This paper establishes a probabilistic approach to assessing the spatial impact of a local activity. Three environmental impact probability effects are distinguished; (1) the effect associated with the facility only, for example size, (2) interaction effects of distance, and (3) the effects of a given environmental state on the population. A combination of these provides a quantitative framework for characterizing probabilistically the environmental impact of a given facility. By means of simplifying assumptions, analytical results are found. For a more practical applicaton of this approach, simulation is required. The importance of the approach to environmental impact accounting is stressed.