Immediately Dangerous to Life or Health (IDLH) describes an atmosphere that poses an immediate threat to life, could cause permanent health effects or would prevent escape, and requires the most protective respirators and rescue standby.
Immediately Dangerous to Life or Health, or IDLH, describes an atmosphere that poses an immediate threat to life, would cause irreversible adverse health effects, or would impair a person's ability to escape from a dangerous atmosphere. That is the definition OSHA uses in its Respiratory Protection Standard, 29 CFR 1910.134, and it determines the type of respirator and the backup procedures required when workers must enter such an atmosphere.
The term is used in two related ways. First, it describes a condition: a tank with too little oxygen, a pit full of hydrogen sulfide or a room filled with smoke is an IDLH atmosphere. Second, NIOSH publishes IDLH values, which are specific airborne concentrations for individual chemicals. These values estimate the level above which a worker without respiratory protection could suffer death or permanent harm, or be unable to escape, within about 30 minutes.
IDLH is not an exposure limit for everyday work. Permissible exposure limits and other occupational limits are set to protect workers over a full shift and a working lifetime. IDLH values are far higher and are used to make decisions about respirator selection, emergency planning and rescue, not to judge whether routine exposure is acceptable.
Normal air contains about 20.9% oxygen. OSHA defines an oxygen-deficient atmosphere as one with less than 19.5% oxygen, and under the respiratory protection standard, oxygen-deficient atmospheres are generally treated as IDLH. Oxygen can be displaced by nitrogen purges, inert gases such as argon, carbon dioxide, or consumed by rusting metal and decaying organic material inside closed spaces. Workers often get no warning, because low oxygen impairs judgment before it causes noticeable symptoms.
Concentrations of a toxic substance at or above its IDLH value create an IDLH atmosphere. For example, NIOSH lists IDLH values of 100 ppm for hydrogen sulfide and 1,200 ppm for carbon monoxide. Some substances, such as hydrogen sulfide, deaden the sense of smell at high levels, so odor cannot be trusted as a warning.
An atmosphere at or near its lower explosive limit is extremely dangerous because any ignition source could set it off. Many entry procedures treat flammable concentrations above 10% of the lower explosive limit as a hazardous atmosphere that prohibits entry.
OSHA requires that an atmosphere be considered IDLH when the employer cannot identify or reasonably estimate employee exposure. In practice, this means any space that has not been tested, such as a sealed vault or a tank after an upset, must be treated as IDLH until monitoring proves otherwise.
Only the most protective respirators are allowed in IDLH atmospheres. Under 1910.134, employers must provide either a full facepiece, pressure-demand self-contained breathing apparatus (SCBA) certified by NIOSH for at least 30 minutes of service life, or a combination full facepiece, pressure-demand supplied-air respirator with an auxiliary self-contained air supply. Air-purifying respirators, which only filter the surrounding air, are not acceptable because they cannot supply oxygen and can be overwhelmed by high concentrations. Respirators used only for escape from IDLH atmospheres must be NIOSH-certified for escape from the specific atmosphere.
When workers enter an IDLH atmosphere, OSHA requires at least one employee to remain outside the area. That person must maintain visual, voice or signal line communication with the workers inside, be trained and equipped to provide effective emergency rescue, and notify the employer or a designee before entering to rescue. The standby must have a pressure-demand SCBA or supplied-air respirator with auxiliary SCBA, and appropriate retrieval equipment or other rescue means.
All of this sits within a written respiratory protection program, which includes medical evaluation, fit testing, training and maintenance. Workers entering IDLH conditions also need training in the specific hazards, the limits of their equipment and the emergency procedures for the task.
Many IDLH situations occur in confined spaces, where OSHA's permit-required confined space rules add requirements for atmospheric testing, ventilation, attendants and rescue. Chemical releases may also bring HAZWOPER emergency response requirements into play.
NIOSH IDLH values are found in the NIOSH Pocket Guide to Chemical Hazards and supporting documentation. Safety professionals use them to:
Because IDLH values are based on a short escape window, they should never be read as safe working levels. A concentration just below the IDLH value is still far above the occupational exposure limit.
At a wastewater treatment plant, a maintenance worker was assigned to clear a blockage in a wet well. The well had been used without incident many times, and his supervisor considered a quick look with a flashlight from the hatch enough preparation. A coworker insisted on testing the atmosphere first. The four-gas monitor showed hydrogen sulfide above 100 ppm near the bottom of the well, along with depressed oxygen. Without the test, the worker would have entered an IDLH atmosphere wearing no respiratory protection.
The plant treated the event as a near miss with fatal potential. It reclassified the wet well as a permit-required confined space, required pre-entry and continuous monitoring, installed forced-air ventilation, and set the rule that any entry with readings at or above IDLH values would require SCBA, a trained standby attendant and a retrieval system. It also changed its procedures so that untested spaces are always treated as IDLH.
No. A permissible exposure limit is the maximum average exposure allowed over a shift and is designed to protect long-term health. An IDLH value is a much higher concentration that could kill, cause permanent harm or prevent escape within a short time. IDLH guides respirator and emergency decisions, not routine exposure.
No. OSHA allows only pressure-demand SCBA or a pressure-demand supplied-air respirator with an auxiliary self-contained air supply for IDLH conditions. Air-purifying respirators cannot supply oxygen and can fail at high concentrations.
Do not enter without the proper respirator and rescue setup. Untrained would-be rescuers account for a large share of confined space deaths. The standby person should call for help, notify the employer, use retrieval equipment from outside if possible, and enter only if trained and equipped to do so.
NIOSH publishes IDLH values in the NIOSH Pocket Guide to Chemical Hazards and on its website. Some chemicals do not have an assigned value, in which case the employer should treat potential high exposures conservatively.
Part of SafetyIQ's EHS glossary: plain-English definitions of workplace health and safety terms.
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