Definition - What does fit factor mean?
A fit factor is the numerical result of a quantitative fit test performed on a respirator's face piece which indicates the effectiveness of the seal against the face. The measure is a gauge of the amount of leakage permitted by a seal while it is in contact with the wearer's face and whether or not the respirator allows too much leakage or not.
SureHire explains fit factor
Fit factor is expressed as a ratio that identifies how many times cleaner the air inside the respirator's face piece is in comparison to the air outside of the face piece. The Occupational Safety and Health Administration (OSHA) has set out a Respiratory Protection Program which requires employers to follow the standards and procedures for fit testing respirators in order to protect the health and safety of their workers using respirators. The accepted fit factor for a half face mask is 100, whereas for a full face mask it is 2000. These ratios indicate that the air inside the face piece is 100 times or 2000 times respectively, cleaner than the air outside of the face piece.