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EPA-Expo-Box (A Toolbox for Exposure Assessors)

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Other Organics

Flame Retardants

Flame retardants are chemicals or other manufactured materials that confer qualities that resist or inhibit the spread of fire. Although some flame-retardant organic compounds have been banned or voluntarily phased out by manufacturers due to health concerns (e.g., many polybrominated diphenyl ethers [PBDEs] and polychlorinated biphenyls [PCBs]), these compounds may persist in the environment.

A wide range of organic chemicals are used in flame-retardant products, but most can be classified as halogenated (i.e., functionalized with bromine or chlorine), phosphorous-based, nitrogen-based, or as a combination of the aforementioned components. Two principal processes are used for incorporating flame-retardant materials into products: reactive and additive. Whereas the reactive processes produce covalent bonds between the flame retardant and a polymer matrix that effectively lock most of the flame-retardant chemical in the product, the additive processes do not form such bonds and therefore increase the potential for the flame-retardant chemical to leach out of the product to which it is applied.

One of the principle mechanisms by which flame-retardant chemicals (particularly of the halogenated variety) confer flame resistance is through gas-phase radical quenching in which the flame-retardant material thermally degrades and releases chemical radicals that are highly reactive with oxygen, thereby reduces the amount of free oxygen available to supply the combustion process. The result of this process is the release of some of the halogen atoms from the flame-retardant chemical and the transformation of the chemical from highly halogenated substances to a lower-halogenated congener.

Example structures selected from the range of traditional and emerging flame-retardant organic compounds are provided below to demonstrate the diversity of compounds in this group.

Example Flame Retardants
Polybrominated Diphenyl Ether (PBDE)
PBDE
Tetrabromobisphenol A (TBBPA)
TBBPA
Tetrabromophthalate (TBPH)
TBPH
Tetrabromobenzoate (TBB)
TBB
Hexabromocyclododecane (HBCD)
HBCD
Melamine
Melamine
Chlorinated Paraffins
Chlorinated paraffins
Tris (1,3-dichloro-2-propyl) Phosphate
(Chlorinated Tris or TDCPP)
Chlorinated Tris or TDCPP
Tributyl Phosphate
Tributyl Phosphate
Triphenyl Phosphate (TPP)
TPP

Physicochemical Properties

The table below provides a summary of key physicochemical factors that are likely to affect partitioning and fate of select flame retardants in the environment. For chemical-specific values, consult the resources provided in the introduction to this module.

Property Fate and Transport Implications
Halogenated (brominated) Flame Retardants
Vapor pressure at 25°C (atm)

Low vapor pressure indicates that BFRs will not readily volatilize from the pure organic state. Vapor pressure decreases with increased degree of halogenation.

Henry’s Law Constant

The Henry’s Law Constant of BFRs corroborates with the low vapor pressure, indicating low volatility.

Solubility in water (mg/L)

BFRs generally have low water solubility, meaning they are not likely to dissolve in surface water.

Octanol-Water Partition Coefficient (log value)

The high Kow values of BFRs indicate high potential to bioaccumulate.

Octanol-Air Partition Coefficient (log value)

High log Koa indicates strong propensity of the compounds to absorb to organic matter such as soil particles, dust, or vegetation rather than remain in air (U.S. EPA, 2010).

Summary: Traditional brominated flame retardants are highly persistent in soil and sediment, and will sorb to organic matter in the water column or air (i.e., dust). These compounds have a high potential for bioaccumulation.
Phosphorous-based Flame Retardants (PFRs)
Vapor pressure at 25°C (atm)

Low vapor pressure indicates that PFRs will not readily volatilize from the pure organic state, where they exist as liquids. If aerosolized, PFRs are likely to exist in the vapor phase.

Henry’s Law Constant

While vapor pressure indicates volatilization from dry soils is unlikely, the moderate Henry’s Law constants of most PFRs indicate that volatilization from moist soil may occur for some PFRs.

Solubility in water (mg/L)

Relatively low water solubility of most PFRs indicates that they will not readily dissolve in water and have high soil adsorption potential.

Octanol-Water Partition Coefficient (log value)

While log Kow values for PFRs vary greatly, most PFRs have positive log Kow values, suggesting that these compounds will preferentially partition to organic matter in soils and sediments, and some could bioaccumulate and biomagnify in aquatic ecosystems and possibly in terrestrial ecosystems and humans.

Octanol-Air Partition Coefficient (log value)

While Koa values have not been identified, vapor pressure indicates that transfers from organic media (i.e., soil) to air are possible.

Summary: Properties of the different PFRs vary greatly. PFRs are generally poorly soluble and lipophilic and will partition to soil, sediment, or organic matter rather than remain in water or air. However, most PFRs are less lipophilic than the brominated FRs and there is wide variation in Henry’s Law Constants and vapor pressure which indicates that the PFRs may occur in any type of media (van der Veen and de Boer, 2012).

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Routes

Exposure to flame retardants occurs mainly through inhalation or ingestion of dust. Food and water contaminated with flame retardants is another source of exposure. Exposure from dermal contact with contaminated soil and dust may also occur. The Routes Tool Set of EPA-Expo-Box provides additional information and resources organized by route.

Route Potential Sources of Flame Retardant Exposure
Inhalation
  • Inhalation of household dust is a potential source of PBDE exposure.
  • In certain occupational settings, such as electronic recycling facilities, dust containing high levels of flame retardants can also be present. In these environments, exposure could occur through inhalation.
Ingestion
  • Settled dust containing flame retardants and subsequent hand-to-mouth contact is a potential source of exposure.
  • Ingestion of contaminated food or water is a potential source of exposure. PBDEs have been detected in meat, dairy products, and fish. Flame retardant chemicals have also been measured in human breast milk.
Dermal Contact
  • Exposure through dermal contact can occur from contaminated soil or water, or anthropogenic surfaces such as asphalt roads or coal tar surfaces.

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Media

PBDEs have been measured in indoor and outdoor air, water and sediment, and soil. They have also been found in household dust. Flame retardants are widely used in consumer products, making indoor environments particularly important for this group of compounds.

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Exposed Populations

Flame retardants are ubiquitous environmental contaminants, resulting in potetnial exposure across the general population. Certain populations may be at risk for higher exposures, for example:

  • Children may be exposed to high levels of dust in homes due to the amount of time they spend on the floor and the higher level of hand-to-mouth contact. Additionally, many consumer products intended for children are intentionally treated with flame retardants, including pajamas, changing pads, and car seats.
  • Workers involved with product manufacturing and product end-of-life processes (recycling, incineration, landfilling) may be exposed to flame retardant chemicals that are applied to numerous consumer products.
  • Firefighters and other emergency personnel who may respond to home, business, or vehicular fires could be at risk for high exposures as flame retardants are released as materials succumb to fire.

See the Lifestages and Populations Tool Set of EPA-Expo-Box for resources related to particular population groups and lifestages.

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Tools

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Measure of a substance’s volatility, or its propensity to partition to the vapor (gaseous) phase from its condensed phase (i.e., solid or liquid). This can be used to predict whether inhalation or other exposure routes are more relevant.

Related to Vapor Pressure. Reflects chemical partitioning between the aqueous, dissolved phase and the gaseous phase.

Measure of a substance’s partitioning between dissolved and insoluble phases. Depends on the solute (e.g., water, alcohol) and other substances dissolved in the solute.

Measure of a substance’s partitioning between dissolved and insoluble phases. Depends on the solute (e.g., water, alcohol) and other substances dissolved in the solute.

Ratio of a chemical that has reached equilibrium in adjacent fractions of octanol and water. This ratio is used frequently to estimate how an organic chemical will partition in the environment (e.g., between dissolved and sorbed fractions in surface water) as well as how it will behave in with respect to human tissues. A compound with a high octanol-water partition coefficient is more likely to bioaccumulate in human tissues, especially fatty tissues.

Ratio of a chemical that has reached equilibrium in adjacent fractions of octanol and air. This ratio is used frequently to estimate how an organic chemical will partition in the environment (e.g., between gaseous and particulate fractions in the atmosphere, between soil organic matter and air) as well as how it will behave with respect to human respiratory tissues. A compound with a high octanol-air partition coefficient is more likely to bioaccumulate in human respiratory tissues, particularly is the log octanol-water partition coefficient moderate.

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