“Respirators with exhalation valves can be used in a healthcare setting when it is not important to maintain a sterile field (an example of an acceptable practice would be when taking the temperature or blood pressure of a patient). Respirators with exhalation valves should not be used in situations where a sterile field is required (e.g., during an invasive procedure in an operating or procedure room) because the exhalation valve allows unfiltered exhaled air to escape into the sterile field.”

https://www.cdc.gov/niosh/npptl/topics/respirators/disp_part...