Human Health Risk Assessment - Raytheon

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Human Health Risk Assessment
DRAFT
USEPA Superfund guidance (USEPA, 1991a) directs that risk managers consider excess
cancer risks within the range of one-in-ten thousand (1 x 10 -4 ) and one-in-one million (1 x 10 -6 )
to be acceptable depending upon site-specific considerations. This range of risk is generally
referred to as the “acceptable risk range” under the USEPA National Contingency Plan. Under
this federal policy, regulators have discretion to require risk management measures.
Incremental risks greater than 1 x10 -4 generally oblige regulators to require some form of risk
management. In contrast, incremental risks less than 1 x 10 -6 are widely considered to be de
miminis risks, not requiring management. A risk level of one in a million is often referred to as a
“point of departure” or a level of risk where the estimated level of risk and its attendant exposure
assumptions and estimated exposure concentrations are taken into account and the need for
risk management is evaluated. In Florida, the risk level of 1 x 10 -6 is the target risk level used in
setting health protective cleanup levels.
Potential Non-cancer Risks. Potential non-cancer risks for individual COPCs are expressed
as hazard quotients (HQs) (USEPA, 1989). HQs are calculated as the ratio of the estimated
daily intake of each COPC to the corresponding route-specific RfD. HQs are calculated as
follows:
where:
HQ
ADD = average daily dose (mg/kg-day)
RfD = reference dose (mg/kg-day)
ADD
=
RfD
When the average daily dose estimated from site-associated soil constituents exceeds the
protective RfD, the HQ exceeds one. This typically is considered a circumstance requiring
further evaluation since it indicates that exposure could be higher than the “no-effect” dose
represented by the RfD. An HQ higher than one does not necessarily indicate a significant
potential for non-cancer effects, however, since both the average daily dose and RfD are
intended to be conservative estimations. An HQ higher than 1.0 suggests that further
consideration should be given to the likelihood for potential non-cancer effects.
When the HQ does not exceed 1.0, the average daily dose estimated from site-related soil
constituents is not greater than the conservative RfD, indicating that exposure is expected to be
below the threshold required to produce effects and the likelihood of realizing a non-cancer
effect from that COPC is negligible.
To summarize potential non-cancer risks for multiple COPCs across complete exposure
pathways, and across receptors, HQs are summed to arrive at a Hazard Index (HI). The
resulting HI serves as a conservative health protective summary of pathway and receptor risks,
since summing all of the individual COPC HQs incorporates the assumption that their risks are
all additive. In fact, different COPCs may act through different mechanisms and on different
target organs. The overall HIs are useful for rapidly excluding pathways or receptors with
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