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

  • Decomposed material can give off deadly hydrogen sulfide gas.
  • The work being performed in a confined space:

Examples of such include welding, cutting, brazing, painting, scraping, sand blasting, degreasing, etc. Toxic atmospheres are generated in various processes.

For example, cleaning solvents are used in many industries for cleaning/degreasing. The vapors from these solvents are very toxic in a confined space.

  • Areas adjacent to the confined space:

Toxicants produced by work in the area of the confined spaces can enter and accumulate in confined spaces.

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Ventilation by a blower, eductor or fan may be necessary to remove harmful gases and vapors from a confined space. There are several methods for ventilating a confined space. The method and equipment chosen are dependent upon the size of the confined space openings, the gases to be exhausted (e.g., are they flammable?), and the source of makeup air.

Under certain conditions where flammable gases or vapors have displaced the oxygen level, but are too rich to burn, forced air ventilation may dilute them until they are within the explosive range. Also, if inert gases (e.g. carbon dioxide, nitrogen) are used in the confined space, the space should be well ventilated and re-tested before a worker may enter.

A common method of ventilation requires a large hose, one end attached to a fan and the other lowered into a manhole or opening. For example, a manhole would have the ventilating hose run to the bottom (see diagram) to exhaust all harmful gases and vapors. An air intake should be placed in an area that will draw in fresh air only.

Ventilation should be continuous where possible, because in many confined spaces the hazardous atmosphere will form again when the flow of air is stopped.

De-ballasting a tank does not guarantee a safe atmosphere. Testing is still required.


A standby person should be assigned to remain on the outside of the confined space and be in constant contact (visual or speech) with the workers inside.

The standby person should not have any other duties but to serve as standby and know who should be notified in case of emergency. Standby personnel should not enter a confined space until help arrives, and then only with proper protective equipment, life lines, and respirators.

It has been closely monitored and observed that: Over 50% of the workers who die in confined spaces are attempting to rescue other workers.

Rescuers must be trained in and follow established emergency procedures and use appropriate equipment and techniques (lifelines, respiratory protection, standby persons, etc.). Steps for safe rescue should be included in all confined space entry procedures. Rescue should be well planned and drills should be frequently conducted on emergency procedures. Unplanned rescue, such as when someone instinctively rushes in to help a downed co-worker, can easily result in a double fatality, or even multiple fatalities if there are more than one would-be rescuers.


Isolation of a confined space is a process where the space is removed from service by:

  • locking out electrical sources, preferably at disconnect switches remote from the equipment.
  • blanking and bleeding, securing valves Cargo, ballast, IGS, pneumatic and hydraulic lines
  • disconnecting mechanical linkages on shaft-driven equipment where possible, and
  • securing mechanical moving parts within confined spaces with latches, chains, chocks, blocks, or other devices.


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