For each human exposure pathway, there exists a unique equation for calculating the total daily intake of contaminants. These equations include:
- Accidental ingestion of contaminated soils;
- Inhalation of contaminated soil particles (dust);
- Ingestion of contaminated drinking water;
- Dermal contact with contaminated soil; and
- Ingestion of contaminated vegetables, fruit or meat.
These equations can be found in the risk calculation tool of this website.
Inhalation of Dust
The inhalation of dust is usually insignificant relative to direct ingestion of soil and water, and to dermal absorption. However, in some cases the inhalation pathway can become important.
In most cases, the concentration of a contaminant in the respirable airborne dust can be assumed to be equal to the concentration in surface soil (maximum or average). The average airborne concentration of respirable (≤ 10 μm aerodynamic diameter) particulate matter can be assumed to be 0.76 μg/m3 (based on U.S. EPA, 1992).
For situations where vehicle traffic on contaminated unpaved roads can be a significant concern, a reasonable dust level created by vehicle traffic on unpaved roads is 250 μg/m3 (down-wind side of the road; Claiborn et al., 1995).
Toxicological reference values (TRVs) for the dermal exposure pathway are not commonly available. Therefore, dermal exposures are generally added to the ingested dose, once adjustments are made accounting for differences in absorption (see Relative Absorption Factors below).
Exposure via Multiple Pathways
In many cases the intake rates for oral, dermal and inhalation exposures are combined and a single TRV is used to evaluate the risks. For example, in cases where only an oral TRV is available, exposures by all routes (oral, dermal, inhalation) may be summed for comparison to the oral TRV.
In cases where TRVs for oral and inhalation exposures are available, calculations for these pathways should be calculated separately.
Relative Absorption Factors (RAFs)
For more refined risk estimates it is desirable to apply relative absorption factors (RAFs) in exposure calculations. For PQRAs, oral exposures are typically assumed to have a relative absorption of 100% (RAF = 1); but if evidence suggests a lower bio-availability, this may be modified with justification. Where inhalation exposures are being summed with oral exposures, the inhalation RAF will generally default to 1 unless there is a good reason for respiratory absorption to be significantly less that 100%.
Where dermal exposures are being summed with oral exposures, the RAF values presented in Relative Absorption Factors and Exposure via Multiple Pathways table (Excel file) should be applied.
Other sources of RAF values include
- the Risk Assessment Information System (RAIS - external link),
- Toxicological Profiles published by the Agency for Toxic Substances and Disease Registry (ASTDR - external link)