In a May, 2016 letter of interpretation, OSHA’s Directorate of Enforcement Programs answered this question:
Question: If an employee with a neatly trimmed goatee is wearing a respirator and it does not interfere with the seal of the face piece or valve function, and has passed a fit test, does this meet the intent of the OSHA’s Respiratory Protection standard?
Response: The Respiratory Protection standard, paragraph 29 CFR 1910.134(g)(1)(i)(A), states that respirators shall not be worn when facial hair comes between the sealing surface of the facepiece and the face or that interferes with valve function. Facial hair is allowed as long as it does not protrude under the respirator seal, or extend far enough to interfere with the device's valve function. Short mustaches, sideburns, and small goatees that are neatly trimmed so that no hair compromises the seal of the respirator usually do not present a hazard and, therefore, do not violate paragraph 1910.134(g)(1)(i).
In general, however, beards present serious problems for tight-fitting facepiece respirators because their texture and density vary daily, causing unreliable respirator fit and, therefore, present a higher potential for leakage. However, some other types of respirators do not require a face seal, and thus, usually can be worn with facial hair, such as loose fitting powered air-purifying respirators and hooded powered air-purifying respirators.
Thomas Galassi, Director, Directorate of Enforcement Programs, OSHA