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What is a confined space: the complete definition + dangers

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Large industrial installations or machines require regular maintenance, cleaning or inspections. Common examples of applications are; sewer cleaning or inspection, maintenance of silos and storage tanks or inspection of cargo or fuel tanks in ships.

Often you need to enter a tight space, where there is little air circulation, narrow passages and additional hazards. This is called an confined space.

Accidents occur regularly because employees do not correctly assess the risks in such spaces. Strangely enough, nowhere is there a literal legislation on how to safely enter a confined space, merely instructions.

In Article 53, the ARAB (General Regulations for Occupational Health and Safety) refers to ‘Work in places where hazardous gases may be present’ and even gives examples, but the word ‘confined space’ do not appear anywhere.

That's why we will give you the most precise definition of a confined space.  Then, we will discuss the 8 most common risks when entering.

What is a confined space?

A confined space is not intented for continuous occupancy by employees, has an enclosed character, a potential hazardous atmosphere and limited access.

That's a whole lot.

Let us take a closer look at these points.

Not intended for continuous occupancy

You should avoid a confined space as much as possible.

When your employees need to enter a confined space, this is considered an exception to the rule, as you are actually entering an unusual work area. Place adequate signage and warning symbols to ensure that no employees enter the space without permission.

If machines or other installations require maintenance or inspection, keep the time spent and the number of employees in the space as low as possible.

Enclosed characteristics

This is a tricky one.
When is a space enclosed and when not? Often, the differences are not immediately recognizable.

An open trench or pit, for example, becomes a confined space when there is too little air circulation. Gases that are heavier than air (for example carbon dioxide or sulphur dioxide) are able to accumulate at ground level.

Even a partially walled space can technically be a confined space. There are cases where a nitrogen leak on one side of an open tent has resulted in fatalities.

Limited or difficult access

Generally, you recognize a confined space by its opening or location. Usually, openings are very narrow (45 cm diameter) and it is difficult to pass through them.

Getting tools through the opening is at least as problematic, and life-saving devices (e.g. fall protection) are not easy to use.

Then there are also larger openings, such as the cargo spaces of a ship. They create different problems. You need ladders or hoists to enter the space, and in emergencies, it is difficult to escape from these spaces.

A hazardous atmosphere

Asphyxiation, fire or explosion are the most common causes of accidents in confined spaces.

Actually, you should look at it this way: the dangers within the space determine whether it is enclosed, not just the space itself.

Measuring is knowledge, so continue to measure throughout the intervention. From the time you enter until you leave.

Usually, several risks occur simultaneously. Not only from hazardous gases, but the risk of injury from impact or moving machine parts are also constant hazards.

8 dangers when enetering a confined space

#1: Asphyxiation

By far the biggest risk in a confined space is asphyxiation. Normal air consists of 78% nitrogen gas, 21% oxygen gas and 1% argon. When oxygen concentration falls below 18%, there is the risk of asphyxiation.

Moreover, your senses do not register this risk. It just happens to you. You lose consciousness and are no longer able to perceive the dangers.

An oxygen deficiency is caused by insufficient ventilation. The oxygen level is reduced through chemical or biological reactions.


  • Gases released from paint drying or from poorly sealed pipes displace the oxygen.
  • Some welding processes also involve hazardous gases. If they cannot be discharged, they become a risk.

The presence of inert gases such as nitrogen or carbon dioxide also cause oxygen deficiency. In some cases, this can prevent a fire or explosion hazard (because the amount of air should also not be too high).

#2: Fire or explosion

If the oxygen concentration is higher than 21%, there is a risk of fire or explosion. In a confined space with 30% oxygen, for example, one spark is enough to burn cotton overalls within a minute.

One of the most common causes of an excessive concentration of oxygen is incorrect use of welding or cutting equipment.

Flammable and explosive substances that can be present in confined spaces include paints, solvents, (residues of) materials and cleaning rags, gas or oxygen escaping from gas and oxygen cylinders that have not been properly sealed, or from the leftover contents in the confined space.

Discharge of static electricity or using tools that are not antistatic will cause ignition.

#3: Poisoning

Symptoms of poisoning occur when you are exposed to hazardous substances. Health effects due to exposure often occur immediately, but sometimes the effects only become noticeable over a longer period of time.

These substances infiltrate your body through your lungs, mouth or skin.

Detergents, paintwork, electrical welding or work on contaminated soil can release dangerous substances or gases.

#4: Elektrocution


Electrocution occurs when you touch objects carrying electric voltage. When power tools or cables are damaged, for example from crushing, the risk of electrocution is high.

And when damaged cables make contact with the metal parts or walls of a confined space, those also become live.

#5: Heat stress

Heat stress occurs when you are no longer able to regulate your body temperature due to high temperatures and humidity. The causes are various: radiant heat, humidity or protective clothing (gas-tight suits).

Your body heats up, causing 'heat disorders' such as heat exhaustion or cramp.

If you work in a hot space (e.g. engine room), it is important to drink plenty of water to compensate for perspiration (every 15 to 20 minutes).

#6: Trapped

As we explained earlier, the opening of a confined space is often narrow.

It is possible to become trapped, due to:

  • Narrower entrances and exits
  • Limited space in the work area
  • Transport of materials
  • Use of special PPE, such as gas-tight overalls that take up a lot of space, but are crucial.

#7: Noise

Equipment such as compressed air tools or welding machines generate a lot of noise.

Because of the high resonance (sound reverberation) in a confined space, the noise quickly exceeds 80 decibels.

Good hearing protection is therefore essential when entering a confined space.

#8: Tripping and falling

You may need to work at height because access to the confined space is also at height. Racks or ladders are regularly used inside the space.

Wearing fall protection and checking for handrails is crucial when entering a confined space.

When working on different levels, tools, loose objects or debris can fall. Be sure to wear a helmet to protect yourself from falling objects.


As you can see, a confined space is a very risky and unstable environment.

Apart from these 8 dangers, there are undoubtedly other risks not mentioned here. During your risk analysis, you must pay careful attention and look for the best solution.

Discuss everything with your colleagues, identify the potential hazards and practise thoroughly! Every confined space is different, and unfortunately there is not one single solution for accessing all these different confined spaces.


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