If possible, it is always recommended to replace N95 or KN95 face masks.
In times of limited supply, the CDC has the following recommendations for extended use and limited reuse of N95 masks:
Minimize the number of individuals who need to use respiratory protection through the preferential use of engineering and administrative controls;
Use alternatives to N95 respirators (e.g., other classes of filtering facepiece respirators, elastomeric half-mask and full facepiece air purifying respirators, powered air purifying respirators) where feasible;
Implement practices allowing extended use and/or limited reuse of N95 respirators, when acceptable; and Prioritize the use of N95 respirators for those personnel at the highest risk of contracting or experiencing complications of infection.
If reusing face masks, the CDC also recommends the following steps to reduce contact transmission after wearing:
Discard N95 respirators following use during aerosol generating procedures.
Discard N95 respirators contaminated with blood, respiratory or nasal secretions, or other bodily fluids from patients.
Discard N95 respirators following close contact with any patient co-infected with an infectious disease requiring contact precautions.
Consider use of a cleanable face shield (preferred3) over an N95 respirator and/or other steps (e.g., masking patients, use of engineering controls), when feasible to reduce surface contamination of the respirator.
Hang used respirators in a designated storage area or keep them in a clean, breathable container such as a paper bag between uses. To minimize potential cross-contamination, store respirators so that they do not touch each other and the person using the respirator is clearly identified. Storage containers should be disposed of or cleaned regularly.
Clean hands with soap and water or an alcohol-based hand sanitizer before and after touching or adjusting the respirator (if necessary for comfort or to maintain fit).
Avoid touching the inside of the respirator. If inadvertent contact is made with the inside of the respirator, discard the respirator and perform hand hygiene as described above.
Use a pair of clean (non-sterile) gloves when donning a used N95 respirator and performing a user seal check. Discard gloves after the N95 respirator is donned and any adjustments are made to ensure the respirator is sitting comfortably on your face with a good seal.
Masks should only be extended or reused by same wearer since asymptomatic wearers could potentially spread infection.