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

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Other Organics

Dioxins, Furans, and PCBs

Polychlorinated dibenzo dioxins (PCDDs) and furans (PCDFs) and polychlorinated biphenyls (PCBs) are examples of persistent organic pollutant (POPs).

Dioxins, furans, and PCBs share a similar base structure of two 6-carbon rings with variable chlorine substituents. The term "dioxin" can be somewhat ambiguous, however, because the term is used to describe various groups of chemicals as well as one chemical, in particular. "Dioxins" generally refer to a group of seven PCDDs, ten PCDFs, and twelve "dioxin-like" PCBs, which are grouped together based on similar structural characteristics and mechanisms of action. The specific PCDD congener 2,3,7,8-TCDD is the most widely studied dioxin and is therefore often referred to simply as "dioxin".

PCDDs and PCDFs are not commercially produced or manufactured chemicals; rather, they are released into the environment as a by-product of combustion (burning) or other industrial processes. PCBs, on the other hand, were manufactured until 1979 for use in numerous commercial and industrial applications. The flame-retardant and insulating properties of these compounds were exploited for use in transformers, capacitors, cable insulation and other electrical equipment, hydraulic systems, plastics, paints, and carbonless copy paper, among other applications. Before they were banned, PCBs were released into the environment during manufacture and use and are still released into the environment through poorly maintained hazardous waste sites or leaks from old equipment.

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Physicochemical Properties

PCDDs, PCDFs, and PCBs are persistent, halogenated, aromatic hydrocarbons characterized by a basic structure of two joined, chlorinated, 6-carbon rings. PCBs have anywhere from 2 to 10 chlorine atoms bonded to the total 12 carbon atoms, resulting in 209 different congeners. PCDDs and PCDFs are different from PCBs in that the two carbon rings are joined together by oxygen atoms, creating a three- ring structure. Chlorine atoms bond to the remaining available carbons to create 75 distinct PCDD congeners and 135 PCDF congeners. Of the 75 PCDDs, 7 of them (including 2,3,7,8-TCDD) are especially toxic. Ten of the 135 PCDF congeners and 12 of the 209 PCBs have "dioxin-like" properties. The number and configuration of the chlorine compounds play a large role in determining properties of each PCDD, PCDF, and PCB congener. For example, melting point and lipophilicity increase with increasing degree of chlorination and vapor pressure and water solubility decrease.

Examples of Organochlorine Structures: Dioxins, Furans, and PCBs
PCDDs
PCDDS
PCDFs
PCDFs
PCBs
PCBs

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

Property Fate and Transport Implications
Dioxins and Furans
Vapor pressure at 25°C (atm)

Low vapor pressure indicates that PCDDs and PCDFs will not readily volatilize from the pure organic state.

Henry’s Law Constant

Henry’s Law Constants indicate that volatilization of PCDDs and PCDFs from water to air might be a significant transfer mechanism during warmer temperatures, which could result in seasonal volatilization/deposition and long-range air transport.

Solubility in water (mg/L)

Low water solubility indicates that PCDDs and PCDFs will not readily dissolve in water.

Octanol-Water Partition Coefficient (log value)

Moderate-high log octanol-water partition coefficients indicate that PCDDs and PCDFs will preferentially partition to organic matter in soils and sediments, bioaccumulate, and biomagnify in aquatic ecosystems and possibly in terrestrial ecosystems and humans.

Octanol-Air Partition Coefficient (log value)

High log octanol-air partition coefficients indicate that PCDDs and PCDFs will preferentially partition to organic substances from air and bioaccumulate and biomagnify in terrestrial ecosystems and humans.

Summary: Based on the physicochemical properties of PCDDs and PCDFs, these compounds are expected to accumulate in soils, sediments, and both aquatic and terrestrial biota. These substances might be driven to escape from water, which will lead to sorption to organic matter and volatilization to air, particularly during warmer weather. The expected seasonal volatilization/deposition cycle of these chemicals is expected to result in long-range transport in air and ultimate deposition at the poles.
PCBs
Vapor pressure at 25°C (atm)

Low to moderate vapor pressure indicates that some PCBs will not readily volatilize from the pure organic state, while others are expected to be semi-volatile.

Henry’s Law Constant

Henry’s Law Constants indicate that volatilization of PCBs from water to air is expected to be a significant transfer mechanism which could result in seasonal volatilization/deposition and long-range air transport.

Solubility in water (mg/L)

Low water solubility indicates that PCBs will not readily dissolve in water.

Octanol-Water Partition Coefficient (log value)

Moderate log octanol-water partition coefficients indicate that PCBs will preferentially partition to organic matter in soils and sediments, bioaccumulate, and biomagnify in aquatic ecosystems and possibly in terrestrial ecosystems and humans.

Octanol-Air Partition Coefficient (log value)

Moderate-high log octanol-air partition coefficients indicate that PCDDs and PCDFs will preferentially partition to organic substances from air and bioaccumulate and biomagnify in terrestrial ecosystems and humans.

Summary: Based on the physicochemical properties of PCBs, these compounds are expected to accumulate in soils, sediments, and aquatic and terrestrial biota. These substances might be driven to escape from water and volatilize from the pure organic form, which could result in accumulation in air and long-range transport, particularly of semivolatile PCBs. The expected seasonal volatilization/deposition cycle of these compounds is expected to result in ultimate deposition at the poles.

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Routes

Dioxins and PCBs are ubiquitous in the environment and partition readily to the fatty tissue of animals, resulting in accumulation in the food chain. Humans are primarily exposed to these compounds through ingestion of contaminated foods such as meat, dairy, fish, and shellfish. In utero exposure and exposure due to ingestion of breastmilk also occur and are exposure pathways of concern. For PCBs that are semivolatile, inhalation exposure is also possible. However, the relatively low volatility of dioxins, furans, and some PCBs under typical environmental conditions suggests that direct inhalation might be a less significant route of exposure.

The Routes Tool Set of EPA-Expo-Box provides additional information and resources organized by route.

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Media

Dioxins, furans, and PCBs are often found in sediment and soils. They are slow to degrade and tend to accumulate in living organisms, so can be found in biota and food, particularly dairy, meat, and fish products. The Media Tool Set of EPA-Expo-Box provides additional information and resources organized by media.

Media Sources of Dioxins, Furans, and PCBs
Air
  • Anthropogenic releases of dioxins to air include a variety of industrial processes, especially those involving combustion/incineration and the production of bleached paper products. Chlorine bleaching processes used to whiten paper pulp, as well as uncontrolled burning of chlorine-bleached paper products, release dioxins. Uncontrolled burning of other types of solid waste and hospital waste can also release dioxins.
  • Incineration of PCB-containing waste (such as carbonless copy papers or electrical equipment) can release PCBs into the air. Leaks from landfilled electrical equipment can also result in volatilization of PCBs.
  • Certain older appliances and fixtures such as fluorescent lights, air heating/cooling units, and refrigerators found in homes, schools, offices, and other public buildings, as well as discarded in landfills, can contain PCBs in ballasts and transformers. The PCBs can leak and volatize, contaminating indoor or outdoor air.
  • Natural sources of dioxin release include volcanic eruptions and fires.
  • PCBs are constantly recycled through the environment as a result of numerous natural physical processes. PCBs continuously re-enter air as a result of volatilization from water and soil (ATSDR, 2000).
Water
  • A number of pesticide and herbicide manufacturing processes can release dioxins as a byproduct, and this release can result in contamination of surface waters. PCBs historically contaminated surface water through similar release following manufacturing processes, however, PCBs are no longer intentionally manufactured and are not a byproduct of other processes.
  • Production of bleached paper products can result in wastewater and pulp effluent that is contaminated with dioxins. These effluents can be released into surface water bodies, resulting in widespread contamination of surface and groundwater (ATSDR, 2000).
  • Production and recycling of carbonless copy papers can result in release of PCBs into wastewaters. Leaks from hydraulic fluids in factory machinery can also result in release of PCBs into wastewaters (ATSDR, 2000).
Soil
  • PCBs and dioxins settle to soil and sediments, and can remain in these media for extended periods. Soil and sediment represent the largest sink of these compounds, compared to air or water.
  • Historically, dioxins and PCBs were used in pesticide products and applied directly to soils. Historic disposal of industrial wastes and paper mill sludge also resulted in soil and sediment contamination (ATSDR, 2000).
  • Sewage sludge used as fertilizers in farm settings can contain small levels of PCBs, resulting in contamination of soils.
Food
  • Food, particularly fish, represents one an important source of PCB exposure for humans. Vegetables grown in PCB-contaminated soil might also contain high levels of PCBs.
Aquatic Biota
  • PCBs released to water readily bioaccumulate and biomagnify in aquatic biota. Consumption of seafood from contaminated water bodies can result in exposure to PCBs.
Consumer Products
  • PCBs are no longer manufactured in the US for use in consumer products, however, some older appliances found in homes or other buildings may contain PCBs in hydraulic fluids or lubricant oils, which could leak.

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Exposed Populations

Background exposure to ubiquitous dioxin and PCB mixtures results in body burdens of these compounds in all humans. The general population is further exposed to dioxins and PCBs through ingestion of contaminated food. Specific populations might experience increased exposures to the compounds due to lifestage, geographical location, or behavior. These populations include developing fetuses, breast-feeding infants, children, human and ecological populations living in certain regions, and certain occupational workers.

  • Long range atmospheric transport can result in higher environmental contamination levels in certain regions of the globe; people who live in these regions may have higher levels of exposure.
  • The lipophilic nature of dioxins and PCBs contributes to the accumulation of these chemicals in breast milk. Infants who are breastfed may be exposed to the chemicals through ingestion.
  • Individuals who consume high amounts of fish or aquatic mammals (i.e., seal, whale), may be exposed to dioxins and PCBs, particularly if the fish is sourced from contaminated waters. Fatty cuts of beef or other meats may also contain measureable dioxin or PCB contamination.
  • Certain occupations may result in higher exposure levels to workers.
  • PCB-waste is often labeled as industrial waste and treated as hazardous material. People who live near or work at hazardous waste landfills or hazardous waste incineration sites with PCBs may be at risk for higher exposure, especially if proper controls are not in place (ATSDR, 2000).

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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