Given the absence (or impracticability) of well-validated and sophisticated models, we may need to resort to simpler methods for the purpose of screening.  In some cases simple alternatives may already exist; in other cases they may need to be developed.  In either case, the guiding principle should be that the models provide a reliable and robust approximation of the processes and phenomena of interest. 

In reality, developing simple models is not as difficult as might be expected.  The behaviour of many environmental systems is dominated by a relatively small number of variables, so if these can be approximated then the overall responses of the system can be estimated with some degree of accuracy.  Whatever the medium, for example, pollutant dispersion tends to be determined by just three factors: the distance (or time) between source and receptor, the persistence or stability of the pollutant, and the dilution or filtering capability of the medium or micro-environment.   Reflecting this, pollutant concentrations typically show quite clear spatial patterns that reflect, first and foremost, the source distributions and, secondly, the direction and gradient of dispersal.  As a consequence, general pollution patterns can often be predicted relatively reliably from information on source distribution or intensity and basic environmental characteristics (e.g. wind direction, water flow, slope angle).  Likewise, exposure or intake of any pollutant depends primarily on the extent to which the pollutants can penetrate or accumulate within the micro-environments where people spend their time, and the amount of time people spend there.  This, too, can usually be approximated from simple information on the character of the micro-environment (e.g. ventilation conditions) and general time-activity data.

A useful, encompassing framework for this type of rapid estimation is given by the concept of Intake Fractions.  This expresses the causal chain between source and intake in the form of a simple equation, and because of its generality previous estimates of intake fractions can be used to approximate exposures under different scenarios.  A range of tools is also available for modelling links within this chain (e.g. from source to concentration, or concentration to exposure).  Links are provided under 'See also' below.