Quantitative life cycle assessment (lca) is a method allocating the environmental impacts of the whole life cycle of a product to the functioning of that product. The scientific basis of the method is still being elaborated. In this paper a proposal is made to improve the scientific basis of one specific step of the methods: the aggregation of potentially toxic emissions of substances in one score for human toxicity and two scores for ecotoxicity. The aggregation is based on multimedia environmental models of Mackay simulating the behaviour of substances in the environment, and on toxicity data such as acceptable resp. tolerable daily intake (adi resp. tdi) and no observed effect concentration (noec) per substance. It is proposed to apply models describing the environmental fate of toxic substances in lcas of products. In addition, it is proposed to adopt the concept of a reference substance, as used in the ozone depletion potential (odp) and the global warming potential (gwp), to assess and aggregate emissions of potentially toxic substances.