Mechanical Code…
Immediately Dangerous to Life and Health.
A concentration of airborne contaminant’s, normally expressed in parts per million (ppm) or milligrams per cubic meter (mg / m3), that represents the maximum level from which one could escape within thirty (30) minutes without any escape-impairing symptoms or irreversible health effects. This level is established by the National Institute of Occupational Safety and Health (NIOSH).
(Uniform Mechanical Code® 211.0)