"The manufacture, handling, and use of new chemical substances often require a level of personal protection that includes respirators. Because of the need for these respiratory protection devices, processes to evaluate penetration of full and half facepiece, negative-pressure respirators were studied. The precision and bias were determined for five methods of sampling for inboard penetration through different areas of the face seal. The sampling procedures evaluated were: continuous, low sampling rate, flush on the respirator, mid-nose-mouth probing (CLF); continuous, high sampling rate, deep front-of-mouth probing (CHD); pulsed, exhalation, deep front-of-mouth probing (FED); exhalation valve discharge (EVD); and pulsed, inhalation, deep front-of-mouth probing (PID). The CLF procedure represents current in-face-piece sampling practice in the United States. Based on evaluation with nine full facepiece respirators, the mean sampling biases were CLF: -21%; CHD: -3%; PED: 0.7%; EVD: -14%; and PID: -12.3%. For five half facepiece respirators, the mean sampling biases were CLF: -26%; CHD: -13%; PED: -4%; EVD: -2%; and PID: -24%. To some extent, the location of the face seal penetration, and the design of the respirator affected the bias of each method."