A plumber must enter a sanitary sewer manhole to make a connection. This is a permit-required confined space. What atmospheric hazard is the leading concern, and what is required before entry?

Giải thích

A sewer is a permit-required confined space where hydrogen sulfide, methane, and oxygen deficiency can be immediately dangerous, so the atmosphere must be tested for oxygen, flammables, and toxics and the space ventilated before and continuously during entry. An attendant, retrieval equipment, and a permit are required. Oxygen must be between 19.5 and 23.5 percent.