Dose Reconstruction for Toxic Torts
Dose reconstruction is the determination of historical
doses, whether by inhalation, ingestion, or entry through
the skin. Doses depend on exposure levels, which are determined
by the concentration of chemicals in air, water, soil, food,
etc. The dose at which toxic effects occur varies from chemical
to chemical. It is a toxicological adage that “the dose
alone determines the poison.” Thus, quantitative chemical-specific
doses are required. If concentration data were available for
the time period and location at which exposure is being claimed,
estimating exposure and dose for each chemical would be straightforward,
although issues of bioavailability or chemical speciation
might still be present. However, this is usually not the case.
More typically, either no concentration data exist or the
exposure period of interest predates any data. Exponent provides
a full range of dose reconstruction services to attorneys
and industry, serving as both testifying and consulting experts
in the following areas:
Air dispersion modeling and analysis of air monitoring
data
Unlike chemicals in groundwater or soil, chemicals in air
do not persist. Thus, when there are no monitoring data during
the claimed period of exposure, reliance has to be on mathematical
modeling of exposures. A number of indoor and outdoor air
dispersion models are available, including EPA-approved models
that are appropriate for specific uses. Chemical emissions
estimates for use with these models are case-specific. Mass
balance calculations, emissions models, and compilations of
emissions factors for similar sources can assist in developing
appropriate emissions factors.
Groundwater modeling and analysis of groundwater data
Chemicals in groundwater can undergo a number of attenuation
processes including biodegradation, hydrolysis, dispersion,
and volatilization with time. Thus, it is possible for plaintiffs
to claim that historical concentrations were much greater
than current concentrations. Rates for these loss processes
are generally site-specific and modeling historical concentrations
requires that site-specific data be available. Concentrations
of biodegradation or hydrolysis products can sometimes be
used to estimate original concentrations of “parent”
compounds.
Estimation of bioconcentration factors in fish, livestock,
or produce
Some chemicals can accumulate in fish, livestock, or vegetables.
Thus, even though concentrations in water may be low, concentrations
in food may be high. Laboratory or field studies can be used
to determine bioconcentration rates either based on the chemical
and species of interest or on appropriate surrogates. In choosing
chemical surrogates, the propensity to bind to organics is
an important factor. The plant or animal part used in the
study is also an important factor (e.g., fat or muscle; leaf
or root, etc.).
Quality assurance of chemical data
It sometimes happens that only a few high concentrations have
been recorded. Under these circumstances, it may be important
to examine raw data, such as chromatograms, to see that correct
chemical identifications and concentrations have been determined.
It may also be appropriate to extrapolate high chemical concentrations
measured over very brief time periods to durations of toxicological
interest.
Unlike risk assessments conducted for EPA approval, conservative
“health-protective” factors are not required in
making these dose estimates. Many of the chemicals that are
the focus of toxic torts also occur in consumer products or
have a natural (e.g., lead) or anthropogenic (e.g., dioxins)
background. Therefore, if realistic exposure levels will be
estimated, these should be put in perspective by documenting
the exposures to the same chemicals that people normally receive
(e.g., from residential indoor air or while driving).
Other Exponent services related to dose reconstruction include:
top |
