Direct exposure methods for testing native atmospheres
In vitro studies of adverse cellular effects induced by inhalable substances face a number of problems due to the difficulties in exposing cultured cells of the respiratory tract directly to test atmospheres composed of complex gases and particulate compounds. This paper discusses the characteristics of in vitro work and summarizes the use of different in vitro technologies to determine the adverse effects of inhaled pollutants. The exposure of cells to test atmospheres requires accurate control of the pollutant levels, as well as the close contact of cells and gas without interfering with the medium. Systems which rely on the solution of the gas in the medium overlay do not resemble the exposure conditions in vivo, and may not be suitable for studying, for example, the effects of poorly soluble gases. Exposure to gases or complex mixtures can be performed with roller bottles or flasks on rotating and rocking platforms and, using these techniques, the cells are periodically exposed to the test atmosphere. However, the most promising approach is based on a biphasic cell culture technique, where cells are grown on microporous membranes at an air-liquid interface. Here the cells are nutrified from the basal side of the membrane whilst the apical part with the cultivated cells is in direct contact with the test atmosphere. Based on this culture technique, different exposure systems have been developed and these are described and discussed. Exposure of cells from the respiratory tract to gases or particles is responsible for cell injury or cell activation associated with an overexpression of mRNA and the release of bioactive mediators. Therefore, in vitro studies using such a strategy, in combination with relevant and efficient exposure devices, open up new ways to test native complex gases and aerosols. Furthermore, such an experimental approach is not only suitable for cultivated cells, but it can also be used for exposing bacteria to inhalable test compounds. It is possible to analyze the mutagenic potency of in- and outdoor pollutants and several attempts have been made to determine the induction of revertants in a modified Ames assay after exposure to single gases or complex mixtures.