By Marcelo M. Hirschler, Ph.D.
Fire Protection Engineering
SMOKE TOXICITY, HEAT RELEASE AND FIRE HAZARD
retardants are incorporated into materials to improve their fire
performance, normally by slowing fire development. They are either added
into an existing polymeric material (natural or synthetic) or reacted
with other raw materials to create a new material so that the resulting
material exhibits improved fire performance. This typically results in a
decrease in the amount of combustion products released in a fire.1
toxicity and heat release are key factors in fire hazard, together with
flame spread and ignitability. In fact, "inhalation of combustion
products” is listed as the cause of death for some 2/3 of all fire
victims. It is rare for multiple fire fatalities to occur in fires that
have remained small. In the U.S., more than 83% of fire deaths in
building fires happen in fires that have become very large. Such fires
are large enough that they extend beyond the room of origin, and thus
generate too much toxic smoke for survival.2 The inherent
toxic potency of smoke resulting from burning most combustible materials
falls within a narrow range, so that there is no nontoxic smoke.
Therefore, releasing lower mass of combustibles is essential to lower
the overall toxicity of a fire atmosphere. Moreover, the key fire
property controlling the loss of human tenability in fires is the heat
release rate of the burning materials, 3 which governs the intensity of a fire and can vary by orders of magnitude for common combustibles. 4 Thus, toxic hazard is a more direct function of heat release rate rather than of the toxic potency of the smoke.
Chair (base case)
> 10 min
Chair igniting twice as fast
> 10 min
Chair with twice as high toxic potency
> 10 min
Chair with twice as high heat release rate
Table 1. Effects of Fire Properties on Survival Time
shows predicted survival time from an upholstered chair fire in a
standard room. The data show the different results from varying toxic
potency of smoke versus heat release and the dramatic survival time
declines for the latter. This is a very important concept, because it
puts into perspective the importance (or lack of it) of smoke toxic
potency data in terms of fire hazard assessment, or simply of fire
During the 1970s and 1980s,
there was a belief that burning plastic materials produced smoke that
was far more toxic than smoke from burning natural products such as
wood, wool, or cotton. A number of studies have been done to compare the
amount of carbon dioxide, carbon monoxide, and hydrogen cyanide
produced by natural and synthetic materials under flaming and nonflaming
conditions in order to model smoke toxicity. This work resulted in the
development of multiple small-scale smoke toxicity test methods, all of
which gave varied and narrow rankings for materials, resulting in
limited applicability to predicting outcomes of fire events.
Toxicologists studying toxicity data consider that ranges of toxicity
are measured in orders of magnitude step changes, while most combustible
materials produce data that are comparable and differences between
materials is generally of minor importance to the overall toxicity of
smoke.5 In other words, the smoke toxicity of virtually all
materials, natural or synthetic, is almost identical, within the margin
EFFECTS OF INDIVIDUAL COMBUSTION PRODUCTS ON FIRE VICTIMS
pair of studies involving more than 5,000 fatalities (between fire
victims and non-fire victims or carbon monoxide (CO) inhalation)
addressed: (a) a period between 1938 and 1979 in a localized area
(Cleveland, OH) and (b) a countrywide study in the early 1990s.6
They found remarkable similarities between the populations of victims:
they all died primarily of CO asphyxiation. Other studies have shown
that the fraction of any combustible converted into CO in a large
(typically flashover) fire is approximately 0.2 g/g.7,8 By combining the conclusions of the studies above and others it can be concluded9 that:
is excellent correlation between fire fatalities and the concentration
of carbon monoxide absorbed in blood as carboxyhemoglobin (COHb).
concentrations in blood are the same (when comparing populations of the
same type) in fire and non-fire CO deaths (e.g., defective space heater
Fatalities can be linked to COHb levels as low as 20%, and any COHb level above 30-40% is usually lethal.
toxicity of fire atmospheres is determined almost solely by the amount
of CO, since there is no difference in the COHb levels in the blood of
victims of poisoning by pure CO or in fire victims, once other exposure
factors have been considered.
The concentration of CO in fire atmospheres is roughly 20%,10 irrespective of what materials have burned.
is rarely important to measure individual toxic gases for hazard
assessment purposes, for any materials, including flame retardant
The primary usefulness
of measuring toxic gases issued by burning materials is usually in
terms of material development so as to understand its fire performance.
most immediately dangerous chemicals produced during all fires are
those that behave as chemical asphyxiants, such as CO, responsible for
most deaths in fires, and hydrogen cyanide, along with smaller
contributions by irritants such as hydrogen halides or oxides of
Moreover, the smoke toxicity of virtually all materials is almost identical. 9, 10
overall conclusion from a large body of research is clear: fire
fatalities are overwhelmingly associated with heat release because when
heat release rate increases it leads to more CO generated. Thus, as
fires become bigger, they have higher smoke toxicity.
TYPES OF FLAME RETARDANTS AND THEIR EFFECTS
key chemical elements are known to interfere or disrupt combustion:
chlorine, bromine, phosphorus, aluminum, boron, antimony, and nitrogen.11
These elements are not used as such but provide the essential
functionality into substances known as flame retardants. A flame
retardant could contain one or more of these elements. Flame retardants
act by various mechanisms, including free radical gas phase quenching,
physical barrier formation by charring or contribution of water. Flame
retardants improve fire performance by interfering with the availability
of fuel, oxygen, or ignition source (fire triangle components).
Effective flame retardant designs are rarely composed of a single flame
retardant and may (depending on the substrate) contain a multiplicity of
Two of the elements
mentioned (chlorine and bromine) are known as halogens (another halogen
exists, fluorine, which is not used in flame retardant additives but is
found as part of polymers known as fluoropolymers). Halogenated flame
retardants are generally considered the most effective and can be used
at some of the lowest concentrations. During combustion, free radicals
containing bromine or chlorine quench the fuel source in the gas phase.
On the other extreme, some purely inorganic materials (such as metal
hydroxides: alumina trihydrate or magnesium hydroxide) are used to
provide water, released during combustion to lower gas phase
temperatures. Such flame retardants are the highest volume products used
commercially but have limited applicability because they need to be
used at very high concentrations, often resulting in deleterious effects
to properties of the substrate material, such as flexibility. Between
these extremes are phosphorus-containing materials, which can form
protective char barriers on the surface of a burning material. Their
performance and applications are often improved by including other
elements, such as nitrogen or a halogen. Flame retardants are also used
in combination with other additives (to improve the functionality of the
base flame retardant) that affect fire performance or lower smoke
release. They include materials based on molybdenum, tin, zinc, and
cannot make materials "fire proof.” Flame retardants are an important
first line of defense (in the case of fire) by slowing the combustion
process (or even preventing it) and by lowering the resulting heat
release and flame spread. A large and sustained heat input can overwhelm
the effect of flame retardants and the material can still burn. Flame
retardant materials are being developed continually, and the total
number of flame retardant additives that have been used commercially can
be counted in the thousands, since the first one used commercially in
SMOKE TOXICITY OF FLAME RETARDED MATERIALS
overall smoke toxicity of materials containing flame retardants is not
significantly different from that of materials that do not contain flame
retardants (as discussed above). In fact, properly flame retarded
materials will generate less mass of smoke and combustion products, thus
causing fire atmospheres to be less toxic (as shown in a famous NBS
study15). Thus, the use of flame retarded materials will not
alter the smoke toxicity in fire atmospheres. The basic function of
flame retardants in interfering with the combustion process means that
there will be more incomplete combustion. However, as discussed above,
in large fires the fraction of burnt material converted into CO is
fairly constant, at 20%7-9 so that there is no significant effect of flame retarded materials in actual fires.
materials (including ones with halogenated flame retardants) will
contribute halogenated effluents, including acid gases, that will
contribute to the acute toxicity of fire atmospheres, although it is
normally overwhelmed, as discussed above, by the toxicity of CO. In some
cases, the thermal decomposition or combustion of halogenated materials
generates small amounts of polyhalogenated dioxins and furans as
components of the associated smoke. The composition of emitted gases
will depend not just on the material burnt but also on the presence of
catalysts (including metals like copper) and the fire intensity. The
concentrations of these gases are so small that they are not associated
with acute smoke toxicity but with the chronic effects resulting from
fires. In fact, advances in analytical and detection techniques mean
that scientists can now detect the presence of materials, or derivatives
of materials, at levels so small as to not be meaningful. Thus, they
may affect primarily those facing repeat exposures like firefighters.
of these halogenated dioxins and furans fall into the category of known
human carcinogens, and thus research has analyzed smoke and soot
residues to determine their concentrations during and after fires. A
plethora of research has shown that all fire atmospheres contain large
amounts of known carcinogens, especially polynuclear aromatic
hydrocarbons (PAH), including benzo[a]pyrene [BAP], formed by all
burning materials. In fact, BAP is the one combustion product with the
highest level of toxic carcinogenicity. Therefore, work has been done
comparing the toxic effects of dioxins and furans with those of PAHs. It
was found that the concentrations of dioxins and furans in particulate
residues were at levels 4,000 times lower than those of PAHs.16-19
Moreover, analysis of pollutant data gathered from two well-documented
German catastrophic fires found that PAH levels were thousands of times
higher than those of polyhalogenated dioxins and furans.20
Essentially, all reports to date indicate that dioxins and furans pose
only a very minor exposure risk while the exposure risk to known human
carcinogenic components, like PAHs, is extremely high and unaffected by
the presence of halogenated compounds in a fire.
INHERENT TOXICITY ISSUES ASSOCIATED WITH INDIVIDUAL FLAME RETARDANTS
vast majority of flame retardants are not carcinogens, mutagens or
reproductive toxins, and are neither bio-accumulative nor have acute
toxicity. In 2000, the U.S. National Research Council (Committee on
Toxicology, Subcommittee on Flame-Retardant Chemicals, NRC) presented
findings to the U.S. Consumer Product Safety Commission (CPSC) and the
U.S. Congress.21,22 The work analyzed the inherent toxic
effects of individual flame retardants or flame retardant classes, both
on their immediate effects (acute) and on their long-term effects
(chronic), with primary focus on the latter. This was done by analyzing
the toxicological and exposure data of 16 key flame retardant chemicals
to assess potential health risks to consumers (primarily in residential
furniture). The subcommittee was also asked to identify data gaps and
make recommendations for future research. NRC made assessments to
determine whether causal relationships existed between the dose of each
chemical and each adverse health effect by reviewing human
(epidemiological studies, clinical observations, and case reports) and
laboratory animal data on neurotoxicity, immunotoxicity, reproductive
and developmental toxicity, organ toxicity, dermal and pulmonary
toxicity, carcinogenicity, and other local and systemic effects. NRC
also reviewed in vitro data to determine the potential for genotoxicity
as well as other toxic effects and to understand the mechanisms of toxic
action. Toxicokinetic studies were also reviewed to understand the
absorption, distribution, metabolism, and excretion of the FR chemicals.
For some types of toxic effects, notably most cancers, the subcommittee
conservatively assumed that no threshold for a dose-response
relationship exists or that, if one does exist, it is very low and
cannot be reliably identified. Therefore, the subcommittee’s
risk-estimation procedure for carcinogens was different from that for
non-carcinogens: the relationship between the incidence of cancer and
the dose of a chemical reported in an epidemiological study or an
experimental animal study was extrapolated linearly to much lower doses
at which humans might be exposed in order to overestimate conservatively
the excess lifetime risk of cancer resulting from lifetime exposure to a
chemical at a particular dose rate. This procedure does not provide a
"safe” dose with an estimated risk of zero (except at zero dose),
although at sufficiently low doses, the estimated risk becomes very low
and is regarded to have no public-health significance. The actual risk
is also highly likely to be lower than the upper bound, and it might be
zero. In the final phase of the risk assessment process, NRC integrated
the data to determine the probability that individuals might experience
adverse effects from a chemical under anticipated conditions of
exposure, by calculating a hazard-index to judge whether a particular
exposure would be likely to present a non-cancer toxicological risk.
going into detail, for most of the most widely used flame retardants,
NRC concluded that the hazard indices for non-carcinogenic effects are
less than 1 for all routes of exposure for all flame retardants studied,
meaning that they are not a concern. Carcinogenic risk assessments
performed on the flame retardants that were found to be or likely to be
carcinogenic indicate that some of the estimated excess cancer risks may
be greater than 1×10 -6. However, the NRC committee
concluded that actual carcinogenic risk is likely to be much lower
because of the extremely conservative (high) exposure estimates. Several
of the flame retardants analyzed were actually chemical classes rather
than single compounds. In those cases, one chemical was selected as a
surrogate on the basis of representativeness. Conclusions were based on
the properties of the surrogate and the risk that other members of the
class might be different from the risk of the surrogate. It is important
to point out that this study (as opposed to many other studies) did not
focus exclusively on halogenated flame retardants but discussed all
types of chemistries.
intentionally overestimated exposure levels as a precautionary approach
to the protection of public health and concluded that the following
flame retardants can be used on fabrics for residential furniture with
minimal risk, even under worst-case assumptions:
conclusion, the NRC committee found no significant risk concern with
any of the flame retardants assessed, which covered a broad range of
some of these materials, additional studies were performed after the
NRC work, much of which was done for European Union risk analyses, and
it filled in some of the gaps identified. Two brominated flame
retardants, not directly studied by NRC, have been clearly associated
with potential health issues and withdrawn from the market:
pentabromobiphenyl oxide (pentaBDE) and octabromobiphenyl oxide
(octaBDE). When neither chemical has proven health effects (including
carcinogenic effects) on humans but the chemicals are bioaccumulative
and do have proven health effects on animals, they should not be used.
PentaBDE and octaBDE may enter the body by ingestion or inhalation and
they are stored mainly in body fat. EPA studied pentaBDE in detail in
2008.23 Following a comprehensive risk assessment, the European Union banned the use of both pentaBDE and octaBDE since 2004.24
In the U.S., as of 2005, "no new manufacture or import of” pentaBDE and
octaBDE "can occur...without first being subject to EPA evaluation” and
in May 2009, both were added to the Stockholm Convention on Persistent
Organic Pollutants as it meets the criteria for the so-called persistent
organic pollutants of persistence, bioaccumulation, and toxicity.
December 2009, all manufacturers voluntarily phased out production of a
flame retardant in the same family as the last two, decabromobiphenyl
oxide (decaBDE), in spite of the lack of proven health effects. The main
reason for this action by the manufacturers is that many of the
properties of decaBDE are similar to those of pentaBDE and octaBDE even
if the health effects are not.
earlier, the first flame retardant found with negative health effects
(carcinogenicity), and voluntarily withdrawn from the market (for
children’s sleepwear) in the 1970s, was brominated tris [tris
(2,3-dibromopropyl) phosphate], which has not been used commercially
since. This is a different material from the chlorinated tris flame
retardant used more recently for furniture, and which is being
misidentified in the press as the same material.
(HBCD) has been found to have the potential for ecotoxicity but no
demonstrated effects on humans have been reported. The Stockholm
Convention recommended its inclusion in a list of persistent organic
pollutants, an action not completed as of 2015. However, in 2014
manufacturers of HBCD, extensively used as a flame retardant for
polystyrene thermal insulation, in conjunction with the manufacturers of
the polystyrene foam itself, have decided to replace HBCD in the foam
with a polymeric brominated flame retardant (polyFR),25 which has very low bioavailability and intrinsic toxicity and is, thus, not bioaccumulative.
other flame retardant has, at least until 2015, been demonstrated to
have such an effect on risk to humans that it was deemed necessary to
eliminate it from the market.
HEALTH EFFECTS OF FLAME RETARDANTS
retardants, as discussed above, do not significantly contribute to
acute toxicity in fires. Toxicologists comparing acute toxicities use a
toxicity classification scale for inhalation that places LC50 (toxic potency) values of 10 to 100 in the highly toxic category and values of 10 or less in the extremely toxic category.26
The smoke toxic potency values of flame retarded materials are so
similar to those of the same materials without flame retardants that
they are not statistically significantly different. Moreover, as
properly flame retarded materials will generate lower masses of
combustion products, they will often cause fire atmospheres to be less
toxic. Thus, the use of flame retarded materials will not alter smoke
toxicity in fire atmospheres. With regard to the effects on the health
of firefighters, it is undoubtedly true that firefighters should have
special concerns because the rates of many chronic diseases, including
cancers, are higher among firefighters than among the general
population. These health effects on firefighters will be minimized by:
(a) the continued use of self-contained breathing apparatus both during
firefighting and during overhaul operations (after the fire has been
brought under control) and (b) improvements in the effective treatments
of firefighter protective clothing after each use. However, there is no
evidence that this is associated with the use of flame retardants. In
fact, there is significant evidence that the added effect of the
combustion or thermal decomposition products of flame retardants have an
insignificant added effect on toxicity.
regard to halogenated flame retardants, the data discussed above showed
the extremely minor contributions to the concentrations of carcinogens
in smoke and soot that polyhalogenated dioxins and furans (resulting
from halogenated flame retardants) make relative to the extremely large
contributions from PAH.
retardants are based on many individual chemical components, including
not just halogens. Some of them are also used in household applications
unrelated to fire safety. Thus, any scientifically based discussion of
the toxicity and/ or health effects of flame retardants needs to address
the specific material of potential concern and not a generic catch-all.
While it is essential to ensure that materials with negative health
effects are not used, this cannot be interpreted as a blanket attack on
flame retardants in general or even on brominated and/or chlorinated
flame retardants. Every flame retardant offered for commercial use
should always be investigated and those materials proven to be toxic or
harmful should be prohibited from use.
flame retardants offer an important way to maintain robust fire safety
in product and building designs. They are an essential first line of
defense in terms of passive fire protection. Flame retardants are a
broad class of materials with unique functionality, hazard
characteristics, and impacts on fire events.
data overwhelmingly shows that flame retardants do not contribute
significantly to either acute or chronic fire toxicity in real fires.
While some flame retardants have been removed from the market in recent
years, the vast majority in commercial use do not present significant
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