Where oxygen deficiency (atmospheres containing less than 19.5 percent oxygen) or a hazardous atmosphere exists or could reasonably be expected to exist (e.g., excavations in landfill areas, where hazardous substances are stored nearby) the atmospheres in the excavation shall be tested before employees enter excavations greater than 4 feet (1.22 m) in depth (§1926.651(g)(1)(i)). 

OSHA

1926.651(g)(1)(i)