In order to use mobility as a criterion for priority setting
amongst environmental chemicals the characteristic spatial range
of a substance has been introduced. Roughly spoken, the spatial
range is the distance a chemical can travel before degradation---after
release from a point source in an isotropic environment with the
same average geo-chemical properties as the earth. The present
contribution
starts with a formal definition of spatial ranges in terms of
exposure. Then a simple isotropic model of global circulation
is set up in analogy to previous unit-world models, covering global
long-range transport and first-order degradation. Afterwards,
the model is solved analytically, and a closed formula for spatial
range is obtained. The formula is then evaluated for a selection
of representative environmental chemicals. The characteristic
spatial ranges thus obtained are identical with results obtained
by M. Scheringer on the basis of a more detailed computer model.
It is stressed that the present result allows for a classification
of environmental chemicals with respect to spatial range simply
by inserting known measurable quantities into a given formula.
The whole approach was designed to make the assessment procedure
as workable as possible.