8
Quantifying Persistence
INTRODUCTION
A chemical that is persistent, that is, that does not degrade or decompose, will
accumulate in the environment over time, thereby increasing our exposure as
compared with a chemical that rapidly degrades. A measure of how rapidly a
chemical decomposes is the half-life (HL) or the time needed for half of the
chemical to degrade. This Chapter presents data on the HLs of the Toxics Release
Inventory (TRI) chemicals and a method for converting these to a persistence
factor (PF).
PERSISTENCE FACTOR
The developed PF is based on how much of a chemical would accumulate in the
environment over an average 70-yr life span or the residual if 1 lb of the chemical
were released to the environment each year for 70 yrs. If none of the compound
degrades (as indicated by an infinite HL or at least a HL much longer than 70 yrs),
then, at the end of 70 yrs a residual of 70 lb would accumulate in the environment.
If the HL were short, or about a day or less, then at the end of a year, although
there would be an insignificant remaining residual concentration, there would still
be exposure of the amount released at the initial time of release. Also, while a toxic
chemical may degrade in the environment, the compound is not likely to degrade
in use, either in a manufacturing process or in the home or workplace when using a
product containing the compound. Therefore, minimum impact would be exposure
from the chemical at the time of use or release, with no impact due to accumulation,
equating to a minimum residual of 1 lb.
The developed PF is based on the amount of chemical initially released plus
the residual amount that would accumulate through release of an additional pound
each year over the remaining 69 yrs. Specifically, the chemical- and media-specific
HLs (in days) were obtained for the organic (nonmetal) TRI chemicals for water,
soil, sediment, and air from the U.S. Environmental Protection Agency (USEPA)
Persistent, Bioaccumulative, and Toxic (PBT) Profiler (USEPA 2006). For inorganics
(metals), HL data are not available; however, it is known that metals do have long HLs,
so the HLs for inorganics were set at 10 million years. These HLs were converted
into units of years by dividing the HLs (in days) by 365 days per year, as applicable.
Subsequently, the media-specific HLs were averaged to obtain a chemical-specific
average HL. Using these chemical-specific average HLs, the residual or remaining
amount of chemical after 70 yrs was calculated using the following formula:
105
Quantifying Persistence
INTRODUCTION
A chemical that is persistent, that is, that does not degrade or decompose, will
accumulate in the environment over time, thereby increasing our exposure as
compared with a chemical that rapidly degrades. A measure of how rapidly a
chemical decomposes is the half-life (HL) or the time needed for half of the
chemical to degrade. This Chapter presents data on the HLs of the Toxics Release
Inventory (TRI) chemicals and a method for converting these to a persistence
factor (PF).
PERSISTENCE FACTOR
The developed PF is based on how much of a chemical would accumulate in the
environment over an average 70-yr life span or the residual if 1 lb of the chemical
were released to the environment each year for 70 yrs. If none of the compound
degrades (as indicated by an infinite HL or at least a HL much longer than 70 yrs),
then, at the end of 70 yrs a residual of 70 lb would accumulate in the environment.
If the HL were short, or about a day or less, then at the end of a year, although
there would be an insignificant remaining residual concentration, there would still
be exposure of the amount released at the initial time of release. Also, while a toxic
chemical may degrade in the environment, the compound is not likely to degrade
in use, either in a manufacturing process or in the home or workplace when using a
product containing the compound. Therefore, minimum impact would be exposure
from the chemical at the time of use or release, with no impact due to accumulation,
equating to a minimum residual of 1 lb.
The developed PF is based on the amount of chemical initially released plus
the residual amount that would accumulate through release of an additional pound
each year over the remaining 69 yrs. Specifically, the chemical- and media-specific
HLs (in days) were obtained for the organic (nonmetal) TRI chemicals for water,
soil, sediment, and air from the U.S. Environmental Protection Agency (USEPA)
Persistent, Bioaccumulative, and Toxic (PBT) Profiler (USEPA 2006). For inorganics
(metals), HL data are not available; however, it is known that metals do have long HLs,
so the HLs for inorganics were set at 10 million years. These HLs were converted
into units of years by dividing the HLs (in days) by 365 days per year, as applicable.
Subsequently, the media-specific HLs were averaged to obtain a chemical-specific
average HL. Using these chemical-specific average HLs, the residual or remaining
amount of chemical after 70 yrs was calculated using the following formula:
105