The risk to human health posed by a chemical agent is not fixed at the moment, or by the amount, of exposure.  It depends, also, on what happens thereafter as it enters into, and is absorbed by, the body:  the uptake-elimination processes, the associated changes in the chemistry of the agent, and eventually the contact between the biologically-active form of the chemical and the critical target receptor in the body that elicits a toxic response.  These processes vary greatly between individuals, and between different circumstances, so are a major source of variability in the ultimate health effects seen across a population.  To understand these variations, therefore, often requires information on the internal (i.e. biological) fate of contaminants.

Two primary approaches are available to elicit this information:

• Human biomonitoring provides measured data on dose at different points in the intake-uptake chain, and can thus be used to assess variations, or track changes, in intake and uptake across a population;
• Pharmacokinetic and toxicokinetic models provide a means of analysing and simulating the processes involved, and thus of predicting changes in dose under different scenarios. 

Both are integral to understanding the mechanisms by which toxicants affect human health; both, also, play an important role in health impact assessments, by taking account of the biological factors that intervene between external exposure to an agent and its health effects. 

Links to further information on these approaches are provided on the panel to the left.