Toxic persistence rating: A novel approach to estimate environmental impact
Öberg, T.
Presentation vid the Annual Meeting of the Society for Risk Analysis, Baltimore, Maryland, 3-6 december 2006

Abstract
Many chemicals can inflict damage if released into environmental media (air, soil and water). The current risk paradigm calls for individual consideration and evaluation of each separate compound, even though mixture exposure and additive effects could be of major importance for the overall toxicity. Furthermore, in reality there is no distinct demarcation between persistent and non-persistent compounds. Both these observations put a sequential, one-compound-at-a-time, risk assessment approach in question. An alternative approach is to create a continuous scale where characteristics of baseline toxicity and persistence are combined into a joint toxic persistence rating (TPR). The reciprocal LC₅₀ values can be used as a convenient scale to compare additive toxic potencies, similar to the toxic equivalence factors (TEFs) used for dioxin-like compounds. However, it is also necessary to consider the persistence and multiplying with the estimated atmospheric half-life can achieve this. The more toxic and persistent a chemical is, the greater the TPR and vise versa. TPR = Atmospheric half-life (days) / LC₅₀ (mg/L). The environmental half-lives are proportional to the time-integrated exposure for the amounts emitted and baseline toxicity is assumed to be an additive property. The amount or concentration of each compound considered can thus be multiplied by its TPR and added together to estimate the total environmental burden from mixture exposure to organic environmental pollutants. To facilitate comparison, this estimated quantity can without difficulty be normalized and expressed as a suitable compound equivalent. We have screened and ranked 50,000 compounds using the TPR concept. Baseline toxicity is influenced mainly by size and polar interactions, with a steady decrease in the TPR value when adding hydrogen donors. Halogenation determines much of the atmospheric persistence, with a gradual increase in the TPR value for each chlorine or fluorine atom added.