This paper compares available options for the aquatic ecotoxicological effect factor component in life-cycle assessment (LCA). The effect factor is expressed here as the change in risk per unit change in cumulative exposure, Δeffect/Δexposure. The comparison is restricted to approaches linked, implicitly as well as explicitly, to species-sensitivity distributions (SSDs). This draws on recent insights for chemical mixtures and identifies the implications of different model choices. In spite of the many options, assumptions, and areas for further research, it is concluded that a single effect factor basis represents the best available practice for use in LCA at this time, ΔPAFms/ΔC = 0.5/HC50, where ΔPAFms is the change in the (potentially affected) fraction (PAF) of species that experiences an increase in exposure above a specified effect level, accounting for the presence of complex background mixtures (ms), ΔC is the change in cumulative exposure concentration of the chemical of interest, and HC50 is the median, chronic hazardous concentration for regional, multiple-species systems. The resultant aquatic effect factors are risk-based and can be estimated readily for many chemicals using available methods, without the need to describe the entire SSDs and without the need for additional data. For example, the octanol–water partitioning coefficient provides a sufficient estimation basis for about 50% of existing chemicals that have a narcosis mode of action. This also is relevant in LCA for chemicals that are at low concentrations in the environment, concentrations below the biological thresholds at which more specific modes of action would be of relevance.