Once released into the environment, pollutants are subject to a range of transport and transformation processes.  They are dispersed both through bulk transport (i.e. carried along by the surrounding medium) and by diffusion (movement of individual molecules or particles, down the concentration gradient - i.e. from higher to lower concentration zones).  En route, they are diluted by mixing within the transport medium and transformed by both chemical reactions and physical processes such as abrasion, and selectively removed by gravitational settling, scavenging and filtration.  As a result, concentrations tend to decline with both distance and duration of transport, and new (transformed) pollutants tend to develop, which may themselves be potentially harmful for humans.  In order to estimate exposures and risks for human health, therefore, we need to understand and quantify the effects of these transport and transformation processes.

Factors determining patterns and rates of transport and transformation

The strength of the processes involved, and thus the consequences in terms of pollution concentrations, depends on the nature of the environmental medium and of the pollutants concerned.  Typically, bulk transport processes dominate, so how far and how quickly pollutants spread is governed by the rate of movement of the medium in which they occur - by windspeed, water velocity etc.  Many chemical processes are also temperature (and often moisture) dependent, so rates of transformation are affected by climate and weather.  The local micro-environment also has a powerful influence, for example by determing opportunities for filtration and sedimentation.  

Measuring and modelling the effects of transport and transformation

Information on the fate of the pollutants as they pass through the environment can clearly be obtained to some extent from direct measurement - e.g. by monitoring pollutant concentrations at a sample of locations.  A large number of databases exist providing such data, often based on routine monitoring for purposes of policy compliance, and links to a number of sources (and detailed factsheets on a selection) are provided in the Data section of the Toolkit.   As with other aspects of assessment, however, the value of direct measurements are limited, both by the cost and logistical difficulties of monitoring (which mean that data are often sparse and not necessarily representative) and because many assessments require information relating to scenarios which have not yet taken place.  To a large extent, therefore, assessments rely on modelling of transport and transformation processes.  

The models that have been developed vary between different envitonmental media, not only because the processes of transport and transformation differ, but also because the scientific disciplines involved are often somewhat independent and the extent to which modelling has evolved varies.  Generally, modelling is better developed for air pollution than for other media.  Further information on the approaches taken in these different fields, and on the specific modelling techniques used, is given via the links in the panel to the left (see also link to ExpoPlatform, below).     

In recent years, however, efforts to devise more integrated multi-pollutant and multi-media models have also been made.  In addition, many environmental transport and transformation processes can be simulated by relatively simple spatial functions, so a range of more generic modelling methods have been developed using GIS.   Further information on these various modelling approaches, and access to specific methods and tools, is also given via the links to the left.