Have a second person present, who can watch you from outside the confined space. Always secure a lifeline to yourself when entering areas that may be deficient in oxygen. Without a SCBA, this lifeline is the only means others have to rescue you if you become unconscious. Regardless of the type of equipment selected, it must fit properly to ensure complete protection. A device must have a tight seal around the nose and mouth or contaminated air will leak through the seal.
Facial hair can prevent a good seal, even if it is only one day's stubble.
For best results, have equipment fitted to the wearer by qualified personnel and follow the manufacturer's instructions. An easy way to check the seal is to conduct a positive or negative fit check. For a positive check, cover the exhalation valve with the palm of the hand and exhale so that air builds up inside the respirator. If there is a leak, you will feel it against the skin of your face. If there is no leakage, the mask will be drawn tightly to your face.