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Definitions

A brief description of the terms used in the PBT Profiler is provided below

Persistence

Persistence is the ability of a chemical substance to remain in an environment in an unchanged form. The longer a chemical persists, the higher the potential for human or environmental exposure to it. The individual environmental media for which a chemical's persistence is usually measured or estimated are air, water, soil, and sediment.

The PBT Profiler expresses persistence in individual medium half-lives in air, water, soil, and sediment measured in days. These half-lives are for reactivity-based persistence only. The EPA has established a series of proposed thresholds for persistence based on the efforts of scientists, regulators, and other interested parties worldwide in helping to identify and regulate chemicals that may present the greatest health and environmental risks. The PBT Profiler compares the individual-medium half-lives it estimates for air, water, soil, and sediment to the cutoffs proposed by the EPA. If the estimated half-lives are greater than those proposed by the EPA, the PBT Profiler highlights these values automatically by changing the font color for the estimated value from green to orange or red, depending on which cutoff is exceeded. In the black-and-white version, underlines and italics are used to indicate if the criteria are exceeded. The PBT Profiler currently uses the following cutoff values for persistence in individual media:

Environmental Compartment Half-Life
Not Persistent Persistent
Water < 2 months
(< 60 days)
>= 2 months
(>= 60 days)
> 6 months
(> 180 days)
Soil < 2 months
(< 60 days)
>= 2 months
(>= 60 days)
> 6 months
(> 180 days)
Air <= 2 days   > 2 days
Sediment < 2 months
(< 60 days)
>= 2 months
(>= 60 days)
> 6 months
(> 180 days)

In the black-and-white version, these appear as follows:
Environmental Compartment Half-Life (days)
Not Persistent Persistent
Water < 2 months
(< 60 days)
>= 2 months
(>= 60 days)
> 6 months
(> 180 days)
Soil < 2 months
(< 60 days)
>= 2 months
(>= 60 days)
> 6 months
(> 180 days)
Air <= 2 days   > 2 days
Sediment < 2 months
(< 60 days)
>= 2 months
(>= 60 days)
> 6 months
(> 180 days)

More information on the persistence cutoffs can be found on the Criteria page.

To determine the persistence summary, the PBT Profiler considers water, soil, and sediment but not air. This is because bioaccumulation is commonly considered only for these environmental compartments. To determine the Persistence summary, the PBT Profiler first determines the media (environmental compartment) a chemical is most likely to be found in. This is determined by the Level III multi-media model where advective losses (deep sediment burial or atmospheric transport out of the modeled area) are accounted for.

The half-life in the compartment a chemical is most likely to partition to (the predominant compartment) is compared to the criteria discussed above. If this half-life exceeds the EPA criteria, then the persistence portion of the PBT summary is shaded orange or red.

Information on the techniques that the PBT Profiler uses to estimate a chemical's persistence in the environment is available in the PBT Profiler methodology section. Definitions of chemical persistence in air, water, soil, and sediment are briefly described on this page. A detailed discussion on the persistence of a chemical in the environment is contained in EPA's final TRI rule on PBT chemicals.

Overall Persistence To Top
 

It is important to distinguish between persistence in a single medium and overall environmental persistence. Persistence in an individual medium is controlled by transport of the substance to other media, as well as transformation to other molecular species (together, these are referred to as the residence time). Persistence in the environment as a whole is a distinct concept. It is based on the observations that the environment behaves as a set of interconnected media, and that a chemical substance released to the environment will become distributed in these media in accordance with its intrinsic (physical/chemical) properties and reactivity.

Multimedia mass balance models offer the most convenient means to estimate overall environmental persistence from information on sources and loadings, chemical properties and transformation processes, and inter-media partitioning. It is important to note that these models reflect a steady-state situation (that is, input = output) within a defined geographic space (which is why these models are sometimes referred to as box models). On the global scale, a steady-state situation is not likely to occur.

In addition to reactivity-based persistence in the individual media, the PBT profiler estimates an overall environmental persistence for a standardized set of environmental conditions in which net advection and sediment burial have been removed, using a level III multimedia fate model. The purpose of this is to provide to the user a measure of overall environmental persistence based on reactivity only. This may be useful in making pollution prevention decisions since ultimately, on a global scale, there is no advective removal. If advective loss is included, the residence time is reduced and can give a misleading impression of a short persistence. Advective losses merely relocate the chemical; they do not destroy it.

It is important to note the difference between residence time and half-life in each environmental medium when interpreting the overall persistence. The half-life is a measure of how fast a chemical is destroyed in each environmental medium. The residence time in each medium includes the half-life but it also takes into account transport into and out of that medium. The residence time may, therefore, be substantially different than the media-specific half-life for some chemicals. Since the overall persistence is the weighted average of the residence time in all media, it may sometimes be greater than any of the individual medium-specific half-lives for some chemicals (depending on their transport properties).

There is an ongoing debate in the scientific community on the best method for using the overall persistence. At this point in time, however, there is no agreement on the criteria that should be applied to the overall persistence. The PBT Profiler provides the overall persistence on the P2 considerations page to allow users to compare the results for different chemicals or release scenarios when identifying P2 opportunities.

Bioaccumulation To Top
 

Bioaccumulation is the process by which the chemical concentration in an aquatic organism achieves a level that exceeds that in the water, as a result of chemical uptake through all possible routes of exposure. Biomagnification refers to the concentration of a chemical to a level that exceeds that resulting from its diet. Bioaccumulation includes both biomagnification and bioconcentration. In general, chemicals that have the potential to bioconcentrate also have the potential to bioaccumulate.

Bioconcentration in fish can be readily measured in the laboratory and is frequently used to predict the importance of bioaccumulation, which is much more complicated to determine. The potential for bioconcentration in fish is expressed as its bioconcentration factor, or BCF. There are many reliable techniques for measuring or estimating a chemicalís BCF. The PBT profiler estimates a BCF based on a chemical's physical and chemical properties. This technique is described more fully in the methodology section. A detailed discussion of bioaccumulation, bioconcentration, and BCFs can be found in EPA's final TRI rule on PBT chemicals.

The EPA has established a series of proposed thresholds for bioaccumulative chemicals to help identify and regulate chemicals that may present the greatest health and environmental risks. The PBT Profiler compares the bioconcentration factors (BCFs) it estimates to the cutoffs proposed by the EPA. If the estimated BCF is greater than those proposed by the EPA, the PBT Profiler highlights the values automatically by changing the font color for the estimated value from green to orange or red, depending on which cutoff is exceeded. The output of the PBT profiler uses these colors so that the user can quickly identify which chemicals have the highest potential to bioaccumulate in fish and aquatic organisms.

The BCFWIN model used to estimate BCF does not explicitly address a variety of factors that may influence bioaccumulation under field conditions, such as possible metabolism of the chemical in exposed organisms, which could lead to actual bioaccumulation being lower than predicted. The user therefore needs to exercise due caution when interpreting BCFWIN results.

The PBT Profiler currently uses the following cutoff values for bioaccumulation:

Bioconcentration Factor
Not Bioaccumulative Bioaccumulative
< 1,000 > = 1,000 > = 5,000

In the black-and-white version, the BCF criteria appear as:

Bioconcentration Factor
Not Bioaccumulative Bioaccumulative
< 1,000 > = 1,000 > = 5,000

More information on the bioaccumulation cutoffs can be found on the Criteria page.

Toxicity To Top
 

Toxicity is a relative property of a chemical that refers to its potential to have a harmful effect on a living organism. It is a function of the concentration of the chemical and the duration of exposure. Because persistent and bioaccumulative chemicals are long-lasting substances that can build up in the food chain to high levels, they have a higher potential to express toxicity and be harmful to humans and the ecosystem.

The PBT profiler uses a chronic (long-term) toxicity value called a ChV (defined below) to estimate a chemical's relative toxicity. The PBT Profiler currently uses the following cutoff values for toxicity:

Fish ChV (mg/l)
Not Toxic Toxic
> 10 mg/l or
no effects at saturation
< 10 mg/l < 0.1 mg/l

In the black-and-white version, the toxicity criteria are:

Fish ChV (mg/l)
Not Toxic Toxic
> 10 mg/l or
no effects at saturation
< 10 mg/l < 0.1 mg/l

For more information on how the PBT Profiler estimates toxicity, see the PBT Profiler methodology section. More information on the toxicity cutoffs can be found on the Criteria page.

Media To Top
 

These are the four environmental compartments a chemical may be found in; air, water, soil, and sediment. Air is also commonly referred to as the atmosphere or atmospheric compartment. The atmospheric compartment includes the area closest to the Earth's surface (troposphere) and the stratosphere. Water refers to surface water, and includes oceans, rivers, lakes, and ponds. As defined, this medium does not include aquifers found below the Earth's surface (groundwater). The PBT Profiler does not explicitly consider groundwater.

Soil refers to the topmost layer (the first few inches) of the Earth's surface that is not covered by water. Soil includes the horizons near the surface that differ from the underlying rock material. Sediment is the particulate matter found under surface water. For the purposes of the estimation methods used by the PBT Profiler, soil is considered aerobic (oxygenated) and sediment is consider anaerobic (free of oxygen).

PBT strategies typically consider the persistence of a chemical in water, soil, and sediment but not air. This strategy is also used by the PBT Profiler. Nevertheless, the persistence of a chemical in air is important in identifying P2 opportunities because once in the atmosphere, it may travel a long distance from its original point of release.

Half-Life To Top
 

Half-life is the length of time it takes for the concentration of a substance to be reduced by one-half relative to its initial level, assuming first-order decay kinetics. Generally, it is useful to consider "complete removal" as taking approximately six half-lives. The PBT profiler estimates the half-life of a chemical in each environmental medium (air, water, soil, and sediment). These half-lives are for reactivity and do not take into consideration partitioning into or out of the medium, or physical transport (i.e., advection). See the discussion on the PBT Profiler methodology for additional details on how the half-life is estimated.

Percentage in Each Medium To Top
 

Once a chemical is released to the environment, it may move from one environmental compartment to another. The expected distribution of a chemical in air, water, soil, and sediment is expressed as the estimated percentage in each medium relative to the total amount in the environment. The PBT Profiler determines the amount in each medium at steady state using a multimedia mass balance model (also called a fugacity model) that is based on a chemical's physical/chemical properties and reactivity. The method the multimedia model uses to estimate the percentage in each medium is discussed in detail in the PBT Profiler methodology section.

Bioconcentration Factor To Top
 

The bioconcentration factor (BCF) is a measure of the ability for a water-borne chemical substance to concentrate in fatty tissue of fish and aquatic organisms relative to its surroundings.  EPA defines bioconcentration as the net accumulation of a substance by an aquatic organism as a result of uptake directly from the ambient water through gill membranes or other external body surfaces (60 FR 15366). In general, chemicals that have the potential to bioconcentrate also have the potential to bioaccumulate. Because BCF values are much easier to measure (and estimate), the BCF is frequently used to determine the potential for a chemical to bioaccumulate.

The PBT profiler estimates a bioconcentration factor from a chemical's physical and chemical properties. More information on how a BCF is estimated can be found in the PBT Profiler methodology section.

Fish ChV

The following definitions also address the methods the PBT Profiler uses to estimate aquatic toxicity using:

No Effects at Saturation

"No effects at saturation" is the phrase used to describe the situation which occurs when that the estimated Fish ChV (in mg/L) is greater than a chemicals' water solubility (also in mg/L). This indicates that a saturated aqueous solution of the chemical (one where the maximum water solubility has been reached) does not have a concentration high enough to allow potential toxic effects to be expressed.

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Computer Resources Donated by SRC, Inc.          Ver 2.000     Last Updated September 4, 2012