The paper describes a method of environmental assessment of industrial discharges based on multiplicative models. The severity, S, of a substance in an industrial discharge is defined as the ratio of substance concentration, either at the source or at some ambient point of interest, to a maximum specified 'safe' concentration level. The source is considered 'clean' unless S is expected to exceed unity on more than a given acceptably small proportion of time. Otherwise, it is 'dirty'. A source is classified as clean or dirty based on (a) measurements of factors such as stack emission characteristics and (b) possible knowledge of the statistical properties of other factors (such as meterology). Standard statistical decision techniques are used, with some novelty, to take account of the forms of variation present (time fluctuations, measurement errors, etc.) and to best incorporate existing prior knowledge of the statistical parameters involved. Log-normal distributional assumptions are used, coupled with multiplicative transport models in ambient cases.