Studies have shown that during heavy exertion, it is possible for the facepiece pressure to become momentarily negative in relation to the outside atmosphere, potentially resulting in facepiece leakage. The consequences of facepiece leakage for a positive pressure mask can be extremely serious since these types of devices are more likely to be used in extremely hazardous environments; even brief leaks can cause serious exposure.
Wearers also may believe that they can afford to take less care when donning their facepiece when using a breathing apparatus that appears to be highly protective; they may ignore face seal checks and correct strap tensioning because they are relying on airflow to overcome any leaks. Fit testing demonstrates to wearers the need to don the facepiece properly.