We conducted a national-scale survey on the haloacetic acids (HAAs) in the typical components of the Japanese diet. Combined with the findings of a previous study on multi-route HAA exposure, we estimated the actual relative contributions of drinking water ingestion to total HAA exposure and in this paper we discuss the necessary allocation factors for setting drinking water quality standard values of HAAs. The currently applied allocation (20%) was found to be unrealistically low and in need of appropriate adjustment. After determining the probability distribution of the relative contribution of each HAA, the rounded values corresponding to 0.05 and 0.1 cumulative probabilities were recommended for dichloroacetic acid (40%), trichloroacetic acid (30%), bromochloroacetic acid (30%) and bromodichloroacetic acid (60%) as their allocation factors. The direction of future investigations is discussed along with an overview of various sources of uncertainty. Ingestion exposure via diet and daily drinking water consumption were identified as priority factors.