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4 common hazards of confined spaces

[fa icon="calendar"] 12/2/17 6:00 PM / by SAMTRAC


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We’ve covered the definition of confined space, as well the basics for conducting a risk assessment in one. Today we look at the most common hazards you should keep an eye out for when working in a confined space.


Type 1: Hazardous atmospheres

The atmosphere in a confined space may be hazardous for several reasons:

  • The air may have too little or too much oxygen
  • The atmosphere may be toxic or explosive


Once a confined space is identified, its atmosphere must be hazard-rated as high, moderate, or low.


High-hazard atmosphere

An atmosphere that may expose a worker to risk of death, injury, or acute illness, or otherwise impair a worker’s ability to escape unaided from a confined space if the ventilation system or respirator fails.


Moderate-hazard atmosphere

An atmosphere that is not clean, breathable air but is not likely to impair a worker’s ability to escape unaided from a confined space if the ventilation system or respirator fails.


Low-hazard atmosphere

An atmosphere that is shown by pre-entry testing or is otherwise known to contain clean, breathable air immediately prior to entry into a confined space, and that is not likely to change during the work activity.


Oxygen: too little or too much?

Lack of oxygen is a leading cause of death among workers entering confined spaces. Low oxygen levels cannot be detected by sight or smell. You must test the air for this hazardous condition. A very low level of oxygen can damage the brain and cause the heart to stop after a few minutes.


Type 2: Toxic atmospheres

Contaminants in the air can result in an atmosphere that is toxic to workers and may result in injury or death. In the past, miners would take canaries down into coal mines, since these small birds react quickly to carbon monoxide, a deadly gas. If the canaries breathed a small amount of the gas, they would sway on their perches before falling. This gave miners warning that the deadly gas was present. Today, miners have monitors to let them know when there are toxic substances in the atmosphere.


The concentration of the substance inside the confined space must be determined using a recently calibrated and properly set up air monitor with the correct sensor. Such an air monitor may sound an alarm that will alert the worker before the allowable exposure limit is reached.


In most cases, mechanical ventilation such as fans must be used to ventilate the space, bringing in clean outside air. Additionally, the harmful substance must be eliminated wherever practicable. Air testing and ventilation are the best ways to ensure that workers are not placed at risk from hazardous atmospheres.


Type 3: Explosive atmospheres

Three elements are necessary for a fire or explosion to occur:

  • Oxygen
  • Flammable material (fuel), and
  • An ignition source.



Air normally contains 20.9% oxygen – enough oxygen for a fire. However, a higher level of oxygen increases the likelihood of material burning. Air is considered oxygen-enriched at levels above 23%. Enrichment can be caused by improper isolation of oxygen lines, ventilation of the space with oxygen instead of air, or leaks from welding equipment.



Fires and explosions in confined spaces are often caused by gases or vapours igniting. Coal dust and grain dusts may explode when a certain level of dust in the air is reached.


Note: Two or more chemicals may react with each other and become explosive. Containers of fuels, such as gasoline and propane should not be taken into a confined space as fuel can easily burn or explode. Here are some other common substances that can cause explosions or fires in confined spaces:

  • Acetylene gas from leaking welding equipment
  • Methane gas and hydrogen sulphide gas produced by rotting organic wastes in sewers or tanks
  • Hydrogen gas produced by contact between aluminium or galvanised metals and corrosive liquids
  • Grain dusts, coal dust
  • Solvents, such as acetone, ethanol, toluene, turpentine, and xylene, which may have been introduced into the space through spills or by improper use or disposal


A trained person must test the atmosphere for gases and vapours that will burn or explode.


Ignition sources

Ignition sources include:

  • open flames
  • sparks from metal impact
  • welding arcs
  • arcing of electrical motors
  • hot surfaces
  • discharge of static electricity
  • lighting
  • chemical reaction.


Many processes can generate static charge, including steam cleaning, purging, and ventilation procedures. To reduce the risks from these ignition sources, use non-sparking tools and ensure all equipment is bonded or grounded properly.


Type 4: Physical hazards

Loose and unstable materials

Whenever unstable solids made of small particles like sand or grain are stored in enclosures, there is a danger of the materials flowing onto workers and trapping or burying them. Examples of these confined spaces are sand bins, wood chip or sawdust bins, storage or grain silos, and potash feed systems. Granular materials, particularly if moist, can form bridges (or shoulders) above workers. If jarred, these can collapse onto a worker. Bins and hoppers in which materials are conveyed or augured into the bin are particularly dangerous. A worker may be trapped or crushed when material is accidentally discharged into an empty bin or hopper. The design of these confined spaces may increase the danger of being trapped or buried. For example, in an empty hopper with a floor that slopes steeply to a vertical chute, a worker can slide into the chute and become trapped there. Wherever there are loose, unstable materials that could trap or bury you, a qualified person must inspect the space and assess the hazards. Do not enter until the hazard has been eliminated or controlled. Specific training and safety precautions must be in place before you enter.


Slip, trip, and fall hazards

The space you are about to enter may have a hatchway that is difficult to squeeze through, and ladders for ascending or descending. You are therefore at risk of falling while getting into the space as well as while you are inside. In addition, the flooring of tanks or other wet environments or the rungs of a ladder may be very slippery. If the hazard cannot be eliminated and there is a danger of falling from a height, a fall protection system (such as guardrails or a harness and lifeline) may be needed.


Falling objects

In a confined space there may be the danger of being struck by falling objects such as tools or equipment, particularly if access ports or workstations are located above workers. If workers might be exposed to the hazard of falling objects, safe work procedures must be put in place to prevent this. For example, schedule work activity so that no worker is working above another, and lower equipment and tools into the space before workers enter and remove them after workers leave the space.


Moving parts of equipment and machinery

Mechanical equipment such as augers, mixers, or rotating tanks can be dangerous if activated or not secured. Residual energy, such as gravity or accumulated pressure, may also pose a risk unless the equipment is locked out and de-energized. This must be done by following a written lockout procedure that is specific for each piece of equipment and that states each place where a lock must be applied. Even when the power is shut off and the equipment is locked out at control points, unsecured equipment can move, especially if it is out of balance. Before doing any work in confined spaces:

  • Shut the power off
  • Ensure that the equipment is locked out at control points
  • Test the lockout
  • Secure any equipment that can move, even when it has been locked out


Electrical shock

Electrical shock can result from defective extension cords, welding cables, or other electrical equipment. Work done in metal enclosures or in wet conditions can be particularly dangerous. Install ground fault circuit interrupters (GFCIs) or use assured grounding where there may be a danger of electrical shock. All electrical sources that pose a hazard to workers inside the space must be locked out following the written lockout procedure for the particular confined space.


Substances entering through piping

Piping adjacent to a confined space could contain liquids or gases or other harmful substances. If these substances enter the confined space, the hazards may include:

  • Toxic gases
  • Burns from hot substances
  • Drowning
  • Being trapped, crushed, or buried


Substances must be prevented from entering the confined space through piping. This is done by ‘isolating’ the piping from the confined space. The method often involves disconnecting the piping or putting solid plates to block off the piping from the confined space. If valves are used to isolate the piping, a special double-block system must be used so that nothing can leak into the confined space. Under special circumstances, a professional engineer can certify that a valve isolation system is safe for a worker to carry out intended work in a confined space.


Poor visibility

Poor visibility increases the risk of accidents and makes it harder for a standby person to see a worker who may be in distress. If poor visibility results from inadequate lighting, the light levels should be increased (although area lighting is not always required). If activities such as sandblasting or welding result in poor visibility, appropriate ventilation may be needed to reduce harmful substances in the air. If portable lighting is used where there may be an explosive atmosphere, the lighting must be ‘explosion-proof’.


Emergency lighting, such as flashlights or battery-operated area units must be provided where necessary, so that workers can locate exits and escape


Temperature extremes

Special precautions are needed before workers enter equipment such as boilers, reaction vessels, and low-temperature systems. A qualified person must provide these procedures. Allow enough time for cooling of confined spaces that have been steam-cleaned.



Noise produced in confined spaces can be particularly harmful because of reflection off walls. Noise levels from a source inside a small confined space can be up to 10 times greater than the same source placed outdoors. If the noise levels cannot be reduced, proper hearing protection must be worn where necessary.


Risk of drowning

Confined spaces should be fully drained or dry when entered. Spaces that are not fully drained or dry may pose a risk of drowning. The risk of drowning in a vat or tank with a large amount of liquid is easily recognised. However, workers have drowned in small pools of liquid. For example, insufficient oxygen, the presence of a toxic gas, or a blow to the head can make workers unconscious. Workers who have fallen face-down into a small pool of water have drowned.


The benefit of a hierarchy of controls







Topics: hazards, Confined spaces



SAMTRAC International is the leading occupational health, safety and environment (HSE) management e-learning training course that provides students with the foundational and fundamental knowledge critical to occupational HSE management.

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