Immediately Dangerous to Life or Health (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. OSHA uses this definition in its respiratory protection standard, and NIOSH publishes IDLH values for hundreds of chemicals.
IDLH matters because it marks the point where ordinary air-purifying respirators and routine precautions are no longer enough. Entering an IDLH atmosphere without the right supplied-air respirator, standby support and rescue plan can kill a worker within minutes, and would-be rescuers are often among the victims.
What Does IDLH Mean?
OSHA defines IDLH in 29 CFR 1910.134(b), the respiratory protection standard. An atmosphere can be IDLH for several reasons:
- Toxic contaminants at concentrations that can cause death, serious injury or permanent harm after a short exposure, such as high levels of hydrogen sulfide or carbon monoxide.
- Oxygen deficiency. OSHA requires employers to treat all oxygen-deficient atmospheres, meaning air with less than 19.5% oxygen by volume, as IDLH, with a narrow exception for atmospheres where the employer can show oxygen will stay within the ranges in Table II of the standard under all foreseeable conditions.
- Unknown conditions. Under 1910.134(d)(1)(iii), when an employer cannot identify or reasonably estimate employee exposure, the atmosphere must be treated as IDLH.
- Escape impairment. Contaminants that cause severe eye or respiratory irritation, confusion or loss of coordination can prevent self-rescue even if they are not immediately lethal.
How NIOSH Sets IDLH Values
NIOSH publishes chemical-specific IDLH values, which appear in the NIOSH Pocket Guide to Chemical Hazards. They were first developed in the 1970s to support respirator selection, were reviewed in the 1990s, and NIOSH has used an updated derivation methodology since 2013.
IDLH values are based on the effects that could occur from a 30-minute exposure. That does not mean 30 minutes at the IDLH value is safe or allowed. The values exist to guide respirator selection and to identify conditions where only the most protective respirators are acceptable. Examples of NIOSH IDLH values include:
- Hydrogen sulfide: 100 ppm
- Carbon monoxide: 1,200 ppm
- Ammonia: 300 ppm
- Chlorine: 10 ppm
Always confirm the current value in the NIOSH Pocket Guide or on the chemical's safety data sheet before relying on it.
OSHA Respirator Rules for IDLH Atmospheres
Under 29 CFR 1910.134(d)(2), employers must provide one of the following for employees in IDLH atmospheres:
- A full facepiece pressure demand self-contained breathing apparatus (SCBA) certified by NIOSH for a minimum service life of 30 minutes, or
- A combination full facepiece pressure demand supplied-air respirator with an auxiliary self-contained air supply.
Respirators used only for escape from IDLH atmospheres must be NIOSH-certified for escape from the atmosphere in which they will be used. Air-purifying respirators are not acceptable for entry into IDLH atmospheres.
Standby Person and Rescue Requirements
Section 1910.134(g)(3) sets additional procedures for IDLH atmospheres:
- At least one employee, or more when needed, must be located outside the IDLH atmosphere.
- Visual, voice or signal line communication must be maintained between the employees inside and outside.
- The employees outside must be trained and equipped to provide effective emergency rescue.
- The employer or designee must be notified before a standby employee enters the IDLH atmosphere to provide rescue.
- Once notified, the employer or designee must provide necessary assistance appropriate to the situation.
- Standby employees must be equipped with a pressure demand or other positive pressure SCBA, or a positive pressure supplied-air respirator with auxiliary SCBA, and with appropriate retrieval equipment or an equivalent means of rescue where retrieval equipment is not required.
Many IDLH entries take place in a confined space, where the permit-required confined space standard adds its own testing, attendant, permit and rescue requirements.
How to Manage Work in a Potential IDLH Atmosphere Step by Step
- Identify the hazard. List the contaminants that could be present and check oxygen levels, using the chemical's IDLH value as a reference.
- Eliminate the need for entry. Consider whether ventilation, isolation, purging or remote tools can remove the IDLH condition or the need to enter.
- Test the atmosphere. Use calibrated, bump-tested direct-reading instruments, testing oxygen first, then flammables, then toxics.
- Treat unknowns as IDLH. If exposure cannot be identified or reasonably estimated, assume IDLH conditions.
- Select the right respirator. Provide a pressure demand SCBA or a combination supplied-air respirator with an auxiliary escape supply, and confirm users are medically cleared, fit tested and trained.
- Set up standby and rescue. Assign a trained, equipped standby person outside, establish communication and stage retrieval equipment.
- Authorize the work. Use a permit that records test results, equipment, personnel and rescue arrangements.
- Monitor continuously. Keep monitoring during the work and evacuate if conditions change or alarms sound.
Common Mistakes to Avoid
- Using an air-purifying respirator in an oxygen-deficient or unknown atmosphere.
- Treating the IDLH value as a safe exposure limit for 30 minutes.
- Assigning a standby person who is not trained or equipped for rescue.
- Untrained rescuers entering to help a collapsed worker without respiratory protection.
- Relying on a single atmospheric test taken before work began.
How SMS360 Helps
SMS360's Work Permits module helps teams issue and approve permits for high-risk work, recording atmospheric test results, assigned personnel and rescue arrangements before entry. Training Management keeps records of respirator and rescue training, and Risk Assessment helps document the hazards and controls for tasks that could involve IDLH conditions.
Frequently Asked Questions
What does IDLH stand for?
Immediately Dangerous to Life or Health. It describes an atmosphere that poses an immediate threat to life, would cause irreversible health effects or would impair a person's ability to escape.
Is an oxygen-deficient atmosphere IDLH?
Under OSHA's respiratory protection standard, employers must treat all oxygen-deficient atmospheres (below 19.5% oxygen) as IDLH, unless they can show that oxygen will stay within the Table II ranges under all foreseeable conditions.
Can you use an air-purifying respirator in an IDLH atmosphere?
No. Entry into an IDLH atmosphere requires a full facepiece pressure demand SCBA or a combination pressure demand supplied-air respirator with an auxiliary self-contained air supply.
Who sets IDLH values?
NIOSH publishes chemical-specific IDLH values. OSHA's respiratory protection standard defines IDLH atmospheres and sets the respirator and standby requirements for working in them.
