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

Part C, Section 4
4.1.1 Threshold Contaminants
To derive a guideline, it is necessary to ascribe some allowable proportion of total chemical exposure to
the soil medium. One approach is to base allowable contaminant apportionment to the various media in
proportion to the distribution expected if the various media were in equilibrium in terms of contaminant
intermedia distribution (Travis and Hattemer-Frey, 1989). This assumes that the air, water, soil and
food to which most Canadians are typically exposed exist in such equilibrium. However, much of the
food consumed in Canada is imported or grown in areas distant from the residence (agricultural lands
being an exception). Further, many Canadians consume water that has been treated and supplied by a
municipal source. Canadians also spend a majority of time indoors where air is heated, cooled, and
often contaminated by emissions from various consumer products, building materials, and lifestyle
activities. Therefore environmental media are not likely to be in equilibrium. Apportionment of
allowable exposure between media based on theoretical equilibria is therefore not appropriate.
The SEQCCS has proposed a simple, arbitrary, and practical solution to this problem which recognizes
that no single medium should deplete the entire tolerable daily intake or even the entire residual tolerable
daily intake (RTDI). The RTDI is the difference between the tolerable daily intake and the estimated
daily intake (RTDI = TDI - EDI). With five primary media to which people are exposed (i.e., air,
water, soil, food, and consumer products) the SEQCCS proposes that 20% of the residual acceptable
daily intake be apportioned to each of these five media (Figure 19).
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