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Limitations of risk assessment Toxicity Assessment
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Toxicity Assessment establishes the relationship between the contaminant/s of
concern and the receptor. It asks the question 'What does the contaminant do to the receptor?' The objective
of Toxicity Assessment (also known as hazard assessment) is to determine
Regardless of whether you are concerned with the effects of a group of contaminants or a specific contaminant, you should be able to demonstrate a link between the contaminant/s and human health effect or a biological response . Adverse biological responses to contaminants vary in scale both within and between
individuals, populations and communities and over time scales ranging from seconds to
decades or centuries. The following indicates the kind of responses that might be
expected at each scale for ecological receptors:
For more information on each of these aspects, we recommend reading Section 6.4 of
Environment Canada’s ERA Framework (1994). Toxicity assessment will necessarily involve a
degree of extrapolation between the information provided in the literature and the actual
situation occurring at the site of concern. This extrapolation may introduce sources of
error and uncertainty to toxicity estimates. Error and uncertainty can
significantly affect the validity of models to which these estimates contribute, or the
appropriateness of risk management decisions made on the basis of toxicity estimates. For
discussions on the uncertainty associated with extrapolation, refer to Section 6.6 of
Environment Canada’s ERA Framework (1994) and Section 4.3.1.3 including Text Boxes
4-19, 4-20 and 4-21 in USEPA (1998). To fully understand how toxicity assessment in RA is conducted, it
is important to familiarise yourself with the following basic terms and
concepts: And for ecological receptors: Tier 1 Toxicity Assessments are screening level assessments. In the
Tier 1 Toxicity Assessment, chemicals of potential concern are identified
if they are found at concentrations greater than a regulatory value. These regulatory values (which are also referred to as acceptance
criteria, environmental standards, or environmental quality criteria but referred
to generically in this web site as guideline values) are numeric values designed to help
users assess whether the contaminant is present in sufficient
concentration to pose a potential risk to humans or a specific part of the
environment. They are generally conservative. NOTE: The terms 'criteria', 'guidelines' and 'standards; are
often used interchangeably and confusingly, and one author's 'guidelines'
may be another's 'criteria'. Additionally, documents presenting
'recommendations' (i.e. guidelines) sometimes use the term 'criteria' in
their title. See the Glossary for definitions
of criteria, guideline and standard as used in this web site. Exceedance of these values does not necessarily mean the chemical poses
an actual or significant risk, but rather it signals that a higher level
of Risk Assessment may be required to further examine the potential for
harm to human health or the environment. As stated in the ANZECC 2000 Water Quality
Guidelines, "for some
environmental values the guideline number provided may be an adequate
guide to quality (e.g. for recreation or drinking). For other specific
environmental values the guideline can be just a starting point to trigger
an investigation to develop more appropriate guidelines. Invariably, the process of refining these guidelines to local conditions will result in numbers for toxicants at least, that
are less conservative and hence less constraining on surrounding
activities. It is good practice to check the assumptions under which the guideline
values are most relevant to ensure that they are similar to the actual
situation at your site. Most guideline documents outline the assumptions and
methods used in deriving the guideline values (e.g. aggregating toxicity
test data for individual species or from the most sensitive species), or
refer to the specific research or regulatory authority recommended
guideline values that are considered to be appropriate. As a result, the guideline values will be more appropriate in
some circumstances and less appropriate in others, and the document should
outline the relevant circumstances limiting the use of the guideline
values. For example, a guideline value may assume that workers on an industrial
site located near a landfill may be exposed to landfill gas emissions for
only 8 hours per day for 5 days per week. However, on your site, the
workers may work in shifts of 10-12 hours over a 6 day week. This
effectively increases their exposure time at the site by more than 50%,
which means that a guideline value for landfill gas emissions may not be
sufficiently protective of the health of the workers. You need to take into account a variety of factors when
using overseas values such as, for example, differing soil properties and
rainfall conditions, and the differing sensitivities of New Zealand species. This information may be found in the text accompanying the guideline
values, and may include detailed information on the data or testing from
which the values were derived, assumptions used, any uncertainty values
applied, and guidance on how to apply the values. These should be noted to
ensure that the assumptions and data used match the specific circumstances
of your site so that the guideline values are valid in relation to the
results you obtain from Site Investigations. Follow this link for an explanation
of toxicity values. Another important consideration is the function of background
concentrations. 'Background' is defined as the level of contaminants found in
the vicinity of a locality, but away from a specific activity or site. In
other words, they are an indication of the concentrations of contaminants
typically found in the area and put the concentrations of contaminants
found at your site into the context of the surrounding environment. In the
great majority of cases the concentrations shown in background samples
will not represent "pristine" concentrations. The background
samples are likely to have a direct bearing on the risk management
decisions made, particularly in relation to clean-up goals. Two regional council documents give background levels of metals in
soils in specific regions: Although not intended as such, some data on background soil quality for
a range of New Zealand soils is presented in the paper
by Sheppard et al. on contaminant mobility, and ARC have assessed
background contaminant concentrations in soils in the Auckland region,
Auckland Regional Council Working Report 96 (2002), Auckland Regional
Council Technical Publication 153 (2001). Because the natural chemical composition of surface and groundwater is
determined by the hydrochemistry of underlying strata, site specific
conditions, and catchment land uses, background water quality should be
determined on a site-by-site basis. For surface water samples, contaminant concentrations
are generally compared to their applicable water quality guideline (WQG)
for protection of aquatic organisms. Because releases from contaminated sites are often continuous and long-term, concentrations are
compared directly with the chronic WQG, when available. If, however, the
majority of the exposure is from intermittent events (such as with
stormwater), then the concentrations may be compared with acute
guideline values. Groundwater concentrations are also screened against
WQGs. However, given the dilution expected during migration and upon
discharge of groundwater to surface water, a screening value of 10 times
the WQG would be recommended for this assessment. If available, suitable
site-specific dilution factors should be used. Maximum contaminant
levels applicable to drinking water sources must also be considered in
making these assessments if a pathway to people is likely. A number of approaches have been applied for
derivation of acute and chronic water quality guidelines (WQG) and
criteria for marine and freshwaters. These methods aggregate data for
individual species toxicity tests to generate �community�
sensitivity distributions from which guideline values are derived. Sediment quality guidelines have been derived from the
biological effects database compiled by NOAA (Long et al., 1995).
The chemical concentrations were sorted, and the lower-10 percentile and
median concentrations were identified along with an apparent effects
threshold. The lower-10 percentile data are identified as Effects
Range-Low (ER-L), and the median as Effects Range-Median (ER-M).
Acceptance criteria for these data are described in the ANZECC 2000
guidelines (ANZECC / ARMCANZ 2000), and in the Canadian protocol (CCME
1995). In the Canadian guideline both threshold effects level (TEL), and
probable effects level (PEL) are given. Derivation and guidance for
their usage are also presented in CCME (1995). Soil quality values generally used in New Zealand
include the �environmental investigation soil quality guidelines�
given in the ANZECC guideline (ANZECC, 1992b), as well as the �health-based
acceptance criteria� of specified chemicals contained within
industry-specific guidelines for contaminated gasworks sites (MfE,
1997), timber treatment chemicals (MfE/MoH, 1997) and petroleum
hydrocarbon contaminated sites (MfE, 1999). These values are generic,
intended as an initial measure to determine the significance of
contamination. The Canadian soil quality guidelines are also often
referred to (CCME, 1997). In the ANZECC guidelines, where an
environmental investigation level has not been nominated for a specific
chemical, reference is made to the Dutch B guidelines that define �risk
limits� for specific chemicals. Click here for other sources of soil
quality guidelines. Benchmark values are sometimes applied where
regulatory guidelines or standards to protect ecological receptors are
not available (Sample et al., 1998). Benchmarks used in a risk
assessment are based on the �assessment end points� the risk
assessor / risk manager has selected, according to the ecological
receptors of concern. Benchmarks should not be used as remedial goals nor
should they be interpreted as concentrations that pose significant
risks. They should not be applied in the risk assessment process without
a reasonable understanding of their development and limitations. For
example, the Effects Range-Low (ERL) values for sediment (Long et al
1995) are calculated as the lower 10th percentile
concentration of the available sediment toxicity data that had been
screened and designated toxic by the original investigators. These are
not LC10 values and are not to be used interchangeably. Detailed information is available on benchmark values
for aquatic biota (Suter & Tsao, 1996), sediment-associated biota
(Jones et al., 1997), plants (Efroymson et al., 1997a),
soil invertebrates and soil microbial processes (Efroymson et al.,
1997b) and wildlife (Sample et al., 1996). Refer to the ORNL
website for more information. The drawback to applying generic values such as
benchmark values or environmental quality criteria is that: Follow links for tables of environmental quality
criteria or benchmark values for CCA and BTEX in soil
and receiving
waters. When applying generic values for toxicity assessments
it is important to have an understanding of how mixtures of chemicals
can interact, and how exposure to chemical mixtures can affect toxicity.
It is usually assumed that the toxicity of chemical mixtures will be
additive (additive toxicity).
However sometimes toxicity may be substantially greater than (synergistic
toxicity) or less than (antagonistic
toxicity), the simple addition of individual toxicities. In general,
where compounds have a similar site of impact and mechanism
of action, their combined toxicity will be additive. In cases where
a compound may interfere with the toxicokinetics
of another, for example, by inhibiting its detoxification pathway,
potentiation of toxicity (synergism) can occur. Or, a compound may
inhibit the activation pathway of another, thus diminishing the toxicity
of the individual compound (antagonism). |
