M3 AM Ecological Risk Assessment (Part 1)
Monday, 14 November 2005: 8:00 AM - 11:40 AM in Ballroom 3

(HAR-1117-823333) Evaluating Risk of Essential and Biologically Important Substances.

Harrass, M¹, ¹ Rio Tinto Borax, Valencia, California, US

ABSTRACT- Hazard characterization describes what adverse effects are associated with exposures to substances of concern. The relevant information is typically displayed as an S-shaped curve, showing how the frequency of effects increases as exposure increases. Typically the curve is assumed to pass through the origin, i.e., no adverse effects at zero exposure. However, for substances that are essential or biologically important, deficiencies may result in an increasing incidence of adverse effects as exposures decrease. For such substances, the exposure/response pattern is a U-shaped distribution, with adverse effects increasing as exposures approach zero. Standard approaches to risk characterization do not easily accommodate a U-shaped distribution of effects. For example, a PNEC is typically calculated using divisors (application factors or uncertainty factors) and a selected point along the toxicity distribution. Such values may be below exposures where adverse effects due to deficiency occur. Several modifications of hazard characterization and risk characterization may be considered. One option is to ignore adverse effects due to deficiency during hazard characterization and perhaps address deficiencies when considering risk management options. A second is to incorporate total risk into the derivation of an endpoint, e.g. to explicitly consider risk of deficiency as well as risk of toxicity. A third approach is to modify selection of the application or uncertainty factor to reflect the range between regions of toxicity and regions of deficiency. Boron, known to be essential for plants and some animals while also being toxic at higher concentrations, will be used to illustrate these approaches.

Key words: hazard characterization, essential substances, deficiency