Regarding the risk evaluation of mixtures in the workplace, the Working Group discussed whether there is a need to consider combination effects at the workplace and whether there is sufficient information on combination toxicology to suggest a scientific strategy for the evaluation of any mixture at the workplace. It was concluded that analytical definition of the mixture is a basic prerequisite for evaluation. The Working Group agreed that there is a special situation at the workplace because chemical compounds may be present at concentrations close to their effective threshold level; therefore, combination effects have to be taken into consideration, as shown by the data presented during the conference. Furthermore, observed-effect levels for individual substances have to be modified if the substances have the same mode of action or have the same target organs; no modification is necessary if the opposite has been shown. It was also concluded that the scientific database to date is insufficient for the proposal of a general approach. The assumption of additivity in all cases lacking data may more or less over-estimate the risk.