This guide explains the new regulations in 42 CFR 84 for testing and certifying nonpowered, air purifying, particulate filter respirators, and to provide valuable information for selecting and using the new respirators certified by NIOSH. The new set of regulations specifies a more demanding certification test than under 30 CFR 11. The changes affect only nonpowered, air purifying, particulate filter respirators; certification requirements for all other classes of respirators have been transferred to Part 84 without changes. The new Part 84 provides for nine classes of filters, three levels of filter efficiency (95%, 99%, and 99.97%), and three categories of resistance to filter efficiency degradation (not resistant to oil, oil resistant, and oil proof). The class of filter must be clearly marked on the filter, filter package, or respirator box. The new classes of nonpowered particulate respirators will require new decision logic for selecting the proper one. The selection process will depend on the presence or absence of oil particles. The selection of filter efficiency depends on how much filter leakage can be accepted. The selection of facepiece depends on the level of protection needed. Detailed guidelines for use are presented in this report.