Risk Assessment - Toxicology of Metals - page 291

toxicological concern (TTC) assessment (Kroes et al .,
2005). This method is based on classifying chemicals
according to their chemical structure and estimating
their toxicity, when unknown, from the toxicity of
other chemicals with similar chemical structure. Obvi-
ously, such a procedure is very uncertain and cannot
be used as a replacement for actual risk assessments.
It will not be considered further in this text.

to the risk assessment and risk control measures (for
further discussion see WHO/IPCS, 1999).

6 RISK MANAGEMENT AND RISK
COMMUNICATION

The scientifi c database available for the various
steps in risk assessment varies considerably among
metals as is evident when reading the 31 chapters on
individual metals in this handbook. For many of the
metals, WHO/IPCS Environmental health criteria doc-
uments are available, as well as national documents
from various countries (e.g., ATSDR Tox profi les) cited
it the various chapters of this topic. However, there are
a substantial number of metals for which documenta-
tion is limited, making it diffi cult to make a reliable
hazard identifi cation and quantitative dose-response
analysis. This consideration that is included in the risk
characterization is important in risk management.

5 RISK CHARACTERIZATION

There are both qualitative and quantitative dimen-
sions of risks, and the risk characterization should
address both aspects. Some critical effects are severe,
like cancer, and a defi ned numerical risk of such an
effect needs to be considered in a different way than
the same risk for a critical effect implying an increased
urinary excretion of an enzyme or protein without
immediate consequences for development of disease.
As mentioned in Section 4.1.2, it is desirable to focus
dose-response analysis on the critical effect (i.e., the
earliest, often subclinical, adverse effect). However,
in some cases, particularly when carcinogenesis is
involved, such early biomarkers may not be available,
and the cancer risks in themselves are the basis for the
risk characterization.
The combination of exposure assessment and dose-
response analysis generates quantitative estimates of
how many persons exceed safe levels derived from
NOAEL or benchmark doses and thus are at risk of
displaying the critical effect. A concept frequently
used in evaluations by expert committees convened by
EU is the “margin of safety” (MOS). This is the ratio
between NOAEL in animals and the human exposure
levels in exposure assessment. Usually MOS of 100 is
considered acceptable for noncarcinogenic effects. The
considerations as to whether safe levels are exceeded
should focus on defi ned sections of the population (i.e.,
those most exposed and those most sensitive). Particu-
lar attention should be given to estimates of exposure
in sensitive groups like pregnant women and children.
These sections of the population are particularly sensi-
tive to neurotoxic metals like methylmercury and lead.
Risks calculated for such groups often are of crucial
importance for risk management.
The risk characterization should also provide infor-
mation to the risk manager about the quality of the
data that has been used in the risk assessment for the
manager to be able to judge whether it is reasonably
good for decisions about regulatory action. If there
are obvious data gaps in the scientifi c evidence, these
problems must be pointed out for the risk managers
to be prepared to answer possible questions relating

6.1 Managing Human Exposures by
Emission Control, Substitution, Labeling,
or Restrictions in Use
Regardless of the risk assessment, exposures are
sometimes limited by control of emissions. These con-
siderations are often taken when constructing new
manufacturing plants with new production technol-
ogy and increased automation and encapsulation of
productions processes, resulting in considerable reduc-
tion in exposure to workers (see also Chapter 16). The
“best-practical-means” or “best-available-technology”
approach is also used for emissions to the general envi-
ronment and implies that an industry must use the
best available technology to reduce emission of haz-
ardous substances, taking into consideration, however,
economic aspects.
The best-practical-means approach, if properly
used, carries with it the advantage that resulting con-
centrations in different media (e.g., in ambient air and
soil) around a point source will be as low as possible
using the available cleaning techniques. Under certain
circumstances, this might mean that, for example, con-
centrations in the environment may be lower than the
levels permissible from the toxicological point of view.
Large safety margins may thus sometimes be achieved.
The resulting ambient concentrations should, how-
ever, be checked against health criteria and other
general considerations relating to ecology. The “best-
available-technology approach” has been implemented
in Swedish environmental legislation since the 1970s
and has been effective in limiting exposures of popu-
lation groups living in the vicinity of point sources of