwater fishes, Pimephales
promelas, Oncorhynchus mykiss (Hamilton et al. 1996) and
Oncorhynchus tshawytscha (Buhl and Hamilton 1998), at different life stages; it was observed that at the eyed egg stage
they were less sensitive due to their protective cell layers.
The results of the toxicity tests showed that fire retardants
applied directly into streams require great dilution to be nonlethal for aquatic organisms; for example, a dilution in the
range of 100–1750 times was necessary to approach a safe
concentration for rainbow trout (Gaikowski et al. 1996). It
is essential, therefore, to avoid aerial retardant delivery near
streams.
Dose–response studies are important because of their
application in simulation models developed to estimate the
effect of direct aerial delivery on water and fish mortality.
Norris et al. (1978) developed an empirical mathematical
model using data from drop tests (Northern Forest Fire Laboratory, USDA Forest Service) and on fish mortality (Blahm
et al. 1974). According to Norris et al. (1978), the fish mortality caused by direct applications into streams (normally due to
an accidental application) depends on three variables: delivery characteristics, the specific characteristics of the area, and
the properties of the stream.
The simulation of the toxicity effects of aerial delivery
requires the construction of three models: the aerial retardant
9
release, the dilution of the retardant as it moves downstream
and, finally, the effect of the retardant concentrations on
fish mortality. The first model not only takes into account
parameters that affect the retardant drop, but also introduces
stream parameters such as the leaf area index, thus taking
into account interception by streamside vegetation. To estimate the downstream dilution of the retardant the dilution
rate of Phos-Chek XA (DAP) in a stream was compared
with the dilution of fluorescent dye, in different stretches
of stream (Norris et al. 1978). Finally, in the third phase of
the simulation, the mortality of fishes was related to the retardant concentration. In this phase a new term was introduced,
the tolerance time, which indicates the maximum length of
time the organism can be exposed to a given concentration
of ammonia with negligible mortality (Norris et al. 1978).
Moreover, the model gives the variation in mortality over time
by taking into account changes in ammonia concentration for
a specific point and time.
Norris and Webb (1989) studied the effect of the direct
release of fire retardant (Phos-Chek XA) in four rivers; the
results corroborated the model developed by Norris et al.
(1978). Changes in the retardant concentration were registered up to a distance of 2700 m downstream, and mortality
was not noticed during the first 24 h for the amount and
release rate of the fire retardant applied in this study.
Recent studies showed that the toxicity of long-term fire
retardants could be heightened in the presence of UV-B radiation and water (Little and Calfee 2000). These authors studied
the impact of UV-B radiation on the fire retardant product and
its effect on Oncorhynchus mykiss and Rana sphenocephala.
The toxicity of fire retardants caused by UV-B radiation is
due to sodium ferrocyanide (a corrosion inhibitor). This product, in the presence of natural solar or laboratory-synthetic
UV-B and water, can decompose by photoactivation to yield
HCN. This substance is toxic for aquatic organisms when it
releases free cyanide; so the toxicity of fire retardants containing sodium ferrocyanide increased significantly when they
are exposed to UV-B radiation.
Further work was carried out (Little and Calfee 2002a)
on the main toxic components of certain retardant products
(total ammonia, un-ionized ammonia, nitrate and nitrite and
sodium ferrocyanide). The inclusion in these studies of other
factors served to enhance the knowledge of the effects of
fire retardants on the environment. These effects included the
environmental persistence of these toxic agents (Little and
Calfee 2002b) and the ability of organisms to avoid exposure
(Little et al. 2002). These three studies are reviewed in Little
and Calfee (2002c) and some data are summarized in Table 3.
These studies carried out by Little and Calfee demonstrated the dangers of using retardants that contain YPS
(sodium ferrocyanide), and observed that photoenhanced
cyanide has a very low LC50 : 50 µg L−1 (Little and Calfee
2002b).According to these authors, the environmental impact
of retardants depends on their environmental persistence and
Pimephales promelas
(60–90 days
post-hatch)
Oncorhynchus mykiss
(90 days post-hatch)
Little and
Calfee
(2002b)
Little et al.
(2002)
weak acid dissociable.
Pimephales promelas
(30–60 days after
yolk absorption)
Little and
Calfee
(2002a)
A WAD:
Oncorhynchus mykiss
(all life stages)
Buhl and
Hamilton
(2000)
Phos-Chek D75-F (MAP–AS)
Two foams
Daphnia magna
Hyalella azteca (invertebrate)
Selenastrum capricornutum (alga)
Oncorhynchus tshawytcha
(all life stages)
Fire-Trol LCG-R (APP)
Oncorhynchus mykiss (all life stages)
Buhl and
Hamilton
(1998)
Fire-Trol GTS-R (AS)
Pimephales promelas (all life stages)
Hamilton
et al. (1996)
Fire-Trol GTS-R (with and
without sodium ferrocyanide or
ferrous oxide colorant)
Phos-Chek D75-R (not containing
sodium ferrocyanide)
Fire-Trol GTS-R (with and
without sodium ferrocyanide)
Phos-Chek D75-R (not containing
sodium ferrocyanide)
Fire-Trol GTS-R (with and
without sodium ferrocyanide)
Phos-Chek D75-R (MAP–AS)
Phos-Chek 259F (DAP)
Five foams and LAS and SDS
(anionic surfactants)
Fire-Trol LCM-R (APP)
Fire-Trol LCA-F (APP)
Phos-Chek D75-F
Two foams
Fire-Trol LCG-R
Fire-Trol GTS-R
Fire-Trol 100 (AS)
Fire-Trol 931 (APP)
Phos-Chek 202 A (DAP)
Phos-Chek 259 (DAP)
Oncorhynchus kisutch (fry and fingerling)
Salmo gairdneri (fry and fingerling)
Pimephales promelas (fingerling)
Lepomis macrochirus (fingerling)
Micropterus salmoides (fingerling)
Gammarus pseudolimnaeus (mature scud)
Johnson and
Sanders (1977)
Retardants
Species
Authors (year)
Avoidance/attractance behaviour of fish
using a counter-current avoidance
(DeLonay et al. 1996)
Toxicity test (LC50 at 96 h, ASTM
1998), under different light treatments.
Determination of the ammonia, nitrate
and WAD cyanide concentration and the
persistence of retardant when weathered in
different substrates and in diluted solutions
Percentage of fish mortality in a field
experimental stream. Determination of
the total and unionized ammonia, nitrate,
and WADA cyanide concentration
Toxicity test expressed as LC50 , at 96 h,
in soft and hard water. (ASTM 1989)
Determination of the ammonia,
nitrite and nitrate concentration
Toxicity test expressed as LC50 ,
at 96 h, in soft and hard
water (ASTM 1989, 1990)
Determination of the ammonia,
nitrite and nitrate concentration
Toxicity test expressed as LC50 , at
24, 48 and 96 h, in soft and hard water.
ASTM (1989), ASTM (1990)
Determination of the ammonia, nitrite
and nitrate concentration
Toxicity test expressed as LC50 , at
24 and 96 h. APHA (1971);
EPA (1975); Mount and Brungs (1967)
Method
Table 3. Toxicity tests of aquatic organisms
Rainbow trout avoided at concentrations 10% of LC50
in water treated with GTS-R and less of 1% with D-75-R.
Fish did not avoid YPS when this was tested
alone. Salinity of the test water might be the sensor
that induced avoidance response.
Low LC50 values showed enhanced toxicity
induced by UV photoactivation. Environmental
persistence of retardants depended on the type of
substrate; persistence was lower when percentage
organic matter was higher. Dilute solutions
were not toxic after 7 days of weathering
100% of mortality occurred in sunny conditions (at
128 mg L−1 of GTS-R with YPS), but no mortality
occurred in cloudy conditions. 100% of survival
in tests with GTS-R without YPS and D75-R
There is an important variability of the
toxicity values of fire retardant evaluated
Foams have more toxicity than long-term retardants.
The greatest mortality was in the first 24 h
For all life stages the most toxic products is
Phos-Check WD-881 and the least is Fire-Trol LCG-R
The life stage less sensitive is eyed-eggs for
all fire retardant products in soft and hard water
The Fire-Trol mixtures have more toxicity
on algae than invertebrates, as was also
observed in growth tests
Generally these five fire retardants
have the same toxic effect on fishes
Fire-Trol mixtures have less toxicity
than Phos-Chek mixtures, for these species
Results
Long-term forest fire retardants
their application rate. The first aspect was evaluated by field
tests in two different experiments. First, the retardants FireTrol GTS-R (AS) and Phos-Chek D75-R (MAP–AS) were
watered for several days (up to 45), on different substrates
ranging from non-porous polypropylene surfaces to forest
soils with both low and high organic matter content. Second,
field tests of the persistence of diluted aqueous solutions prepared at LC50 were carried out on fish (Table 3) and added
to substrates. As a result, they showed that the soil type and,
above all, the cation exchange capacity (CEC), which determines the ability of soils to exchange ions with the substances
in contact with the soil in such a way that leaching is reduced,
was likely to be a significant variable in the persistence of the
retardants.
The behavior of fish was studied by Little et al. (2002). The
results showed an avoidance response to low concentrations
of the retardants tested6 (Table 3). This was definitely not in
order to discard the hazard of using fire retardant, since many
factors could inhibit this behavior (chemical substances,
temperature of water or pH values). This avoidance behavior
could affect the habitat negatively, altering aquatic ecosystems and causing significant biological and economic injury
to natural resources (Little et al. 2002). This study also tested
a solution of YPS with UV-B, which the fish did not avoid.
The increased toxicity of fire retardants in the presence
of YPS has been demonstrated, mainly in the work of Little
and Calfee (2002a). The absence of mortality during the tests
with the formulation that did not contain YPS is evidence of
the role of this corrosion inhibitor. On the other hand, the
absence of mortality under cloudy conditions while testing
the formulation with YPS proves that a minimum level is
necessary for the release of cyanide from the YPS to occur.
Finally, other fire-related substances such as ash, which, at
low concentrations clogs gill surfaces and leads to respiratory
failure, may have a higher toxicity than the fire retardants
themselves. In comparison with ashes, the hazard posed by
fire retardants may be negligible in view of large amounts
of ash that are likely to enter aquatic systems from rainwater
runoff (Little and Calfee 2002b).
Toxicity effects on vegetation
When fire retardant is dropped in excess, these chemical products can enter plant organisms and soil and have toxic effects
on the vegetation (Bradstock et al. 1987). Up to now, few
studies have been carried out to determine the impact on
the environment, and each of them deals with different plant
species and employs different methods.
Larson and Duncan (1982) studied the vegetation growth
of a burned area where fire retardant salt (DAP) was airdropped during extinction operations. The main plant species
present in the plot were Bromus mollis, B. diandrus, Vulpia
6
11
megalura and Erodium botrys. The area was divided into different types of plots: unburned plots with DAP, burned plots
with DAP, burned plots without DAP, and unburned plots
without DAP (these last two types of plots being the control
area). During the first year, both area treated with retardant
yielded 12 000 kg ha−1 , twice as much as the unburned control area. In the second year this difference in the yield was not
maintained, the highest yield being 4171 kg ha−1 , which was
produced by the treated burned plots, whereas the unburned
plots without DAP had the lowest yield, 2076 kg ha−1 . The
unburned control plots had a higher herbage yield than the
unburned, treated one. According to the results of the study,
the use of fire retardants increases forage yield in the first
year after a fire, but causes a drop in native clover production
for 2 years. However, in order to determine the true effect of
the retardant on these plants over a long period of time, longterm studies of at least 10 years would be needed. Although
these results might be attributed to the fact that retardants
are fertilizers, this work allowed better understanding of the
role of these products in species richness and the growth of
vegetation.
The most specific study was carried out by Bradstock et al.
(1987), and described the toxic effects of a solution of ammonium sulfate and Kelzan (an algae-based thickening agent)
on a particular vegetation type (Eucalyptus forest). The tests
were conducted in the field and in the greenhouse to determine the effects of each component of the fire retardant on
leaves and to ascertain to what extent rainfall can reduce
ammonium sulfate damage. The results showed that ammonium sulfate is the agent that damages and kills leaves. In the
field it was also determined that there had been changes in the
coverage level of the vegetation and the number of species,
but the changes observed over a year were assumed to be
natural changes (Bradstock et al. 1987).
Larson et al. (1999) studied the acute toxicity of a longterm fire retardant (Phos-Chek G-75 (MAP–AS)) and foam
on bush vegetation in two different habitat types (riparian
and upland), in burned and unburned areas. The parameters
measured were growth, resprouting, flowering, and incidence
of galling insects mainly on Chrysothamnus viscidif lorus and
Artemisia tridentata, which were not affected by the chemical
product.
Community characteristics (species richness, evenness
and diversity) and the number of stems m−2 in woody and
herbaceous plants were also measured. The majority of these
characteristics showed no response to the chemicals over the
course of the application (Larson et al. 1999). The lack of
a response in growth during the treatment seemed to disagree with the results obtained by Larson and Duncan (1982).
However, this difference was attributed to the weather, specifically a lack of precipitation. The relatively short duration
of the study was a limiting factor in the determination of
Fire-Trol GTS-R with and without sodium ferrocyanide or ferrous oxide colorant and Phos-Chek D75-R did not contain sodium ferrocyanide.
12
A. Giménez et al.
the real effect of the application of fire retardants on the
vegetation.
Toxicity effects on humans
The majority of the work done to determine the impact of
retardant use on people is reported by Labat Anderson Inc.
(1994), where hazard analysis was employed to determine an
acceptable dose level for the various commercial fire retardants. This dose level was compared to the estimated doses
to which fire-fighters were exposed. The results of the comparison classified the hazard of the fire retardants approved
by the USDA Forest Service as negligible. The risk was considered to be significant only when the fire retardant comes
into direct contact with people. Some cases of skin and eye
irritation have been detected.
As discussed above, corrosion inhibitors are considered
toxic substances; the toxicity test conducted on animals (rats)
has determined the lethal dose of fire retardant mixtures. For
a fire retardant mixture that contains sodium dichromate7 as a
corrosion inhibitor, the lethal dose considered is 3 L of retardant mixture for a man weighing 90 kg (USA Department of
Health, Education and Welfare Toxic Substance List).
Conclusions
This bibliographic analysis has shown that works on longterm forest fire retardants encompass a significant variety of
parameters and this fact means that the results obtained from
these studies were different according to the factor evaluated.
On the other hand, if one considers the number of possible
variables, the works can be considered quite consistent. Each
parameter has a different influence on the effectiveness of
long-term forest fire retardants.
The various methodologies and the parameters to be evaluated to determine the quality of a fire retardant (chemical
and physical properties) are known, and these procedures are
established in various internal specifications (USDA Forest
Service). However, there are differences between the parameters evaluated in studies on aerial fire retardant delivery
due to the difficulty of controlling some of them, and sometimes external considerations may override the experimental
measures affecting the results.
The P2 O5 available in the DAP or MAP is known to be
the substance that makes these retardants more effective than
AS mixtures on combustion, particularly glowing combustion. The parameters to be studied in order to evaluate the
effectiveness of fire retardants during fuel bed burning are
also known. Most studies have served to determine the effectiveness of retardants during indirect attacks, because this is
their primary use. The works of Blakely (1985, 1990) contributed a new evaluation methodology, during direct attack,
including new parameters for evaluation. These two papers
7 The
gave a detailed account of the amount of time the effect
on combustion is maintained in relation to the amount of
water, thickening agent and retardant applied. Blakely (1985)
corroborated the fact that using a retardant is an improvement
on plain water, but no further studies to determine the mechanism that extinguishes combustion recovery, so as to improve
the effectiveness of fire retardant use, were carried out.
Simulation models of aerial delivery of fire retardant were
carried out to obtain distribution patterns providing a description of several coverage levels. The effective coverage for
other fuel types or models linked retardant coverage levels
to effectiveness are defined using the range of NFDRS; even
so the adaptations do not provide the best basis for aerial
drop guidelines with a view to obtaining a known effective
coverage for other fuel models.
According to Gale and Mauk (1983), 70% of the aerial
retardant delivered is by way of indirect attack.Although most
of the effectiveness studies were made in indirect attack, the
results obtained were not evaluated with the purpose of giving operational and technical data for improving chemical
firebreaks building; only one study was found about building
of firebreaks (Xanthopoulos and Noussia 2000). The results
obtained in the laboratory were not transferred to full scale,
which would have provided guidelines for an effective firebreak. In that case the firebreak was simulated by leaving a
gap rather than considering aerial delivery of fire retardant.
This bibliographic analysis shows that there are no specific
studies on the building of chemical firebreaks even though
in the United States the aerial delivery of fire retardant is
generally for indirect attack.
Another issue that must be taken into account with regard
to the use of fire retardants is their environmental impact. The
toxic effects of fire retardants on streams and aquatic organisms are considered the most significant, and for this reason
there are a significant number of toxicity studies in comparison with works on the toxic effect on vegetation (Table 1).
The amount of fire retardant and the place where it is delivered are the two main parameters determining the degree of
environmental impact. By taking these two factors into consideration it is possible to reduce the fish mortality caused
by fire retardants. The most serious environmental impact of
fire retardant use can be mitigated by taking preventive action.
For example, the USDA Forest Service recommends avoiding aerial delivery of retardant near streams as a performance
guideline.
Lastly, it should be emphasized that most of the environmental impact and toxic effect studies on long-term fire
retardants have appeared over the last 10 years (Table 1),
whereas studies dealing with the effectiveness and application of fire retardants are older. Nowadays, there is not so
much debate, and few studies aimed at determining the factors that affect aerial retardant delivery or in studies providing
use of this corrosion inhibitor was discontinued in the early 1970s, and it was never in most retardants.
Long-term forest fire retardants
an improvement in their use (Table 1) (when, where and
how to apply retardant) of forest fire retardants and their
effectiveness in the extinction of forest fires.
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