An example of a tight-fitting, half facepiece powered air purifying respirator is shown in A (CleanSpace Technology; Australia); the unfiltered exhalation valve is in the center of the facepiece. In B, another view of the battery and fan unit is shown. In C, a view of the accessory N95 filter attachment for the exhalation valve is shown; when used, filtration of the exhalation valve provides “source control,” preventing the wearer of the respirator from exhaling respiratory pathogens into the surrounding air. This respirator is intended to make a highly effective seal against the face, and should be fit tested before use, just as with a negative pressure elastomeric respirator or filtering facepiece respirator. Air is drawn through a high-efficiency particulate air filter (shown in D) and pushed into the respirator under positive pressure by the battery-powered fan located behind the wearer’s head. Exhalation occurs through an unfiltered valve on the front of the respirator, unless the accessory N95 filter is attached to the exhalation valve (the replaceable N95 filter is shown inside of the filter holder in E). This respirator has at least three advantages compared to a loose-fitting powered air purifying respirator. First, because this respirator has both a tight seal against the face and positive pressure produced by the battery powered fan, the Assigned Protection Factor is 50, twice that of a typical loose-fitting powered air purifying respirator, or five times that of a negative pressure elastomeric respirator. Second, it is possible to filter the exhaled air, which is not possible with a loose-fitting powered air purifying respirator. Third, if the power source or fan fails (for example, if the battery is exhausted), the respirator will function as an ordinary negative pressure elastomeric respirator, with the wearer pulling inhaled air through the high-efficiency particulate air filter; if a loose-fitting powered air purifying respirator power source or fan fails, there is no respiratory protection.