Most research on PAHs has been limited to the homocyclic polycyclic aromatic hydrocarbons (PAHs). The number of heterocyclic PAHs known, however, is about twice as much as that of homocyclic PAHs. Furthermore, heterocyclic PAHs are better soluble in water, which may enhance their bioavailability. Azaarenes form such a group of heterocyclic PAHs.

Biotransformation of azaarenes has been shown in experiments with algae, molluscs, midge larvae and fish. Acute LC50-values for midge larvae are similar to concentrations found in the environment. Biotransformation decreases this acute toxicity, but transformation products can show a higher genotoxicity than the parent compound. In addition, azaarenes as well as their biotransformation products induce a delay in the life cycle of midge larvae at concentrations (far) below acute effect concentrations.

This diversity in toxic effects obviously complicates the description and/or prediction of such effects by molecular and/or physico-chemical properties of the toxicants. The search for relationships between toxicity and compound specific properties is further complicated by the strong differences in toxicity found for strongly related compounds (isomers) that hardly differ in molecular parameters.

To gain insight in membrane interactions, one of the toxic mode of actions, this mechanism was studied in detail using a computerised model. Although this approach indeed enhanced the insight in this mechanism, no appropriate parameter was found to describe toxicity.

Based on the findings from this thesis it is concluded that biotransformation is not only compound but also species specific. Therefore, biotransformation not only enhances the number of (toxic) compounds in the environment, but may also alter the mode of toxic action.

In this respect, the present environmental standards fail to protect against the toxicity of a vast number of polyaromatic structures. To enhance these standards, risk assessment should include heterocyclic compounds as well as biotransformation products and their role in toxicity.