The purpose of fit-testing is to make sure the respirator you use will not leak under normal use and working conditions. Almost any respirator you use will appear to fit, but individuals wearing a respirator may not always be able to detect a leak. Therefore a good fit testing program is necessary. The new standard allows respirator users to continue using and purchasing air-purifying and particulate-filter respirators certified under the old standards. Before an employee may be required to use any respirator with a negative or positive pressure tight fitting face piece the employee must be fitted with the same make, model, style, and size of respirator that will be used. The employer shall ensure employees using a tight fitting face piece respirator pass an appropriate qualitative fit test, or quantitative fit test. Fit testing a respirator determines the quality of the respirator to face seal, and is the only way to screen out poorly fitting face pieces. A fit test should not be confused with a fit check or user seal check. The fit check or user seal checks or used only to verify that a respirator is seated properly before a fit test, or before entering a work area with a previously fitted respirator.

The two basic types of fit test are quantitative and qualitative. You can remember the difference between the two by remembering the word quantitative or quantity. Quantity in that something must be measured with an instrument. Quantitative fit testing provides a numerical or scientific measurement of the respirator fit. It is the most accurate type of fit testing. Quantitative fit testing is performed by challenging the seal of a probed respirator equipped with P100 HEPA filters to a challenge aerosol that is easily detected by the fit test instrument or by measuring the pressure inside the face pieces during a controlled negative pressure fit test. The words challenge aerosol refers to approved materials such as smoke or other aerosols that can be detected by a human being. A probed respirator is one that has a probe inside the respirator for measuring pressure. The challenge aerosol is something that is not harmful, but it can be easily detected with sensitive instruments. A fit testing instrument will monitor the aerosol concentrate both inside and outside the face piece during this test, or the instrument will calculate the pressure differences between the inside and outside to determine if there is any leakage.

Next the instrument will either report these concentrations or calculate the wearer's fit factor. The fit factor is the ratio of the concentration outside the face piece divided by the concentration inside the face piece, or a measure of the leakage rate. While this fit factor represents the actual fit of the respirator in a test environment an assigned protection factor or APF is a limit set by an applicable regulatory agency. When the assigned protection factor and the fit factor are compared the fit factor must exceed the APF by at least 10 times in order for the fit to be adequate. Let's say that again, when the assigned protection factor and the fit factor are compared the fit factor must exceed the APF by at least 10 times in order for the fit to be adequate. This means you need to know and understand assigned protection factors or APFs to make sure you have a proper respirator for the potential hazard. For example, a half mask respirator has an assigned protection factor of 10. Therefore the wearer must achieve a fit factor ten times greater than the APF, or 100. This safety factor may be set higher at the discretion of the respiratory program administrator. A probed respirator is a respirator that has been modified by the manufacturer to provide a sampling port through which internal air can be monitored. This probed respirator can only be used for fit testing and can never be used for respiratory protection.