What are the Hazards Involved in Entering and Working in Confined Spaces?

Hazardous Atmospheres

The atmosphere in a confined space may be extremely hazardous because of the lack of natural air movement. This characteristic of confined spaces can result in

  1. Oxygen-deficient atmospheres,
  2. Flammable atmospheres, and/or
  3. Toxic atmospheres.

An oxygen-deficient atmosphere has less than 19.5% available oxygen (O2). Any atmosphere with less than 19.5% oxygen should not be entered!

The oxygen level in a confined space can decrease because of work being done, such as welding, cutting, or brazing; or, it can be decreased by certain chemical reactions (rusting, paint drying) or through bacterial action (fermentation).

The oxygen level is also decreased if oxygen is displaced by another gas, such as carbon dioxide or nitrogen, resulting from inerting or fire suppression. Total displacement of oxygen by another gas, such as carbon dioxide, will result in unconsciousness, followed by death.


Two things make an atmosphere flammable:

  1. The oxygen in air, and
  2. A flammable gas, vapor, or dust in the proper mixture.

Different gases have different flammable ranges. If a source of ignition (e.g., a sparking or electrical tool, static electricity, sand blasting) is introduced into a space containing a flammable atmosphere, an explosion will result.

An oxygen-enriched atmosphere (above 22%) will cause flammable materials, such as clothing and hair, to burn violently when ignited. Therefore, never use pure oxygen to ventilate a confined space. Ventilate with normal air.


Unless a Chemist has certified a space as Safe. assume that any substance (liquids, vapors, gases, mists, solid materials, and dusts) in a confined space can be hazardous.

Toxic substances may range from fast acting poisons to long term cancer causing carcinogens. Toxic substances can come from the following:

  • The product stored in the space:

The product can be absorbed into the structure and give off toxic gases when removed or when cleaning out the residue of a stored product, toxic gases can be given off.

Example: Removal of sludge or mud from a tank

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