Module 3: Toxicology - Section 3: Linking Chemical Exposures and Health Effects

TOX 3.3: Fate of Chemicals Entering the Body

FATE OF CHEMICALS ENTERING THE BODY:

Once the chemical enters the body, it is transported by blood to organs, such as the liver, where it is converted through various metabolic processes into a less toxic (or occasionally a more toxic) water-soluble metabolite before being excreted via the kidneys and/or liver (See Figure 1). The rate at which substances are absorbed into the body and the rate at which they are distributed to different tissues, metabolised and excreted, can differ markedly between substances.

The biological half-life (t_½) of a substance or its metabolite is the time taken for its concentration to fall to 50% of its original value after the end of exposure. Half-life can be measured in minutes, hours or days. Some chemical substances have a shorter biological half-life and a shorter residence time within which to exert their toxic effects e.g. benzene, carbon monoxide. Other substances have low excretion rates, a long biological half-life and may therefore persist in body tissues for a long period of time (months or years) causing chronic ill-health e.g. lead, mercury. It is this dose fraction of the chemical that enters and persists in the tissue of the target organ that causes impaired functioning and the manifestation of clinical disease. Biological monitoring procedures are designed to detect the presence of these absorbed substances in body fluids such blood/plasma and urine.