Probit functions are used to calculate the risk of toxic substances in Seveso-establishments. These functions show the relationship between human lethality and toxic substance concentration. They are based on animal experiments and assumptions like inhalation studies and animal to human extrapolation. The methodology to derive probit functions for Flanders and The Netherlands is described, focusing on differences. The probit function for hydrogen chloride (HCl) is studied in detail, including the consideration of a specific baboon study.