Microbial indoor air quality is an important issue in particular in the professional sector. This thesis aims to investigate the conditions leading to microbial development on to fibrous filters and to microbial release down stream of filters that could decrease air quality. The first part of the thesis was realized on laboratory and consisted in the filtration of a microbial consortium composed with Staphylococcus epidermidis (bacterium specie) and Penicillium oxalicum (fungi specie). The effects of three parameters on the microbial behavior were studied : the relative humidity (RH) of the air, the filter material, the airflow presence/absence. Whatever conditions, S. epidermidis did not grow up. However, P. oxalicum has demonstrated its ability to develop itself when RH was close to 100% and some P. oxalicumspores were released downstream of filter after growth, when ventilation was restarted. The second part of the thesis consisted in working with a semi-urban outdoor air. Two air handling unit (AHU) have operated during 5 months. The ventilation of one AHU was stopped each week-end and restarted each beginning of week. Temperature and RH of the air, filters pressure drop and total concentration of PM in air before filtration were monitored. Concentration of total cultivable microorganisms upstream and downstream of both filters was also measured each week, in particular at the restart of ventilation for one AHU. According to seasonal variations of microbial concentrations, results have revealed for instance that the filtration efficiency of cultivable bacteria was particularly weak, and sometimes negative, for the AHU operating continuously.
