How synergistic or antagonistic effects may influence the mutual hazard ranking of chemicals

The presence of various agents, including humic materials, nanomaterials, microplastics, or simply specific chemical compounds, may cause changes in the apparent persistence, bioaccumulation, and/or toxicity (PBT) of a chemical compound leading to an either increased or decreased PBT characteristics and thus an increased or decreased hazard evaluation. In the present paper, a series chloro-containing obsolete pesticides is studied as an illustrative example. Partial order methodology is used to quantify how changed P, B, or T characteristics of methoxychlor (MEC) influences the measure of the hazard of MEC, relative to the other 11 compounds in the series investigated. Not surprisingly, an increase in one of the three indicators (P, B, or T) lead to an increased average order and thus an increased relative hazard as a result of a synergistic effect. A decrease in one of the indicator values analogously causes a decreased average order/relative hazard through an antagonistic effect; the effect, however, being less pronounced. It is further seen that the effect of changing the apparent value of the three indicators is different. Thus, persistence apparently is more important that bioaccumulation which again appears more important than toxicity, which is in agreement with previous work. The results are discussed with reference to the European chemicals framework on registration, evaluation and authorization of chemicals (REACH) framework.