Air from a room with specific pollution can only be recycled if it is effectively purified. After recycling, it may only be sent to other rooms if the pollution of all the rooms concerned is of the same nature. In the event of recycling, the concentrations of dust and substances in the atmosphere of the premises must remain below the occupational exposure limit values defined in articles R. 4222-10, R. 4412-149 and R. 4412-150.