Ozone Exposure Indices: Concepts for Development and Evaluation of their Use

Abstract

Most air pollutant research is concerned with quantifying the relationship between pollutant exposure of the organism and the ensuing biological response. The quantifying function for this relationship has been frequently referred to as “dose-response”. For this discussion of exposure indices, or characterizations, we elect to refer to this process as “exposure-response;” a more generic term. Thus, this expression is not limited to the classical definition of dose as the product of the toxicant concentration absorbed by the organism and the duration of exposure. The exposure—response function requires a measure of the biological response and a measure of the pollutant exposure. The biological response is the culmination of a series of events, physical, biochemical, and physiological, beginning with pollutant uptake by the plant and ending with a measurable biological effect on the plant. In the case of crops, what is usually measured is the final agronomic yield. The measure of pollutant exposure has been some expression of the concentration of the pollutant and the time of the exposure period. At present, there is no agreement on a measure of exposure that best relates pollutant exposure over time (e.g. growing season) and the measurable response of crops or trees. A number of indices have, however, been developed, and their inherent assumptions, advantages, and disadvantages will be evaluated and discussed in this review. Defining the appropriate index that best relates exposure to plant response requires consideration of the underlying biological basis for the response, and this perspective will be used to evaluate currently applied indices and formulate new ones.