Summary So many work environments are affected by health issues related to bioaerosols that it is difficult to keep count. Hog houses, dairy farms, peat bogs, sawmills and food processing plants are among the most well-known. Though there are no exposure limits for the different bioaerosol components, apart from recommendations for endotoxins and certain dusts, it is well known that health risks differ in these work environments depending not only on the type of establishment but also from one establishment to the next. Furthermore, the nature of the bioaerosols found in these environments is only partially understood, and this makes implementation of adequate preventive measures difficult. The development of a method that could be used to establish health-risk indices or improve our understanding of air quality determinants could help foster conditions that would promote the respiratory health of workers. Dendritic cells are immune system cells that express various activation markers in the presence of a multitude of exogenous agents. Thus, the general hypothesis of this study is that dendritic cells can be used as biosensors to assess air quality in the work place. The objectives of the project were 1) to establish a method for culture of dendritic cells that would make it possible to discriminate between toxic/immunogenic agents and harmless agents; 2) to test the inflammatory impact of complex environmental samples; and 3) to evaluate the relationship between activation of dendritic cells in vitro and acute inflammation provoked in a murine model. Cultures of differentiated dendritic cells from mouse bone marrow were used for this project. The cells were analyzed for expression of activation markers with flow cytometry and for release of tumor necrosis factor (TNF) with the ELISA (enzyme-linked immunosorbent assay) method after incubation with various immunogenic agents, including endotoxins, peptidoglycan, b-D-glucan or whole microorganisms. Following these validation experiments, the dendritic cells were used to evaluate work environment samples characterized for total dust, endotoxins, total bacteria and total archaea. Various dilutions of these samples were tested on the dendritic cells to obtain a maximum cell activation value relative to an internal control (endotoxins). A dendritic cell activation index was thus obtained, and its ability to distinguish between environments of different health risk levels was explored in a murine model of acute airway exposure to field sample extracts. Modulations in the expression of cell activation markers and TNF measurements provide a wide enough dynamic range for distinguishing highly inflammatory agents from less inflammatory ones and their combined effects. Relative dendritic cell activation was used to stratify the different types of work environments by health risk and inflammatory impact in vivo. Characterization of sample contents showed that total dust and endotoxins were the two main determinants of dendritic cell activation in the field samples. However, the results demonstrate that air sample endotoxin content almost invariably underestimated dendritic cell activation. In addition, when endotoxin levels were low, total dust played a greater role in dendritic cell activation. In sum, a test that can be used to compare the potential of simple, combined and “field” agents to stimulate dendritic cell activation was developed. Dendritic cell activation correlates with the health risk in different types of environments and with their immunogenic impact in vivo. The dendritic cell testing made it possible to measure the relative composition of bioaerosols and to corroborate the concept that measurement of single agents alone is inadequate for development of health-risk indices.