This paper proposes a procedure to establish coherence between the toxicological and the legal definition of hazardous exposure situations to substances with effect threshold. This procedure is useful for defining unsafe body doses or exposure levels at and above which such situations would appear to be "sufficiently probable" in the toxicological as well as the legal sense. We propose the unsafe exposure level to be F-times higher than the safe level. F is the geometric mean of those default factors which are intended to relate the experimental or epidemiological database to the human target population. Our proposal for defining "sufficiently probable unsafe exposures" is based entirely on widely accepted toxicological conventions. It is conservative and, at the same time, flexible enough to allow F to be corrected in concert with improvements of the database. The German Federal Environmental Agency has officially adopted this proposal and is recommending it for risk assessment of environmental contaminants.