Hazardous Atmosphere

Hazardous atmosphere is an atmosphere that may expose employees to the risk of death, incapacitation, impairment of ability to self-rescue (that is, escape unaided from a permit space), injury, or acute illness.

Causes of Hazardous Atmosphere

• Flammable gas, vapor, or mist in excess of 10 percent of its lower flammable limit (LFL).
• Airborne combustible dust at a concentration that meets or exceeds its LFL.  NOTE: This concentration may be approximated as a condition in which the dust obscures vision at a distance of 5 feet (1.52 m) or less.
• Atmospheric oxygen concentration below 19.5 percent or above 23.5 percent.
• Atmospheric concentration of any substance for which a dose or a permissible exposure limit and which could result in employee exposure in excess of its dose or permissible exposure limit.  NOTE: An atmospheric concentration of any substance that is not capable of causing death, incapacitation, impairment of ability to self-rescue, injury, or acute illness due to its health effects is not covered by this provision.
• Any other atmospheric condition that is immediately dangerous to life or health.  NOTE: For air contaminants for which OSHA has not determined a dose or permissible exposure limit, other sources of information, such as Material Safety Data Sheets, published information, and internal documents can provide guidance in establishing acceptable atmospheric conditions.

If hazardous conditions are detected during entry, employees must immediately leave the space.  The employer must evaluate the space to determine the cause of the hazardous atmosphere and modify the program as necessary.

When entry to permit spaces is prohibited, the employer must take effective measures to prevent unauthorized entry.  Non-permit confined spaces must be evaluated when changes occur in their use or configuration and, where appropriate, must be reclassified as permit spaces.

A space with no potential to have atmospheric hazards may be classified as a non-permit confined space only when all hazards are eliminated in accordance with the standard.  If entry is required to eliminate hazards and obtain data, the employer must follow specific procedures in the standard.

• The employer's written program should establish the means, procedures and practices to eliminate or control hazards necessary for safe permit space entry operations.  These may include:
  • Specifying acceptable entry conditions.
  • Isolating the permit space.
  • Providing barriers.
  • Verifying acceptable entry conditions.
  • Purging, making inert, flushing or ventilating the permit space.