Developing biological endpoints for defining virtual elimination: a case study for PCDDs and PCDFs

Defining virtual elimination has created considerable debate. A traditional approach has been to use chemically defined detection limits or levels of quantification that are determined using the best currently available metho dologies. Ever increasing improvements in analytical techniques could lead to corresponding,pressure to reduce the targets for virtual elimination. Th e current Toxic Substances Management Policy in Canada recognizes this and clearly states that it is not the intent of virtual elimination to have a m oving target or to chase down the last molecule of the chemical of concern. Although it may be possible to reduce a chemical to less than some extreme ly sensitive detection limit the chemical may or may not exert biological e ffects at that level. The chemically defined detection limits may be much l ower than background levels in the environment, making it an unrealistic ta rget. Conversely biological responses may result from trace levels of a com pound that are not detectable in effluents or selected compartments of the environment (i.e., water) using current chemical techniques. Alternatively an effect-based approach can establish biologically meaningful endpoints to defining virtual elimination. Polychlorinated dibenzo-p-dioxins (PCDDs) an d dibenzofurans (PCDFs) are used in this study as an example to evaluate th e advantages and limitations of several possible approaches of using biolog ical endpoints to determine the presence of these compounds in the environm ent and ultimately define virtual elimination. A. review of the biological responses to PCDD/PCDFs is included to demonstrate the importance of select ing appropriate biological endpoints. Mixed function oxygenase (MFO) induct ion, although not recommended at this point, is used as an example of a pos sible sensitive endpoint that could potentially be used to detect exposure of biota to these chemicals. Three different approaches are explored: (1) m easuring MFO induction in a sentinel species in the environment; (2) testin g environmental extracts for MFO induction in cell lines; and (3) using bio logical endpoints (MFO induction) to define chemical targets for virtual el imination. While the use of biological endpoints is the most desirable appr oach to defining virtual elimination, there are significant knowledge gaps which limit our selection and application of this approach.