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Flame Retardants and the Associated Toxicity
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Flame Retardants and the Associated Toxicity

By Marcelo M. Hirschler, Ph.D. | Fire Protection Engineering



SMOKE TOXICITY, HEAT RELEASE AND FIRE HAZARD

Flame 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

Smoke 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.

Product Survival time
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 3 min


Table 1. Effects of Fire Properties on Survival Time


Table 13 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 safety.

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 of error.

EFFECTS OF INDIVIDUAL COMBUSTION PRODUCTS ON FIRE VICTIMS

A 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:

  • There is excellent correlation between fire fatalities and the concentration of carbon monoxide absorbed in blood as carboxyhemoglobin (COHb).
  • 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 incidents).
  • Fatalities can be linked to COHb levels as low as 20%, and any COHb level above 30-40% is usually lethal.
  • The 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.
  • It is rarely important to measure individual toxic gases for hazard assessment purposes, for any materials, including flame retardant additives.
  • 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.
  • The 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 nitrogen.
  • Moreover, the smoke toxicity of virtually all materials is almost identical. 9, 10


The 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

Seven 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 chemicals.

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 sulfur compounds.

Flame retardants 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 the 1700s.12-14


SMOKE TOXICITY OF FLAME RETARDED MATERIALS

The 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.

Halogenated 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.


Some 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

The 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.


Without 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.

NRC 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:

  • Hexabromocyclododecane
  • Decabromodiphenyl oxide
  • Alumina trihydrate
  • Magnesium hydroxide
  • Zinc borate
  • Ammonium polyphosphates,Phosphonic acid (3-{[hydroxymethyl]amino}-3-oxopropyl)-dimethyl ester,1
  • Tetrakis hydroxymethyl phosphonium salts (chloride salt)

They also recommended that additional exposure studies be made on the following flame retardants to determine whether toxicity studies need to be conducted:

  • Antimony trioxide
  • Antimony pentoxide and sodium antimonates
  • Calcium and zinc molybdates
  • Organic phosphonates (dimethyl hydrogen phosphite)
  • Tris (monochloropropyl) phosphates
  • Tris (1, 3-dichloropropyl-2) phosphate
  • Aromatic phosphate plasticizers (tricresyl phosphate)
  • Chlorinated paraffins

In conclusion, the NRC committee found no significant risk concern with any of the flame retardants assessed, which covered a broad range of chemical compositions.



For 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.

In 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.

Much 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.

Hexabromocyclododecane (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.

No 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

Flame 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.

With 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.


CONCLUSIONS

Flame 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.

However, 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.

Published 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 toxicological concerns.

Marcelo M. Hirschler is with GBH International.


References:

  1. Hirschler, M.M., "Effect of flame retardants on polymer heat release rate,” in Fire and Materials Conf., San Francisco, CA, Feb. 2-4, 2015, pp. 484-498, Interscience Communications, London, UK.
  2. Gann, R.G., Babrauskas, V., Peacock, R.D. and Hall, J.R., Jr., "Fire Conditions for Smoke Toxicity Measurement,” Fire and Materials, 18, pp. 193-99 (1994).
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  4. Hirschler, M.M., "Heat release from plastic materials,” Chapter 12, in "Heat Release in Fires,” Elsevier, London, UK, Eds. V. Babrauskas and S.J. Grayson, 1992. pp. 375-422.
  5. Hirschler, M.M., "General principles of fire hazard and the role of smoke toxicity,” in "Fire and Polymers: Hazards Identification and Prevention” (Ed. G.L. Nelson), ACS Symposium Series 425, Developed from Symposium at 197th. ACS Mtg, Dallas, TX, April 9-14, 1989, Amer. Chem. Soc., Washington, DC, Chapter 28, p. 462-478 (1990).
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  21. Hirschler, M.M., "Safety, health and environmental aspects of flame retardants,” Chapter 6 in "Handbook of flame retardant textiles,” edited by Fatma Selcen Kilinc- Balci, Woodhead Publishing, Sawston, UK, pp. 108-173, 2013.
  22. U.S. National Research Council, Committee on Toxicology, Subcommittee on Flame- Retardant Chemicals: D.E. Gardiner (chair), J.F. Borzelleca, D.W. Gaylor, S. Green, R. Horrocks, M.A. Jayjock, S. Kacew, J.N. McDougal, R.K. Miller, R. Snyder, G.C. Stevens, R.G. Tardiff and M.E. Vore, "Toxicological Risks of Selected Flame-Retardant Chemicals,” National Academy Press, Washington, DC (2000).
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  24. Directive 2003/11/Ec of the European Parliament and of the Council of 6 February 2003 amending for the 24th time Council Directive 76/769/EEC relating to restrictions on the marketing and use of certain dangerous substances and preparations (pentabromodiphenyl ether, octabromodiphenyl ether). Official Journal of the European Union 15.2.2003.
  25. Lukas, C., Ross, L., Beach, M.W., Beulich, I., Davis, J.W., Hollnagel, H., Hull, J.W., King, B.A., Kram, S.L., Morgan, T.A., Porter, M.E. and Stobby, W.G., "Polystyrene Foam Insulation with a Sustainable Flame Retardant: Transition Update,” in Fire and Materials Conf., San Francisco, CA, Feb. 2-4, 2015, pp. 568-583, Interscience Communications, London, UK.
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