Most hazards are invisible.
The only way to know when a respirator is needed is to test the atmosphere. 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.