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Q&A: Anhydrous Ammonia Leaks

Recently, we received the following question from a subscriber:

What does OSHA say about initial entry into an area with a suspected gas leak of anhydrous ammonia of unknown quantity and the use of an SCBA?

This was our response:

The answer varies depending on the area to be entered. We will assume in this case it is a confined space since you use the word “entry.” There are different requirements for trenches and other work areas that are not confined spaces. However, in all cases you must conduct an evaluation of the area for hazards and test the atmosphere before allowing any entry in order to help determine if entry is the best option and evaluate respiratory protection or other PPE needs.

According to the OSHA rule for confined spaces in general industry (29 CFR 1910.146), before any entry, the space must be evaluated to determine how the hazardous atmosphere developed, conduct atmosphere testing, engineering controls like ventilation implemented where feasible. The testing results and the decisions about what steps must be followed before entry must be evaluated by, or reviewed by, a technically qualified professional like an OSHA consultation service, a certified industrial hygienist, a registered safety engineer, or a certified safety professional.

Rescue-type SCBA is not sufficient for routine entry or work in a permit space.

Rescue services must be available if entry is allowed.

For example, OSHA rules for a non-permit confined space confined space at 29 CFR 1910.146(c)(5) says at least the following precautions must be taken before entry:

Before an employee enters the space, the internal atmosphere must be tested for the following conditions, in the order given, with a calibrated direct-reading instrument: Oxygen content, flammable gases and vapors, and potential toxic air contaminants.

See the OSHA factsheet about atmospheric testing at

There must be no hazardous atmosphere within the space whenever any employee is inside the space.

Continuous forced air ventilation must be used. The atmosphere within the space must be periodically monitored as necessary to ensure that the continuous forced air ventilation is preventing the accumulation of a hazardous atmosphere.

The permit-required space section of the rule at 29 CFR 1910.146(d) has similar procedures as described above but are more detailed.