Human Health Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 242

made, however, if gastro-intestinal absorption is less than 50% (US Environmental
Protection Agency 2004 ).
In spite of the inaccuracy involved with the use of the general equation ( 5.7 ), this
simplified approach is used in many regulations and in guidance for contaminated
sites. Guidance for a more detailed Risk Characterisation is available from other
frameworks (e.g., ECHA 2008 ).
In addition to comparing exposures, concentrations in environmental compart-
ments can be compared with specific limit values. These limit values can be
toxicologically based (e.g., limit value of drinking water, or limit value of indoor
air), but can also have a regulatory basis (e.g., limit values for food stuffs). Although
exceeding these limit values is generally considered unacceptable, it cannot be con-
cluded a priori that there is an unacceptable human health risk involved. Limit values
in food stuffs generally combine a toxicological evaluation with the principle of 'as
low as possible under good agricultural practices'. For these limit values, concen-
trations above the limit values will not result in an unacceptable health risk if total
exposure is below the Critical Exposure value.
Moreover, Toxicological Reference Values are often based on the assumption that
continuous exposure is taking place, while for most scenarios exposure only takes
place during a part of the day. On the other hand, politically-based limit values could
also reflect feasibility and be higher than what would be considered an acceptable
toxicologically based risk limit. In that case, a conflict between the purely toxico-
logical approach for contaminated sites Risk Assessment and politically-based limit
values could arise.
5.5.3 Relevant Time Span
After calamities, human beings can be exposed over a short period to often high
contaminant levels. Analogously, humans are exposed for short periods when they
spend a relatively short time at a contaminated site, for example, when children play
every now and then at a contaminated industrial site or when humans recreate for
short periods in a contaminated area. In many other cases, humans are exposed for
longer periods and on a more continuous basis, from several years up to their whole
lifetime, for example , when they live or work on a contaminated site. In other words:
exposure ranges from acute ,via sub-acute and sub-chronic ,to chronic . There are
no absolute definitions for these adjectives (International Programme on Chemical
Safety 2004 ). However, acute effects often refer to effects that occur within 24 h or
within the 14 days after exposure; sub-chronic and chronic exposure in experimental
toxicology is often defined as 'exposure up to 90 days' and 'considerably more than
90 days', respectively. The phrases 'acute exposure' and 'chronic exposure' are also
often used in the meaning of single exposure and repeated exposure, respectively,
but this distinction does not leave much scope for further differentiations in regard
to exposure duration.
Obviously, it is essential to relate site-specific Human Health Risk Assessment
to actual exposure times. Since the amount of time that humans will spend on a