How to Control Atmospheric Hazards in Confined Spaces

Date First Published on SafetySmart Compliance: March 9th, 2012
Topics: Airborne Contaminants | Confined Spaces |

Perhaps the greatest hazard posed by confined spaces is the air inside. So if you’re allowing workers—either your own or those of a contractor, subcontractor or other outsider—to enter permit-required confined spaces, you need to take steps to control atmospheric hazards. Here’s how.

Air inside confined spaces is a concern because of and what it might contain, such as flammable, explosive and other noxious gasses and vapors. Air inside such spaces is also a concern because of what it might not contain, like oxygen.

Accordingly, a “hazardous atmosphere,” i.e.., one that has the potential to cause death, incapacitation or impairment of self rescue, is one of the criteria that turns an ordinary confined space into a “permit-required confined space” calling for more stringent safety measures. OSHA defines hazardous atmosphere as one containing or potentially containing:

  • A flammable gas, vapor or mist above 10% of its lower flammable limit (LFL);
  • Airborne combustible dust at a concentration at or above its LFL;
  • Oxygen concentration of below 19.5% or above 23.5%;
  • Concentration of a substance above or potentially above its Permissible Exposure Limits (PELs) or dose limit; or
  • Any other atmospheric condition immediately dangerous to life or health (IDLH).


Atmospheric Testing Requirements

You must ensure that atmospheric testing is performed before allowing workers to enter permit spaces containing or potentially containing hazardous atmospheres. Testing must be overseen by the Entry Supervisor; and be conducted before entry and as often as necessary after entry to verify that acceptable atmospheric conditions are present.

Employees may not enter the space until all hazardous atmospheres are removed. If a hazardous atmosphere develops while an employee is inside the space, the space must be evacuate immediately. Where spaces are evacuated, atmospheric testing must be completed again before re-entry.

Employees who perform atmospheric testing must be trained to use the specific instruments and all equipment must be maintained according to the manufacturer’s instructions, including calibration and maintenance;

Testing must be carried out in the following sequence using instruments meant for testing only that condition:

  • First: Oxygen;
  • Second: Combustible gases and vapors;
  • Third: Toxic gases and vapors.

Testing must include all sections of the space, including the top, middle and bottom so that potential pockets of dangerous atmospheres can be identified;

Employees or their authorized representatives have the right to be present during testing and to receive test results. They may also request reevaluation if they have “reason to believe” that the original evaluation isn’t adequate.

Click here if you want to see OSHA`s guidelines on atmospheric testing procedures.


Use of Engineering Controls to Minimize Atmospheric Hazards

Monitoring atmospheric hazards is just the beginning. You also have to control them.

Section 1910.146(d)(3) of the OSHA permit confined spaces standard lists acceptable methods that can be used to eliminate or manage hazardous atmospheres in a permit confined space:


Ventilation, i.e., the use of mechanical systems to force fresh air into the confined space and/or exhausting contaminated air while employees are working in it. If ventilation is used to control atmospheric hazards in the space, the space must be ventilated for a minimum time before entry, typically 15 minutes; when feasible, the space should be ventilated continuously for the duration of the entry.

Ventilation systems must meet designated engineering and design standards and incorporate an audible alarm feature that sounds if the system fails; alternatively, entrants must be equipped with handheld atmospheric monitors that alert them if the atmospheric conditions inside the space deteriorate.

Purging, i.e., the introduction of substances such as an inert gas, steam or water into a confined space to displace or flush out contaminants before workers enter the space. The method of purging must take into account other hazards within the space identified in the hazard assessment and continuous atmospheric testing must be conducted while employees are inside the space.

Inerting, i.e., the introduction of an inert (un- reactive) gas such as nitrogen or carbon dioxide into a confined space to completely displace all oxygen. Although it controls the risk of fire and explosion by removing the source of oxygen, inerting is dangerous because it creates an oxygen deficient atmosphere. Accordingly, when inerting is used to control atmospheric hazards in the space, workers must be properly trained and equipped with Self Contained Breathing Apparatus (SCBA), self‐contained oxygen generating apparatus or Supplied Air Breathing Apparatus (SABA) with an emergency escape bottle. In addition, steps must be taken to ensure that the atmosphere within the space remains inerted at all times while employees are inside.

Isolation, i.e., disconnecting, blanking or blinding or using an equivalent engineered system to prevent a hazardous substance contained in adjacent pipelines from seeping or leaking into the permit space.



Of course, as with any other hazardous operation, confined spaces work should be made safer by utilizing a combination of engineering controls, safe work practices and PPE. That`s why the OSHA standard includes requirements like issuance of entry permits, use of attendants, the provision of rescue services and safety training.