Respirators are not one size fit, so it is recommended to get fit tested. Fit Testings measure tightness between a wearer's face and their facepiece. Any person wearing a tight fitting RPE should get a fit test. This would include disposable and reusable respirators using filters. Suction-based filters require tight fits to prevent contaminated leakage into the facepiece.
They do not take long to complete and should be done at least once a year. However, the frequency of testing will be affected by the gaining or loss of weight, surgical face/mouth procedures, scarring around the mouth, chin, or nostrils. It is important for you to wear everything that you would while working with the facepiece (glasses, nosering, retainers, etc.) to get the most accurate fit for your work conditions.
Attention: Get fitted when clean-shaven. Any amount of facial hair can change the fitting of your mask, putting you in danger of exposure to harmful substances.
Once you are fit tested you are required to use the same respirator model, style, make, and size for your job. These tests are simple to do and can be conducted using a fit testing kit or by a supplier/work professional.