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unless there was more recent strong evidence to suggest that treatment as a non-threshold substance would be
appropriate. In some cases, even if there was reasonable evidence of human carcinogenicity, there was insufficient
dose-response data on non-threshold effects to derive an index dose and the HCV were therefore based on
threshold effects alone.
3.2.2
Selection of recommended TDIs and IDs
As far as possible, SR2 guidance on the selection of TDIs and IDs has been followed. As previously discussed, the
SR2 guidance for derivation of HCV recommends the use of an expert toxicologist. This toxicologist should be able
to evaluate the merits of alternative health criteria and, if required, derive HCV de novo (i.e. derive HCV directly
from toxicological data). As discussed, none of the EIC/AGS/CL:AIRE GAC project volunteers were qualified
toxicologists and therefore no attempt has been made to derive HCV de novo. Rather, TDIs and IDs were selected
from the collated health criteria derived by national and international expert review groups. It should also be noted
that the HCV for contaminants with SGV have in general been derived from existing health criteria from expert
review groups as opposed to de novo. To ensure that a consistent approach was followed, the following protocol
was derived and used for the derivation of HCV:
• TDIs or IDs were not recommended if there were insufficient data with sufficient details on
provenance. For example, for some contaminants, the only health criteria available were reference
doses (RfD), reference concentrations (RfC) or carcinogenic slope factors given in web-based
databases such as RAIS. These databases did not provide sufficient supporting data (such as
details on the study that the health criteria was based on) to have confidence that the health
criterion was suitable for derivation of an EIC/AGS/CL:AIRE GAC . In these cases, no TDI or ID were
recommended.
Methodology
• Information sources labelled as draft with “do not cite or quote” have not been used to derive HCV,
but are referred to in the proformas for information.
• Occupational exposure levels (OEL) have not been used to derive HCV, but OEL data are reported
in the proformas for information.
• Insufficient data on dermal exposure toxicity were identified to enable the derivation of HCV
for dermal exposure. In accordance with the Environment Agency of England and Wales SR2
document, in the absence of data on dermal toxicity the HCV for oral exposure (where available)
have been used for assessing dermal exposure in the CLEA model. For contaminants where no
oral HCV has been derived, route-to-route extrapolation from inhalation data has been considered
as discussed below.
• Route-to-route extrapolation has been considered for substances where an HCV has been derived
for only one route of exposure. The SR2 document cites IGHRC (2006) for guidance on the use of
route-to-route extrapolation. Route-to-route extrapolation may under- (or over-) estimate toxicity
due to differences in absorption, metabolism and mode of action between routes of exposure.
Oral to inhalation extrapolation is only recommended when: (a) the critical toxicological effect is
systemic (rather than at the initial site of contact); (b) first pass metabolism (i.e. metabolism in the
liver) and/or metabolism in the gut are not significant; and (c) there is sufficient information on
the relative acute toxicity and/or bioavailability between routes of exposure. The IGHRC note that
inhalation to oral extrapolation is less likely to underestimate toxicity because absorption in the
gastrointestinal tract is typically lower than in the lungs.
Of the 44 substances researched 19 had a recommended HCV oral
but no HCV inhal
and 3 had a
recommended HCV inhal
but no HCV oral
. An initial review revealed that there were unlikely to be
sufficient data to satisfy all three conditions for the 19 compounds with an HCV oral
and no HCV inhal
.
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