Federal Contaminated Site Risk Assessment In Canada Part I: Guidence on Human Health Preliminary Quantitive Risk Assessment (PQRA)

2.6 Hazard Assessment

Health Canada TRVs should be applied where available (these are presented in a companion document [Health Canada, 2003]). For substances with no Health Canada TRVs, reference doses (RfDs), reference concentrations (RfCs), acceptable daily intakes (ADIs), or minimum risk levels (MRLs) should be obtained from the following agencies, in order of preference:

1) U.S. EPA Integrated Risk Information System (IRIS);
http://www.epa.gov/iriswebp/iris/index.html

2) World Health Organization (WHO); various sources including:
http://www.inchem.org/;
http://jecfa.ilsi.org/index.htm;
http://www.who.dk/air/activities/20020620_1

3) Netherlands National Institute of public Health and the Environment (RIVM);
http://www.rivm.nl/bibliotheek/rapporten/711701025.pdf

4) Agency for Toxic Substances and Disease Registry (ATSDR) (U.S.);
http://www.atsdr.cdc.gov/toxpro2.html

For each contaminant of potential concern, the source of each TRV and the pathway(s) to which it is being applied should be identified.

In some cases, assessors may believe that the TRVs presented by Health Canada (2003) are inadequate or inappropriate for application at the site in question. In these cases, the assessor should discuss his/her concerns with the Client and, where deemed appropriate, alternate TRVs may be employed. However, it is imperative that the PQRA report contain a clear description of the inadequacies of the TRVs presented by Health Canada, along with a convincing rationale (with citations) to support the use of an alternate value. For these cases, risks should be characterized using the prescribed TRV and the assessor's preferred value.

2.7 Risk Characterization

2.7.1 Non-carcinogens: Single-Substance Exposures

For substances presenting risks other than cancer, a Hazard Quotient (HQ; analogous terms include "exposure ratio" and "hazard ratio") will be derived as the ratio of the estimated exposure (for each critical receptor) to the tolerable daily intake (TDI) or tolerable concentration (TC), as follows:

Hazard Quotient = Estimated Exposure (ƒÊg/kg/day) Tolerable Daily Intake (ƒÊg/kg/day)

OR, in the case of air-borne contaminants with a tolerable air concentration in (ƒÊg/m³)^-1:

Hazard Quotient = Air Concentration (ƒÊg/m³) x Fraction of Time Exposed Tolerable Air Concentration (ƒÊg/m³)

Hazard Quotients for individual exposure pathways should be presented where there are pathway-specific TRVs. Where exposures via multiple pathways are being summed for comparison to a single TRV (for example, it is common to sum oral and dermal exposures for comparison to the oral TDI), it is necessary only to display the HQ for the summed exposure.

For purposes of preliminary quantitative risk assessment, exposures associated with a HQ # 0.2 will be deemed negligible. This is consistent with the CCME (1996) and the OMEE (1996a), and has become accepted common practice.

2.7.2 Carcinogens: Single-Substance Exposures

For substances deemed to be carcinogenic, the estimated exposure (amortized as appropriate) will be multiplied by the appropriate slope factor or unit risk to derive a conservative estimate of the potential incremental lifetime cancer risk (ILCR) associated with that exposure. The ILCR is derived as:

ILCR = Exposure (ƒÊg/kg/d) x Cancer Slope Factor (ƒÊg/kg/d)^-1

OR, in the case of air-borne contaminants with a unit risk value in (ƒÊg/m³)^-1:

ILCR = Air Concentration (ƒÊg/m³) x Fraction of Time Exposed x Cancer Unit Risk (ƒÊg/m³)^-1

Where pathway-specific slope factors or unit risks exist, the risks via inhalation and the risks via oral + dermal exposure should be estimated separately. In other cases, the cancer risks posed by simultaneous inhalation/dermal/oral exposure will be estimated.

Cancer risks will be deemed to be "essentially negligible" (de minimus) where the estimated ILCR is # 1-in-100,000 ( 1 x 10^-5). The rationale for this essentially negligible risk level is presented in Appendix B.

2.7.3 Exposure to Mixtures

For simultaneous exposure to multiple chemicals of potential concern, non-cancer Hazard Quotients should be assumed to be additive, and should be summed for those substances determined by the risk assessor to have similar target organs/effects/mechanisms of action. For the purposes of PQRAs, exposures associated with this total HQ # 0.2 will be deemed negligible.

For carcinogens with the same target organ and form of cancer, the risks should be assumed to be additive and thus should be summed. The total cancer risk in such cases will be deemed to be "essentially negligible" where the estimated total ILCR is # 1-in-100,000 (1 x 10^-5).

2.8 Non-standard Assumptions and Toxicological Reference Values

In those situations where assessors have introduced exposure pathways, equations, assumptions and/or TRVs that are different from, or in addition to, those presented in this guidance document, the implications for exposure and risk estimates must be summarized and discussed.

• Were exposures increased, decreased, or essentially unchanged compared to the prescribed procedures?
• Were the resulting risks increased, decreased, or essentially unchanged compared to the prescribed procedures?
• Do the prescribed methods predict negligible risks while the alternate methods suggest that a risk exists? Or vice versa?

2.9 Uncertainties

The uncertainties in the exposure and risk estimates should be discussed. Issues to be addressed should include, but not be limited to:

• the quality and quantity of data;
• use of maximum COPC concentrations (where appropriate);
• factors, assumptions, and models that would likely lead to an overestimation of exposures and risks; and
• factors, assumptions, and models that might lead to an underestimation of risks.

2.10 Conclusions and Discussion

The overall conclusions with respect to the risks posed by the contaminated site should be summarized in this section of the PQRA report. Any other issues that, in the opinion of the assessor, require discussion but have not been presented in other sections, should also be included here.

2.11 Recommendations

List all recommendations that may stem from the results of the PQRA.

2.12 References

The report should be thoroughly referenced to enable peer reviewers to identify and obtain all documents and authoritative sources cited in the report. A complete list of those references is required.