6.4 Exposure Assessment (Step 2)
131
6.4.3 Substance Evaluation in Terms of Persistence and
Long-Range Transport Potential
Even without completing an effect assessment (discussed later in Sect. 6.5), a first,
simplified evaluation of a substance can already be carried out on the basis of
exposure using the two indicators of persistence and long-range transport potential
(LRTP). These quantities describe the tendency of a substance to cause long-lasting
and widespread exposures. Persistence serves as an indicator for the duration of an
exposure and the LRTP as an indicator for an exposure’s spatial extent. Even without
precise knowledge of the effects of such exposures, substances with high persistence
and LRTP can be considered to pose a threat to the environment (Scheringer and
Berg, 1994). According to the precautionary principle, their use should therefore be
avoided.
Chlorofluorocarbons (CFCs), for example, have a persistence of about 100 years
due to their high chemical stability. According to their even global distribution,
they also have a high spatial range of 40,000 km, the circumference of the Earth
(the spatial range is a metric used to quantify the LRTP of chemicals (Scheringer,
2009)). Acid gases, such as NO x and SO x , can also be transported over distances of
several hundred kilometers from, for example, Central Europe to Scandinavia. Both
of these are examples of substantial spatial shifts of environmental exposure (and
consequently effects) across regions.
According to the EU REACH legislation, a substance is considered persistent if
its half-life exceeds a value of 40 days in freshwater or 120 days in soil (ECHA,
2017b). Similar criteria are used under the Stockholm Convention on Persistent
Organic Pollutants. All these criteria are so-called single-media half-life criteria,
and they are the established way of identifying persistent chemicals in chemical
regulation schemes.
However, a chemical’s persistence can also be measured by metrics other than
these single-media half-life criteria. The overall persistence (P ov ) of a chemical is
defined as a residence time or turnover time in a multi-compartment model, i.e., as
the ratio of the total amount of chemical in the model system (the “stock”), M, and
the flow of the chemical through the system, here denoted by E for the emission
rate:
P ov =
M
E
(6.22)
The total amount of chemical in the system is the sum of the amounts in all
individual compartments, i: M =
i m i , and the emission rate is, at steady state,
equal to the total loss rate, which is given by the sum of all losses in the individual
compartments: E =
i k i m i (with loss rate constants k i ). Accordingly:
P ov =
M
E
=
i m i
i k i m i
=
1
i k i f i
=
1
k
(6.23)
131
6.4.3 Substance Evaluation in Terms of Persistence and
Long-Range Transport Potential
Even without completing an effect assessment (discussed later in Sect. 6.5), a first,
simplified evaluation of a substance can already be carried out on the basis of
exposure using the two indicators of persistence and long-range transport potential
(LRTP). These quantities describe the tendency of a substance to cause long-lasting
and widespread exposures. Persistence serves as an indicator for the duration of an
exposure and the LRTP as an indicator for an exposure’s spatial extent. Even without
precise knowledge of the effects of such exposures, substances with high persistence
and LRTP can be considered to pose a threat to the environment (Scheringer and
Berg, 1994). According to the precautionary principle, their use should therefore be
avoided.
Chlorofluorocarbons (CFCs), for example, have a persistence of about 100 years
due to their high chemical stability. According to their even global distribution,
they also have a high spatial range of 40,000 km, the circumference of the Earth
(the spatial range is a metric used to quantify the LRTP of chemicals (Scheringer,
2009)). Acid gases, such as NO x and SO x , can also be transported over distances of
several hundred kilometers from, for example, Central Europe to Scandinavia. Both
of these are examples of substantial spatial shifts of environmental exposure (and
consequently effects) across regions.
According to the EU REACH legislation, a substance is considered persistent if
its half-life exceeds a value of 40 days in freshwater or 120 days in soil (ECHA,
2017b). Similar criteria are used under the Stockholm Convention on Persistent
Organic Pollutants. All these criteria are so-called single-media half-life criteria,
and they are the established way of identifying persistent chemicals in chemical
regulation schemes.
However, a chemical’s persistence can also be measured by metrics other than
these single-media half-life criteria. The overall persistence (P ov ) of a chemical is
defined as a residence time or turnover time in a multi-compartment model, i.e., as
the ratio of the total amount of chemical in the model system (the “stock”), M, and
the flow of the chemical through the system, here denoted by E for the emission
rate:
P ov =
M
E
(6.22)
The total amount of chemical in the system is the sum of the amounts in all
individual compartments, i: M =
i m i , and the emission rate is, at steady state,
equal to the total loss rate, which is given by the sum of all losses in the individual
compartments: E =
i k i m i (with loss rate constants k i ). Accordingly:
P ov =
M
E
=
i m i
i k i m i
=
1
i k i f i
=
1
k
(6.23)