By Vytenis Babrauskas, Ph.D., FSFPE, and Heather M. Stapleton, Ph.D. | Fire Protection Engineering
The Nature of FR Chemicals
Flame retardant (FR) chemicalsA are any compounds specifically added to materials to interfere with their combustion. This interference is normally to achieve some desired fire performance attribute – slower ignition, reduced flame propagation, etc. Vast numbers of compounds have been proposed in the patent literature for this purpose, but only a small portion have come into widespread use. Plastics and wood are the main categories of products to which FR chemicals are often added. The FR chemicals used for treating cellulosic materials (wood, cotton) and treating plastics are generally from completely different families of compounds. Many of the FR chemicals are specifically suited for only one type of product. Within the broad families, cellulosic products are often treated with ammonium phosphate, ammonium sulfate, borax, boric acid, and other compounds based on nitrogen, phosphorus, or boron chemistry.A Most of those compounds have not been found useful for treating plastics. Instead, for a rudimentary level of FR action in plastics, inorganic fillers (aluminum trihydrate, magnesium hydroxide) have been used where they perform through endothermic decomposition resulting in the release of water molecules. Otherwise, halogenated organic compounds have tended to dominate use in plastics.
By the mid-1950s, it was already appreciated that bromine or chlorine atoms can interrupt the complex chain of reactions that needs to occur for combustion to take place.1-3 Molecules containing bromine or chlorine atoms are referred to as halogenated. There are other atoms in the halogen series besides these two; but for various reasons, they have not been found useful for FR purposes. Even so, many Br- or Cl-containing compounds do not constitute successful FR chemicals, so that eventually only a few families of such compounds came to be used. It was found that effectiveness is higher when there are numerous halogen atoms in the molecule, thus practical compounds have been poly-halogenated. In addition, because plastics processing must take place within a narrow range of temperatures specific to the polymer involved, many types of FR chemicals are effective for one type of polymer only.
Brominated and chlorinated FR chemicals that have seen widespread use are listed in Table 1.4 Some of them tend to show up in mixtures, rather than as a single chemical. Also, some come in a very large number of congeners, which comprise structural variations on a basic formula.
Why Are FRS Used?
Halogenated FR chemicals generally cost more than the base polymer into which they are added and do not add to the functionality or perceived value of the product. Thus, they are normally added solely due to a regulatory requirement. In the U.S., the major regulated areas where FR chemicals are likely to be used in plastics are:
Electrical and electronic equipment. These are generally tested according to a large number of specific standards published by UL. But the product-specific standards generally require a certain level of performance under UL 94,6 which is a small Bunsen burner type test measuring primarily flame propagation and production of flaming droplets.
Motor vehicle materials for the interior of the passenger compartment. This is regulated by the MVSS 302 standard6 of the Dept. of Transportation.
Foam insulation for buildings. These are regulated by building codes, but they in turn refer to the Steiner Tunnel (ASTM E84)6 requirement.
Upholstered furniture. From 1975 to 2014, the State of California required that furniture padding materials comply with TB117-1975.8 PUR foams needed to be treated with FR chemicals to pass this test, which was changed in 20139 from an open flame test to a smolder test, which no longer requires the use of FRs in the furniture padding.
Role of FRS in Fire
On casual look, FRs can appear to be highly effective. In the UL 94 test, a piece of plastic that achieves, say, the high V-0 rating (for many uses, much lower performance level, e.g., HB, is permitted) shows slow flame spread and will cease burning soon after the igniting flame is removed. On the other hand, a specimen that shows poor behavior can ignite rapidly and continue burning until all the material is consumed. But this type of favorable result for FR materials only holds for tests where a very small flame is presented to the specimen. In real-life fires, the flames may just as well come from a burning wastebasket as from a cigarette lighter, and an entirely different outcome will ensue.
For years, the FR chemicals industry relied on a 1988 NIST study by Babrauskas et al.,10 claiming that this shows in large-scale that FR chemicals are effective when used in consumer products. But, in fact, the study results were misrepresented. The NIST study was effectively a replication of an earlier study done for NASA11 to show the effectiveness of highly fire-resistant materials (much costlier than typical commercial products). Both these studies showed that if a room is outfitted exclusively with FR products formulated on a "cost-is-no-object” basis, fire development is not going to take place. But apart from NASA and some other institutional niche markets, the amount of FR chemicals added to consumer products is the minimum needed to pass pertinent regulations, not the maximum that engineering technology can offer. Furthermore, apart from such niche applications, the combustibles in most other places will not all be selected to be FR. Consumers will likely not sit in their living room wearing FR-treated pajamas while reading a copy of the newspaper printed on FR paper. In fact, a prior research study done at NIST12 showed what happens when the combustibles are not all intensely FR-treated , the ignition source is not tiny, and combustibles that are FR-treated are at the consumer-level, and not the NASA-level of performance. The tests showed that FR materials gave no improvements in fire behavior, with regards to both heat release rate and toxicity.
Chemical or Chemical mixture
Brominated FR Chemicals
Pentabromodiphenyl ether (mixture)
Banned or withdrawn in numerous locales
Octabromodiphenyl ether (mixture)
Banned or withdrawn in numerous locales
Being phased out
Being phased out
PS foams, textiles
Being phased out
1,2-bis (2,4,6-tribromophenoxy) ethane
Removed from children’s sleepwear
Printed circuit boards, textiles
Bis (2-ethylhexyl) tetrabromophthalate
PUR foams; epoxy wires and cables
Blend of TBB, TBPH, triphenyl phosphate, and triaryl phosphates (mixture)
Chlorinated FR Chemicals
Tris (2-chloroethyl) phosphate
WHO carcinogen; California Prop. 65
PUR foams, textiles
Tris (1-chloro-2-propyl) phosphate
Bis (hexachlorocyclopentadieno) cyclooctane
Tris (1,3-dichloro-2-propyl) phosphate
California Prop 65
PUR foams, textiles
The Problems with Pops
A number of the halogenated FRs listed in Table 1 are considered persistent in the environment, which implies that they are resistant to environmental degradation for long periods of times (e.g., greater than six months). The addition of chlorine and bromine to molecules generally decreases a chemical’s aqueous solubility, and increases its sorptive behavior, both of which can contribute to increasing persistence, particularly in aromatic compounds. PentaBDE, octaBDE, and HBCD are now considered persistent organic pollutants (POPs) under the Stockholm Convention, an international environmental treaty that is designed to restrict and eliminate the use and production of POPs. These three FRs are considered not only persistent, but bioaccumulative and potentially toxic as well. Both field and laboratory studies have demonstrated that PBDEs and HBCDD biomagnify and increase in concentration with increasing trophic level within a food web.13,14 This can be problematic because humans often sit at or near the top of the food web. In fact, it has been estimated that some PBDE congeners have half-lives in human tissues as long as 5-7 years, implying that any health affects associated with exposure could last decades.15
Studies conducted by the Centers for Disease Control and Prevention (CDC) demonstrate that PBDEs are ubiquitously detected in the bloodstream of the general U.S. population, similar to other POP chemicals like DDT and PCBs.16 And more recent studies have found that exposure to some of the organophosphate flame retardants used as replacements for PBDEs, such as TDCIPP, are also ubiquitous in the U.S.17,18 Due to TB117-1975, it is believed that more FRs, particularly pentaBDE, were used in the U.S. and Canada relative to other countries worldwide, which likely explains why the U.S. and Canadian populations have PBDE exposure levels that are an order of magnitude higher than any other area of the world.19,20 In addition, young children, particularly toddlers, appear to have the highest exposure to PBDEs of any age class due to their crawling and hand-to-mouth behaviors that expose them to higher amounts of dust particles in the home.21,22 FRs, particularly additive FRs, can leach out of FR-treated products in the home and accumulate in indoor air and dust particles where people become exposed. In the U.S., several studies have demonstrated that PBDE levels in dust particles are significantly and positively associated with levels in human serum, suggesting that dust exposure is a predominant exposure pathway.23-25
Toxic Effects and Human Health Studies on Halogenated FRS
A number of studies conducted on animals indicate that exposure to halogenated FRs can affect circulating levels of hormones in the body, suggesting they are endocrine disruptors. For example, exposure studies in several different species (rodents, fish, and birds) have demonstrated that exposure to PBDEs, HBCDD, and TDCIPP results in decreasing levels of thyroxine (T4), the pro-hormone secreted by the thyroid gland, which distributes to peripheral tissues to help regulate growth and metabolism.26-31 PBDEs, in particular, have a chemical structure that is very similar to endogenous thyroid hormones, and levels of PBDEs measured in human sera are similar to concentrations of thyroid hormones that circulate in the blood.32 PBDEs and their oxidative metabolites can also inhibit enzymes that catalyze the clearance of thyroid hormones from the body.33,34 However, stronger evidence points to negative associations between PBDE perinatal exposure and neurobehavioral outcomes. In rodent studies, scientists have found that exposure to PBDEs during critical windows of brain development can result in lasting and persistent effects in adulthood.35-37 Some recent studies in fish models also suggest that TDCPP is a neurodevelopmental toxicant;38,39 however, no studies to date have examined perinatal exposure to TDCPP and neurodevelopmental outcomes. Thus, one of the primary concerns with regards to the ubiquitous exposure to these compounds is the effect of these chemicals on growth and development of young children.
To determine if similar effects may be observed in human populations, several epidemiological studies have been conducted to examine associations between exposure to FRs and effects on thyroid hormone levels and neurodevelopmental outcomes. Significant associations between PBDE levels in human serum and significant changes in circulating levels of T4 or thyroid stimulating hormone (TSH) have been observed, although in some cases they are positive associations and sometimes negative associations.24,40-42 These differences in the human epidemiological literature are thought to be due to differences in cohorts (e.g., pregnant vs non-pregnant cohorts) or differences in how the thyroid hormone levels were measured. Several epidemiological studies have now examined perinatal exposure to PBDEs in the U.S. population and have found that higher levels of PBDE exposure are significantly associated with decreases in several cognitive and behavioral effects, including reductions in IQ and hyperactivity.43-45 Combined with results from animal studies, these data suggest that PBDEs are neurodevelopmental toxicants.
Lastly, research conducted by the National Toxicology Program also suggests that chronic exposure to PBDEs and TBBPA can lead to increased incidence of tumors in rodents, suggesting that these halogenated FRs may be carcinogenic.46,47 Further studies are needed to determine if exposure to FRs are associated with cancer rates in the general population; however, one recent study did find a link between levels of some PBDEs in house dust samples with an increased risk of childhood leukemia.48 Taken together, these data demonstrate that current exposure to halogenated FRs, or at least PBDEs, is associated with adverse health outcomes.
Toxicity of Halogenated FRS in Fires
The bulk of research on the toxicity of halogenated FRs involved the chemical in its original form. But of equal concern is the fact that dioxin or furan compounds can be produced during heating.49,50 Dioxins are of extreme toxicological concern, so much so that even researchers who study and promote the use of FRs refer to dioxins as being51 "the most toxic family of [anthropogenic] chemicals ever studied.” Laboratory tests52 show the production of dioxins peaks at temperatures in the range of 700 - 900ºC, which is exactly in the range for flashover in house fires.
Dioxins and furans are more commonly produced from combustion of specific substances; however, they are also found as byproducts/ impurities in chemical formulations, such as PBDE commercial mixtures. But any fire tends to produce polycyclic aromatic hydrocarbons (PAHs), a well-known family of compounds, some of which are carcinogenic themselves. Thus, it is of additional concern that a recent study showed that "fires with products containing brominated FRs” produced an "order of magnitude higher total PAH yields and a more toxic mixture of PAHs” than the combustion of other products. In other words, burning FR-containing plastics both greatly enhances the production of general-purpose carcinogenic compounds, and introduces extremely hazardous families of compounds that are not a normal feature of combustion reactions in general.
History has demonstrated that new formulations of halogenated FR chemicals have been developed and commercialized in the absence of substantive research or data on their toxicity.54 Only after these chemicals are in use does research emerge suggesting there may be serious toxicological concerns. Based on these patterns, a consortium of health and public-interest groups filed a petition55 in March 2015 before the Consumer Product Safety Commission (CPSC) requesting that CPSC ban, as "hazardous substances,” various products that contain non-polymeric, additive organohalogen flame retardant chemicals. This is in recognition of this repeated history with new formulations of organohalogens, and, the basis of the petition is that this whole "class of chemicals is foreign to the mammalian body and inherently toxic, due to its physical, chemical and biological properties.”
In recent years, much concern has been raised about the high rates of cancer among firefighters, including some types not common in the general population. There is some concern that this may be due to their exposure to halogenated dioxins and furans produced by the decomposition of flame retardants in fires.56 Assigning a unique causative factor to cancer is often difficult or impossible, but the concern of firefighters is real. Bates57 found that California firefighters are disproportionately prone to testicular cancer, melanoma, brain cancer, or esophageal cancer. Ma et al.58 found high rates of bladder, testicular, or thyroid cancer among Florida firefighters, while Baris et al.59 found increased risk for colon cancer, kidney cancer, non-Hodgkin’s lymphoma, or multiple myeloma among Philadelphia firefighters. LeMasters et al.60 reviewed some 32 published studies and concluded that, overall, risk of multiple myelomas, non-Hodgkin’s lymphoma, testicular cancer, and prostate cancer was notable. Recently, NIOSH researcher undertook a cancer study involving 30,000 U.S. career firefighters; the initial results61,62 bear out concerns about disproportional incidence of cancers.
Benefits Versus Harm
As with any societally imposed regulation, the quantitative way to assess whether the regulation is appropriate is by comparing the social benefits reaped from the regulation against the costs and harm potentially caused by it. In the general case, this would be a very challenging technical exercise and such work has not been undertaken. But in some cases, valid conclusions can be drawn because benefits are negligible, while harm is potentially significant. Several such cases have already been documented in detail.
In 2011, Babrauskas et al.63 examined the fire safety benefits of the then-current California TB117- 1975. While a serious potential of harm for both human heal th and worldwide ecology was found, it was shown that the fire safety benefits were negligible. This is not surprising, since the state only required that foams be FR treated, and foams in upholstered furniture are covered with fabrics. The outer fabric is the material ignited first, yet it would not resist ignition from even very small flames. The level of FR treatment required to pass the California test was also modest, so that alone would make them to be materials incapable of resisting a larger flame. Finally, fire loss statistics showed that the problem is very small, while losses did not preferentially drop more rapidly in that state. This situation led the State of California to revise its regulation in 2013, replacing open-flame with smolder testing.
Another situation where fire safety benefits are negligible-to-nil concerns the FR requirements for thermal insulation foams inside cavity walls, or buried under concrete inside buildings. A study64 showed that Steiner Tunnel testing cannot give valid results for plastic foams. But it also showed that fire safety is achieved by the already required thermal barrier in front of the foams, and that mandating Steiner Tunnel ratings does not do anything positive to enhance the fire safety.
Perhaps the most extreme example of ineffective fire safety regulations that introduce toxic harm is the MVSS 302 standard. No less a body than the National Academy of Sciences65 concluded in 1979 that this test "is considered by test experts to be almost totally ineffective in providing fire safety in a real fire situation.” But the test remains in force to this day and the automotive materials used to meet the standard have been predominantly halogenated organic FR compounds.
No valid basis for concluding that FR chemicals make a positive contribution to fire safety has been found in any area where detailed studies have been conducted. The requirements were all introduced not because fire safety effectiveness was demonstrated, but on the basis of speculation or small-scale tests that do not reflect real-scale fire hazards. Yet, the scientific evidence demonstrates that the population as a whole is ubiquitously exposed to these potentially hazardous chemicals, particularly young children at sensitive developmental stages. This is particularly true for pentaBDE, in which perinatal exposure is now associated with learning and behavioral deficits in children.
In view of the disparity between benefit versus harm shown in the cases analyzed, any area where FR chemicals are potentially utilized should be scrutinized carefully to demonstrate that benefits do indeed outweigh the harm. Such applications are likely to be few, due to a simple arithmetic fact: fire safety benefits accrue only to individuals who sustain a fire (an ever-shrinking, small part of the population), while adverse health effects challenge the totality of the population.
Vytenis Babrauskas is with Fire Science & Technology Inc. Heather M. Stapleton, Ph.D., is with Duke University’s Nicholas School of the Environment.
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