7.4 Risk assessment methodology for conventional and alternative  (Page 9/17)

Risk characterization

In the last step, a hazard quotient (HQ) as an indicator of risks associated with health effects other than cancer and excess cancer risk (ECR) as the incremental probability of an exposed person developing cancer over a lifetime, are calculated by integrating toxicity and exposure information, as shown below. If HQ>1, there may be concern for potential adverse systemic health effects in the exposed individuals. If HQ ≤ 1, there may be no concern. It should be noted that HQs are scaling factors and they are not statistically based. The EPA's acceptable criterion for carcinogenic risks is based on public policy as described in the National Contingency Plan (NCP) and is the exposure concentration that represent an ECR in the range of 10 ^-4 – 10 ^-6 , i.e. 1 in 10,000 to 1 in 1,000,000 excess cancer cases ( EPA, 1990 ).

Noncancer Risk: $\text{HazardQuotient}(\text{HQ})=\frac{\text{ADD}}{\text{RfD}}$

Excess Cancer Risk (ECR): $\text{ECR}=L(\text{ADD})\text{xCSF}$

To account for exposures to multiple COCs via multiple pathways, individual HQs are summed to provide an overall Hazard Index (HI). If HI>1, COCs are segregated based on their critical health end-point and separate target organ-specific HIs are calculated. Only if target organ-specific HI>1, is there concern for potential health effects for that end-point (e.g. liver, kidney, respiratory system).

Cumulative Noncancer Risk: $\text{HazardIndex}=\text{HI}=\sum _{{\text{COC}}_{\text{NC}}=1}^n({\text{HQ}}_o+{\text{HQ}}_d+{\text{HQ}}_i)$

Cumulative Excess Cancer Risk: $\sum _{{\text{COC}}_C=1}^n\text{ECR}=\sum _{{\text{COC}}_C=1}^n({\text{ECR}}_o+{\text{ECR}}_d+{\text{ECR}}_i)$

Here, o, d and i subscripts express oral (ingestion), dermal contact and inhalation pathways.

As discussed above, the HQ, HI, and ECR estimates are performed for RME and CTE scenarios separately in the case of deterministic risk assessment. Although EPA published the probabilistic risk assessment guidelines in 2001 ( EPA, 2001 ), its application has so far been limited. Proper evaluation of uncertainties, which are associated with compounded conservatism and potential underestimation of quantitative risk estimates (e.g. due to the presence of COCs without established toxicity values), is intrinsic to any risk-based scientific assessment. In general, uncertainties and limitations are associated with sampling and analysis, chemical fate and transport, exposure parameters, exposure modeling, and human dose-response or toxicity assessment (derivation of CSFs/RfDs, extrapolation from high animal doses to low human doses), and site-specific uncertainties.

Conclusion

The improvement in the scientific quality and validity of health risk estimates depends on advancements in our understanding of human exposure to, and toxic effects associated with, chemicals present in environmental and occupational settings. For example, life-cycle of and health risks associated with pharmaceuticals in the environment is poorly understood due to lack of environmental concentration and human exposure data despite extensive toxicological data on drugs. There are many other examples for which either data on exposure or toxicity or both have not yet been developed, preventing quantitative assessment of health risks and development of policies that protect the environment and public health at the same time. Therefore, it is important to continue to develop research data to refine future risk assessments for informed regulatory decision-making in environmental sustainability and to ensure that costs associated with different technological and/or engineering alternatives are scientifically justified and public health-protective. One area that, particularly, requires advancement is the assessment of health risks of chemical mixtures. Current risk assessment approaches consider one chemical at a time. However, chemicals are present in mixtures in the environment. Furthermore, physical, chemical and biological transformations in the environment and interactions among chemicals in the environment may change the toxic potential of the mixture over time. Thus, risk assessment is an evolving scientific discipline that has many uncertainties in all of the four steps. These uncertainties should be thoroughly documented and discussed and the risk assessment results should be interpreted within the context of these uncertainties.