From the air to the water phase: implication for toxicity testing of combustion-derived particles

The biological effects of airborne particulate matter (PM) in humans stimulated many research activities in recent years. One type of particles contributing to PM are those derived from combustion processes. By using in vitro test systems for assessing the toxic effects of those particles, the transfer into liquid media is necessary. Thus the question arises, how the transfer of airborne particles into liquid suspensions should be realized and how the exposure of test organisms/cells to the particles can be done and monitored with regard to concentration and stability. Further, the characterization of the airborne particles regarding their composition seems to be of importance in order to understand the induced biological effects. Hence, this review gathers and evaluates results from in vitro and ecotoxicity studies, where the introduction of airborne particles into liquids is required. The aim was to learn on the current practice and recommend on the design of toxicity studies in the future. In total, about 80 papers were retrieved by literature search that deal with the assessment of biological effects of combustion-derived particles or their constituents. The papers were evaluated in detail with regard to the methodology applied. From this evaluation and based on expert knowledge regarding nanoparticle testing, recommendations for the assessment of PM applying toxicological assays were developed. To provide guidance, essential criteria are developed that should help in preparation, testing, and during the evaluation phase of toxicological assays. We further advise whole particle testing, complemented by testing of particle extracts and washed particles.