Laboratory research has shown that the most efficient filters can show signs of filter efficiency degradation. Increased pressure drop as the filter loads may increase faceseal leakage lowering overall respirator performance. Three methods used to indicate the end of service are warning properties, service life testing and setting of change out schedules, and the use of ESLIs. Each has advantages and disadvantages. Odor is not thought to be the best methods to indicate the end of service.
People vary in their odor thresholds and can easily loose sensitivity.
Change out schedules based on testing in the lab is normally done for single chemicals and may not apply to workplaces where multiple materials are used. Techniques are available to test the actual environment. ESLI's have not been developed for many materials. Supplied Air Respirators Current certification requirements have a wide range of air flows that are allowed. Air flow may be the critical factor in the performance of these types of respirators.