Assessing the impact of different assumptions on the size of a Mixture Assessment Factor (MAF) for chemical mixtures in surface waters using data from three recent monitoring studies.

Mixture Assessment Factors (MAFs) have been proposed in the European Union (EU) as a rapid and simple way of protecting aquatic organisms from the combined effects of unintentional chemical mixtures when regulating industrial chemicals under the REACH (EU Regulation on the registration, evaluation, authorisation, and restriction of chemicals) program. A wide range of values has been suggested for the MAF including values of 20 or larger. In this paper we performed a series of case studies using composition data from 46,546 mixtures reported in three surveys of chemicals in EU surface waters. We determine that much of the evidence indicating a need for MAF values of five or greater is the result of assumptions on the impacts of future mitigations and screening assumptions used when determining combined risk. In this paper we present estimates of the MAF values that are based on more realistic assumptions for the impacts of future mitigation and mixture risk assessments that use data on the specific endpoints caused by chemicals and the modes-of-action (MoAs) by which the endpoints occur. We show that smaller MAFs may be sufficient to protect ecological receptors in >95 % of the mixtures reported in each of the three surveys. We also show that generic MAFs could be tailored to individual chemicals based on the chemicals endpoints and MoAs. Finally, we demonstrate that the use of a large MAF could result in unnecessary concerns for chemical mixtures in many surface waters. These findings suggest that caution should be taken in the use of large MAFs in regulations.