Before anyone argues about whether a substance can cause a disease, someone has to establish that it arrived — by what route, at what concentration, for how long. Most matters are won or lost here.
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Causation arguments get the attention, but they rest on a foundation that is quietly more fragile: the claim that a particular person or property actually received a meaningful dose of a particular substance. That is a physical question, answered with monitoring data, modeling, historical records, and site investigation rather than with medical literature. It asks where the substance originated, how it moved through air, groundwater, soil, or the food chain, whether it reached the receptor at all, and at what concentration and duration. An exposure estimate built on assumption rather than evidence is the most common single point of failure in a toxic tort matter — and it is the first thing a competent opponent attacks.
Three distinct disciplines with their own data requirements, methods, and standards. Start with the one that matches the question in your matter.
Turning concentrations into a defensible dose — route, duration, frequency, and the uncertainty around all three.
investigateHow a substance moved through air, groundwater, or soil — and whether it could physically have arrived where it is claimed.
investigateChemical fingerprinting and age dating — distinguishing one source from another when several are plausible.
investigateExposure work is sequenced so that the physical record is established before it is interpreted — and before positions harden around a number.
Exposure is contact between a person and a substance at some concentration over some period; dose is how much actually entered the body. The distinction matters because the two can diverge sharply — a high airborne concentration in a building nobody occupied produces no dose, and a modest concentration in a home where a child spent every day for a decade may produce a substantial one. Dose reconstruction takes measured or modeled concentrations and combines them with route, duration, frequency, and receptor-specific factors such as breathing rate or water consumption.
That is the normal situation rather than the exception, since exposures in these matters are usually discovered years or decades after the fact. The reconstruction is then built from what does exist: process and purchase records, permits and emissions inventories, contemporaneous complaints, current sampling combined with fate and transport modeling run backwards, and analogous facilities. The resulting estimate carries genuine uncertainty, and the credible version says so explicitly with a range. Estimates presented as precise despite an absent record are the ones that draw a Daubert challenge.
Through chemical fingerprinting and age dating. Many contaminants carry compositional signatures specific enough to separate sources — biomarker ratios in petroleum, congener patterns, degradation-product ratios, and stable isotope composition. Analytical work on hydrocarbon biomarkers, for example, uses GC-MS in selected ion monitoring to compare hopane and sterane distributions, which resist weathering long after lighter compounds have gone. Where signatures overlap, the argument shifts to transport modeling and to timing — whether a plume could physically have reached the location from a given source in the elapsed time.
It depends on the medium and the forum. Exposure assessment commonly follows EPA guidance including the Exposure Factors Handbook and the Risk Assessment Guidance for Superfund; vapor intrusion work follows EPA and ITRC technical guidance; sampling and analysis follow published EPA or ASTM methods appropriate to the analyte, with data quality assessed against project-specific objectives. Occupational exposure work draws on industrial hygiene practice and NIOSH and OSHA methods. The governing point across all of them is that the method must be one the field actually uses, applied the way the field applies it.
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