This study deals with the characterization of the ambience in a food-processing clean room. This clean room, installed at the ＇Ecole Nationale des Ingenieurs des Techniques des Industries Agricoles et Alimentaires (ENITIAA)＇ operates in the cleanliness class 10,000 with two laminar zones in the cleanliness class 100. Firstly, the ambience in this clean room has been studied with no processing taking place in terms of velocities, temperatures and comfort criteria. Results show that the zones at the opposite sides from the laminar zones are relatively stagnant and that the temperature is uniform all over the room. Thermal comfort criteria reveal quite good ambient conditions. In second place, we have investigated the optimization of the air change rate for a cleanliness class 10,000 during the manufacture of a fresh product and the influence of the equipment location on the number of particles inside the clean room. Results show that the usual air change rates do not always ensure a cleanliness class 10,000 during the food-processing. On the other hand, this cleanliness class can be obtained with lour air change rates if the process is suited to the clean room.