Context Dairy farming is associated with an increased risk of lower airways diseases that may be linked to different occupational exposures. Aims AgrExPro is an ongoing study of personal repeated measurements of the morning shift exposure of Breton dairy farmers. The campaign aims at: Identifying determinants of dairy farmer’s exposure to thoracic dust, endotoxins, cultivable mould and bacteria, ammonia and acetaldehyde. Characterising long term exposure to these contaminants for an ongoing epidemiological study on Breton dairy farmer’s respiratory health. Methods Each farmer included in the study is visited for sampling a least 2 times during contrasted season. At term, 150 measurements of each contaminant are expected. Simultaneously to sampling, potential exposure determinants are recorded by an observer in a predetermined record sheet that includes exhaustive lists of occupational task and farm characteristics. To identify exposure determinants, multivariate linear mixed effect models will be used on log transformed concentrations as dependent variable of time by task and farm characteristics. Final models will be used to reconstitute long term exposure. First results Classical multivariate linear models have been applied on 22 measurements of thoracic dust, endotoxins and ammonia recorded during summer 2015. For dusts and endotoxins, two similar 3 parameter’s models have been established: both grain handling and bedding (manual/mechanical) have a significant positive impact on exposure. Adjusted R² is 0.53 for dusts and 0.46 for endotoxins. For ammonia, a 4 parameter’s model has been established. 3 parameters have a significant positive impact on exposure: ratio of slurry surface on the volume of the stable, number of cows and milking in a closed parlour. Ventilation has a significant negative impact on exposure to ammonia. Adjusted R² for ammonia model is 0.64. Conclusion These first results are promising as some exposure determinants have been identified with a small number of measurements