SYMPOSIA PAPER Published: 01 January 1993

The Quotient Method of Ecological Risk Assessment and Modeling under TSCA: A Review


For regulatory purposes, ecological risk assessments should have clear and meaningful objectives. In U.S. EPA's Office of Toxic Substances (OTS), the focus of ecological risk assessments is on natural resources, which are valued by society for one or more reasons. Biotic resources can be affected by toxic substances through adverse effects on mortality, growth and development, and reproduction and by bioaccumulating through the food chain. Effects on growth and development, mortality, and reproduction are presumed to occur at the population level of organization.

In OTS, the Quotient Method of ecological-risk assessment is used. The method compares toxicological-effect concentrations with predicted or measured exposure concentrations of a toxic substance. If the ratio of the two equals or exceeds 1, a risk is inferred; below 1 there is less likelihood of a risk. The advantages of the method are that it is simple to use, is compatible with the hazard and exposure assessments conducted in OTS, and has been used extensively to regulate toxic substances. The limitations are that it cannot address taxonomic or life-stage sensitivity to a toxic substance, cannot objectively address risks at intermediate quotients (near 1), and does not articulate the consequences of certain risks such as indirect toxic effects.

Computer-simulation models can be useful tools in describing the consequences of direct and indirect toxic effects. A conceptual framework model for ecological risk assessment is presented. The use of population models such as the Risk Analysis & Management Alternatives System (RAMAS) and ecosystem modeling approaches such as the Ecosystem Uncertainty Analysis is discussed. These models are still under consideration and their mention does not constitute endorsement or recommendation.

The results of a risk assessment must be communicated clearly. The Tripartite Decision Methodology, which graphically presents the results of one or more analyses is discussed.

Author Information

Rodier, DJ
Office of Toxic Substances, United States Environmental Protection Agency, Washington, DC
Mauriello, DA
Office of Toxic Substances, United States Environmental Protection Agency, Washington, DC
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Developed by Committee: E47
Pages: 80–91
DOI: 10.1520/STP19235S
ISBN-EB: 978-0-8031-5234-2
ISBN-13: 978-0-8031-1860-7