Human Health Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 232

own molecules play in the vital life processes such as cell replication, embryonic
development and the functioning of the endocrine system.
The human body has a whole scale of defence mechanisms for eliminating
adverse effects of contaminants that intrude on the human body. First, there are
several physical barriers such as the skin, and internal (lipid) obstacles such as the
intestine membrane and cell membranes. Second, the body has a number of defen-
sive molecules such as enzymes and vitamins that neutralise toxic contaminants in
the body. The next step in the defence system is formed by the specific body organs
that are specialised in removing contaminants such as the liver and the kidneys.
When the defence system fails and adverse effects result, several repair functions in
the human body such as the DNA-repair enzymes, are able to repair primary dam-
age. In spite of all these mechanisms, a part of the intruding contaminants could
reach the body's own molecules and cause adverse effects. This could happen, for
example, when a surplus of contaminants from a contaminated site enters the body.
Several contaminants that cause severe health damage at high doses may be
innocuous or even essential at low doses.
If, and to what extent, primary damage occurs and repair mechanisms are effec-
tive is dependent on the genetic talent, life style, dietary habits, age and other factors
that determine the physical condition of a human being.
The first step in the Hazard Assessment process is hazard identification which
defines the type and nature of the adverse effects of the contaminant considered.
In the second step, hazard characterisation , these adverse effects are quantified
and this process ideally results in a dose-response assessment. These dose-response
assessments form the basis for a derivation of Critical Exposure values, which are
used in Human Health Risk Assessment for risk appraisal.
5.4.2 Threshold and Non-Threshold Effects
As was explained in Section 5.4.1 , the exposure rate also is a crucial factor in regard
to adverse effects in the human body. In toxicology, an important parameter is the
Toxicological Reference Value , which represents a benchmark for exposure ( Critical
Exposure ) or for a concentration ( Critical concentration ). When this value is not
exceeded, it is assumed that there are no unacceptable health risks. When, on the
other hand, this value is exceeded, this does not automatically imply that health
effects will occur. Exceeding the Toxicological Reference Value generally means
that there is a possibility for health effects (an unacceptable human health risk).
Usually, a further investigation is needed to gain insight into the actual human health
risks.
Toxicological Reference Values are generally derived for the oral and the inhala-
tion routes, separately. In case of systemic effects, one Critical Exposure value can
be used for both routes, independently from the exposure route, after correction for
differences in bioavailability per route. Toxicological Reference Values for long-
term systemic effects through dermal exposure are generally not available, but on
occasion can be derived from oral Toxicological Reference Values.