The respirator mask can't protect you if it doesn't fit your face and If your respirator mask doesn't fit your face properly, contaminated air can leak into your mask facepiece, and you could breathe in contaminated substances.
Quantitative Fit Testing is the most accurate form of respirator fit testing, a data-driven approach to determining the fit of a respirator mask on healthcare worker, this device measures particle concentration in the ambient air and inside of the mask, and determines how well a mask fits by comparing the ratio of these particle concentrations.