Definition:
IDLH, or Immediately Dangerous to Life or Health, is a concentration threshold defined by the U.S. National Institute for Occupational Safety and Health(NIOSH). An IDLH value represents an airborne concentration of a substance that poses an immediate threat to life, would cause irreversible or delayed serious health effects, or would impair a person’s ability to escape.
IDLH values are U.S.-centric and were developed primarily for occupational safety and respiratory protection. In practice, process safety analyses often borrow IDLH values as conservative toxicity thresholds when evaluating consequences of accidental releases.
Note: AEGL, ERPG, and IDLH are all acute inhalation benchmarks and are often shown side‑by‑side in dispersion and emergency‑planning tools, which can cause confusion. AEGL and ERPG are public‑exposure guidelines for one‑time release scenarios, whereas IDLH is a NIOSH worker‑protection value for respirator/entry decisions; they are not interchangeable and must be used only for their intended purpose.
Key Points:
- IDLH values are developed and published by NIOSH (CDC).
- Outputs based on IDLH thresholds are mostly for occupational safety. But it may be borrowed and may feed into hazard and risk assessments (H&RA) and QRA.
- They apply to toxic substances, not to materials that are only flammable or explosive. Substances with no acute toxicity (e.g., methane) do not have IDLH values.
Example:
During an emergency response to a toxic gas release, a safety professional evaluates whether personnel can safely enter an affected area. If predicted airborne concentrations exceed the substance’s IDLH value, only workers equipped with appropriate respirators or supplied‑air protection are permitted entry, or entry is prohibited altogether to ensure escape capability. This application is occupational safety–focused; IDLH is not a process safety or functional safety metric. It could feed H&RA in odd scenarios.
See Also: OSHA, H&RA, dispersion analysis, AEGL, ERPG
Cited Source:
- NIOSH – IDLH documentation and definitions: https://www.cdc.gov/niosh/idlh/default.html
- CDC / NIOSH – Occupational Exposure Guidance: https://www.cdc.gov/niosh