Analyses of air pollution provide evidence that occupational and environmental exposure to airborne contaminants is significantly associated with human health risks, ranging from bronchial reactivity to morbidity and mortality due to acute intense or long-term, low-level repeated exposures [1, 2, 4, 8]. Although an extensive background database from toxicological studies has been developed on animal models, most toxicity data have been obtained from oral and dermal exposures rather than inhalation exposure [22, 105]. Although inhalation studies are technologically complicated and require unique equipment and resources [37], recent studies have shown that in-vitro methods may also have significant potential for assessing the toxicity of airborne contaminants.

An optimal in-vitro exposure system for studying cellular interactions upon chemical airborne exposures must meet several requirements [62, 83, 98, 106]. Such requirements include the direct exposure of target cells to unmodified airborne chemicals without an intervening layer of media, the constant nourishment of cells during the exposure time, the duplication of in-vivo parameters without system-related toxicity, an uncomplicated exposure system, and a reasonable price. Studies have confirmed that both static and dynamic direct exposure techniques at the air-liquid interface have the potential for more extensive use in studying the toxic effects of airborne contaminants under more representative physiological conditions [62, 96, 98].

A diversified battery of in-vitro test methods measuring different cytotoxic endpoints (see Table 4.2), and a multiple human cell-based system may potentially provide a better understanding of mechanisms involved in the toxicity testing of chemicals. However, in order to develop new insights into mechanisms inducing acute cytotoxicity, the implementation of toxicity endpoints such as proinflammatory cytokine production, reactive oxygen species formation, lipid peroxidation, and mitochondrial function are recommended. In order to extend observations to lower doses, further morphological and biological changes that might occur at the cellular membrane, nucleus, specific proteases and DNA level must be considered which may increase the potential for on-site toxicity evaluations.

Dynamic direct exposure techniques that can be operated independently from the cell culture incubator offer an advantage for on-site toxicity assessments. Considering the multitude of airborne chemicals that usually occur in real environments, dynamic direct exposure techniques at the air-liquid interface can be used for the comprehensive toxicity assessment of airborne contaminants including gases, vapors, solid/liquid aerosols, and complex atmospheres. However, studying the toxic effects of airborne particulates requires sampling inlets with specific characteristics and design for the accurate and homogeneous distribution of test atmospheres close to the target cells. In the future, a range of in-vitro bioassays, in conjunction with direct-exposure techniques, may provide an advanced technology for the toxicity testing and biomonitoring of airborne contaminants.

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