There is virtually no information published on the absolute efficiency of baghouses in reducing the emmisions of fine particles of asbestos. This lack of information is unfortunate because serious occupational health problems may result from the common practice of recirculating air to conserve energy. Emission testing has been conducted at five asbestos processing plants where the emissions are controlled by baghouses. The results showed that the mass removal efficiency frequently exceeded 99.00%. Membrane filter samples of the effluent were examined by optical and electron microscope. It was observed that despite the high mass efficiency, the number of fibers emitted, which were greater than 1.5 mum in length, was about 10(4)-10(5) fibers/m3, while the number of fibers less than 1.5 mum was 10(7)-10(8) fibers/m3. The significance of the size of the fibers in terms of probably health impact is briefly discussed.