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What should be done when preparing to enter the confined space?

The important thing to remember is that each time a worker plans to enter any work space, the worker should determine if that work space is considered a confined space. Be sure that the confined space hazard assessment and control program has been followed.

The next question to ask is - Is it absolutely necessary that the work be carried out inside the confined space? In many cases where there have been deaths in confined spaces, the work could have been done outside the confined space.

Before entering any confined space, a trained and experienced person should identify and evaluate all the potential hazards within the confined space. An important step in determining the hazards in a confined space is air testing.

Air quality testing

The air within the confined space should be tested from outside of the confined space before entry into the confined space. Care should be taken to ensure that air is tested throughout the confined space - side-to-side and top to bottom. A trained worker using detection equipment which has remote probes and sampling lines should do the air quality testing. The sampling should show that:

  • The oxygen content is within safe limits - not too little and not too much.
  • A hazardous atmosphere (toxic gases, flammable atmosphere) is not present.
  • Ventilation equipment is operating properly.

The results of the tests for these hazards are to be recorded on the Entry Permit along with the equipment or method(s) that were used in performing the tests.

Air testing may need to be ongoing depending on the nature of the potential hazards and the nature of the work. Conditions can change while workers are inside the confined space and sometimes a hazardous atmosphere is created by the work activities in the confined space.

It is important to understand that some gases or vapours are heavier than air and will settle to the bottom of a confined space. Also, some gases are more light, than air and will be found around the top of the confined space. Therefore, it is necessary to test all areas (top, middle, bottom) of a confined space with properly calibrated testing instruments to determine which gases are present. If testing reveals oxygen-deficiency, or the presence of toxic gases or vapours, the space must be ventilated and re-tested before workers enter. If ventilation is not possible and entry is necessary (for emergency rescue, for example), workers must have appropriate respiratory protection.


How are hazards controlled in confined spaces?

The traditional hazard control methods found in regular work sites can be effective in a confined space. These include engineering controls, administrative controls and personal protective equipment. Engineering controls are designed to remove the hazard while administrative controls and personal protective equipment try to minimize the contact with the hazard.

However, often because of the nature of the confined space and depending on the hazard, special precautions not normally required in a regular work site may also need to be taken. The engineering control commonly used in confined spaces is mechanical ventilation. The Entry Permit system is an example of an administrative control used in confined spaces. Personal protective equipment (respirators, gloves, ear plugs) is commonly used in confined spaces as well.

How is air quality maintained?

Natural ventilation (natural air currents) is usually not reliable and sufficient to maintain the air quality. Mechanical ventilation (blowers, fans) is usually necessary to maintain air quality.

  • If mechanical ventilation is provided, there should be a warning system in place to immediately notify the worker in the event of a hazard or a failure in the ventilation equipment.
  • Care should be taken to make sure the air being provided by the ventilation system to the confined space is ’clean’.
  • Ease of air movement throughout the confined space should be considered, because of the danger of pockets of toxic gases still remains even with the use of mechanical ventilation.
  • Do not substitute oxygen for fresh air. Increasing the oxygen content will significantly increase the risk of fire and explosion.
  • The use of mechanical ventilation should be noted on the entry permit.

The following paragraphs provide information on ventilation, isolation and respirators. All these are important for air quality maintenance and safety of workers. All these are monitored and regulated as per regulations of industrial health and hygiene.

Ventilation by a blower or fan may be necessary to remove harmful gases and vapours 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 in flammable gases or vapours have displaced the oxygen level, but are too rich to burn, the forced air ventilation may dilute them until they are within the explosive range. Also, if inert gases (e.g. carbon dioxide, nitrogen, argon) are used in the confined space, the space should be well ventilated and re-tested before a worker enters.

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 to blow out all harmful gases and vapours (see diagram). The 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 be formed again when the flow of air is stopped.

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

  • Locking out by electrical sources, preferably at disconnected switches remote from the equipment.
  • Blanking and bleeding pneumatic and hydraulic lines.
  • Disconnecting belt and chain drives, and mechanical linkages on shaft-driven equipment where possible.
  • Securing mechanical moving parts within confined spaces with latches, chains, choke, blocks, or other devices.
  • Locking out by electrical sources, preferably at disconnected switches remote from the equipment.
  • Blanking and bleeding pneumatic and hydraulic lines.
  • Disconnecting belt and chain drives, and mechanical linkages on shaft-driven equipment where possible.
  • Securing mechanical moving parts within confined spaces with latches, chains, choke, blocks, or other devices.

Respirators are devices that can allow workers to safely breathe without inhaling toxic gases or particles. Two basic types are air-purifiers, which filter dangerous substances from the air, and air-suppliers, which deliver a supply of safe breathing air from a tank or an uncontaminated area nearby.

Selecting the proper respirator for the job, the hazard, and the person is very important, as is thorough training in the use and limitations of respirators. Questions regarding the proper selection and use of respirators should be addressed to a certified industrial hygienist, or to the NIOSH Division of Safety Research, 944 Chestnut Ridge Rd., Morgantown, West Virginia 26505.


How are fire and explosion prevented?

Work where a flame is used or a source of ignition may be produced (hot work) should not normally be performed in a confined space unless:

  • All in flammable gases, liquids and vapours are removed prior to the start of any hot work. Mechanical ventilation is usually used to
- keep the concentration of any explosive or flammable hazardous substance
- less than 10% of its Lower Explosive Limit
- make sure that the oxygen content in the confined space is not enriched.
- Oxygen content should be less than 23% but maintained at levels greater than 18%. (These numbers can vary slightly from jurisdiction to jurisdiction.)
  • Surfaces coated with combustible material should be cleaned or shielded to prevent ignition.

While doing the hot work, the concentrations of oxygen and combustible materials must be monitored to make certain that the oxygen levels remain in the proper range and the levels of the combustible materials do not get higher than 10% of the Lower Explosive Limit. In special cases it may not be possible, and additional precautions must be taken to ensure the safety of the worker prior to entering the confined space.


If a potential flammable atmosphere hazards are identified during the initial testing, the confined space should be cleaned or purged and ventilated and tested again before entry to the confined space is allowed. Only after the air testing is within allowable limits entry should occur as the gases used for purging can be extremely hazardous.

How are energy sources controlled?

All potentially hazardous energy sources must be de-energized and locked out prior to entry to the confined space so that equipment cannot be turned on accidentally.





Is entry necessary?





Are the instruments used in atmospheric testing properly calibrated?


Was the atmosphere in the confined space tested?


Was Oxygen at least 19.5% - not more than 21%?


Were toxic, flammable, or oxygen-displacing gases/vapours present?

Which of the following gases are identified?

Hydrogen Sulfide


Carbon Monoxide




Carbon Dioxide


Other (list)



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