Protocol for the Derivation of Environmental and Human ... - CCME

Section 3
Exposure to Contaminants
3.1 Exposure to Mixtures of Chemicals
No chemical substance exists alone in the environment; however, toxicological studies are usually
carried out using single chemicals or simple mixtures. Hence, chemical interactions (additivity,
antagonism, synergism, transformation, potentiation) are factors that may alter the risk posed by an
individual chemical.
In some cases, chemicals with similar structures will exhibit the same toxic effect but with different
potency. For example, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) generally occurs in a mixture with
other congeners of the polychlorinated dibenzodioxin (PCDD) family. The overall toxicity of a PCDD
mixture will be greater than just the toxicity of the 2,3,7,8-TCDD it contains. However, the toxicity will
be much less than if all the PCDDs are considered to have the same toxicity as 2,3,7,8-TCDD. In this
case, PCDD mixtures are assessed as toxic equivalents (TEQs) with 2,3,7,8-TCDD being the most
toxic congener. Other congeners are ascribed a toxic equivalent relative to TCDD. This approach is
being investigated for some other chemicals such as Polychlorinated Biphenyls (PCBs) and Polycyclic
Aromatic Hydro-carbons (PAHs), but at this time, combined toxicological assessments of more
heterogeneous mixtures are not possible, and a chemical-by-chemical approach is recommended.
3.2 Determining Estimated Daily Intake
Canadians are exposed to background contamination in the air, food, and water. This background
exposure is quantified by the Estimated Daily Intake (EDI) for a particular contaminant. The EDI
estimates the typical total concurrent background exposure from all known or suspected sources (air,
water, food, soil, consumer products) via all known or suspected routes (inhalation, ingestion, dermal
contact), often termed a multi-media exposure assessment (Appendix G), for the average Canadian
{Figure 17 (a)}. However, it does not include exposures which may occur from a contaminated or
remediated site {see Figure 17 (b)}. This background exposure is with us at all times. Consequently,
risks posed by a contaminated site must be determined in addition to this background exposure.
In general, mean concentrations in various environmental media used in estimating exposure are those
reported to Health Canada by the authors of relevant accounts. Contaminant databases for
environmental media may include non-detectable values, particularly for organic chemicals. Calculating
a mean concentration requires that numeric values be substituted for non-detectable values. Traditional
bounding approaches to this problem, have either been to substitute by zero or to substitute by the
detection limit, respectively under- or over-estimating the true value (Haas and Scheff, 1990; Slymen et
al., 1994). A simple intermediate assumption is that non-detectable values can be estimated by half the
detection limit, although this may lead to an overestimate of exposure. Where appropriate, information
on concentrations of NCSRP environmental contaminants in specific locales may also be used to
estimate background exposure of some high exposure subgroups in the general population.
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