An atmosphere that may expose employees to the risk of death, incapacitation, impairment of ability to self rescue, injury, or acute illness from the following causes:
Oxygen concentration below 19.5% or above 23.5%
Flammable gas, vapor, or mist in excess of 10% LEL (Lower Explosive Limit)
Atmospheric concentration in excess of any substance's published dose permissible exposure limit (PEL) which is capable of producing the above effects
Any other atmospheric condition that is immediately dangerous to life or health (IDLH)