The approach to any potentially hazardous atmosphere, including biological hazards, must be made with a plan that includes an assessment of hazard and exposure potential, respiratory protection needs, entry conditions, exit routes, and decontamination strategies. Because biological weapons are particles, they will not penetrate the materials of properly assembled and fitted respirators or protective clothing. For protecting workers from biological hazards, the paper recommends half-mask or full facepiece air-purifying respirators with particulate filter efficiencies ranging from N95 (for hazards such as pulmonary tuberculosis) to P100 (for hazards such as hantavirus) as a minimum level of protection. First responders typically use self-contained breathing apparatus respirators with a full facepiece operated in the most protective, positive pressure (pressure demand) mode.