The atmosphere of a confined space must be tested immediately before entry - every time the space is entered. Testing may indicate oxygen deficiency, an explosive of flammable atmosphere, or high concentrations of other hazardous substances. Most other potentially hazardous work places must be tested periodically to be sure the atmosphere is safe to breathe. If the atmosphere does not measure up, anyone who works in it will be assigned a respirator to wear. The harmful concentration of most hazardous substances is so low it is measured in parts per million, or even parts per trillion.
Testing is important whenever there is any question about air quality.
It is the only way to be absolutely sure the air is safe to breathe and the only way to determine how much protection is required. If you are told to wear a respirator, wear it- even if the air seems okay to you. 23-1.06 Respirators are your only protection in a hazardous atmosphere. In order to wear a respirator you must be in good health and you must be able to achieve a proper fit. Anyone who is going to have to work with a respirator must first have a medical checkup.