Abstract
It is rigorously shown that the effective decay rate in the
environment of a chemical is between the minimum decay rate in
one of its possible compartments, and an upper value which is
the weighted average decay rates in all compartments. The weights
are the compartments' volumes and the equilibrium concentrations
that would have occurred in the compartment due to transport alone,
with no degradation. This upper value is approached, in the sense
of a general limiting law, if degradation is much slower than
transport. This limiting law, together with an estimate for the
spatial range of a persistent chemical, could serve as a minimal
base for exposure-based assessment of environmental risk. As a
first illustration, the result is applied to DDT and hexachloroethane.
A broader group of chemicals will be discussed elsewhere.