Estimation of population-level effects from effects on individuals

and receptor are commonly expressed as hazard quotients. The quotient method

uses the ratio of a measurement end point to a toxicity reference value (e.g.,

NOAEL) as an approximate index of risk. Hazard quotients (HQ) are defined as:

HQ = (CB/CT)

(10)

where

CB = estimated body burden (mg COC/kg tissue) or dietary dose (mg

COC/kg tissue/day) to receptors of concern

CT = toxicity end point, expressed as body burden (mg/kg) or dietary

dose (mg/kg/day)

For ecological risk assessments, a hazard quotient of less than 1 indicates that

exposure to a particular COC is not likely to be associated with an adverse effect.

Because of the uncertainties involved in the risk estimates, hazard quotients

greater than 1 indicate varying levels of concern.

A number of limitations restrict the utility of the quotient method. For

example, the method does not convey the potential for effects beyond those

predicted by the simple quotient method (e.g., loss of prey species,

biomagnification at higher trophic levels).

Estimation of population-level effects from effects on individuals

In most assessments of the effect of contaminants on biota, there is a disparity

between the types of data available and the types of data needed to assess

population-level effects. Most toxicological data are obtained from short-term

toxicity tests on standard laboratory species. However, the effects of greatest

concern in ecological risk assessments are those resulting from long-term

exposures on the persistence, abundance, and/or production of populations

(Barnthouse et al. 1987). Unfortunately, data on population-level effects of

contaminants are sparse, and population-level effects are difficult to predict. In

some cases, effects on individuals may not be intense enough to be observed at

the level of the population. In other cases, effects on individuals may occur, but

compensatory mechanisms (e.g., increased migration into an area) may preclude

the occurrence of population- or community-level effects. Alternatively, long-

term community-level changes may occur that are not predicted on the basis of

short-term toxicity tests (Swartz et al. 1994).

In some cases, sufficient data may be available to develop a dose-response

function for population-level effects, such as the reproductive potential of female

fish (Barnthouse et al. 1987), or increased mortality of offspring (Moore, Breton,

and Lloyd 1997). Because prediction of community-level responses from

survival, growth, and reproductive end points involves substantial uncertainties,

wherever practical, effects on selected communities should be directly observed

in the field (USEPA 1993c).

Chapter 5 Uncertainty in Tier IV Risk Assessments