Working in Confined Spaces: A Practical Guide to Risk Assessment, Safety and Control

Confined spaces present some of the most serious and immediate risks to health and safety across construction, infrastructure, defence and industrial environments.

Understanding how to identify these spaces, recognise the hazards, and apply appropriate control measures is essential for protecting both workers and those responsible for site safety.

What is a Confined Space?

A confined space is defined as:

"Any place, including any chamber, tank, vat, silo, pit, trench, pipe, sewer, flue, well or other similar space in which, by virtue of its enclosed nature, there arises a reasonably foreseeable specified risk." - The Confined Spaces Regulations 1997

This definition applies across a wide range of industries. In construction, confined spaces may include basements, trenches and service ducts. In infrastructure projects, tunnels, culverts and shafts are common. Within naval environments, confined spaces are frequently found in ship compartments, tanks and engine rooms. In industrial settings, silos, tanks and processing vessels are typical examples.

Reduced oxygen is the most common hazard when working in confined spaces. It is also important to recognise that many incidents occur not during routine work, but during attempted rescues - 50% of injuries sustained in confined spaces involve individuals who entered to help others without proper controls.

Understanding Risk Assessments and Permits

Never enter a confined space unless authorised to do so. A Confined Space Risk Assessment is a critical first step in ensuring safe working conditions and must take into account a wide range of factors.

A confined space risk assessment should detail entry and exit points, and the size, shape and type of environment. It should also consider known residues and contaminants, oxygen levels and atmospheric test data such as flammability and toxicity. In some environments, it will also be necessary to determine whether the space is classified as a hazardous zone, such as Zone 0, Zone 1 or Zone 2.

The activities being undertaken, the materials and equipment being used, and the health and safety control measures in place must all be clearly defined. Importantly, the emergency rescue plan must form part of this assessment, as it is essential that all personnel understand what actions to take in the event of an incident.

Oxygen, flammability and toxicity tests are required when assessing the atmosphere. These measurements provide essential information about whether it is safe to enter and work within the space.

A permit-to-work is a formal check of the Safe System of Work. It authorises specific individuals to carry out defined tasks within a set timeframe and outlines the precautions required to complete the work safely. While not always mandatory, whether a permit is required should be determined by a competent person.

A Safe System of Work must assess the task, identify the hazards, define safe methods, implement those methods and monitor them throughout the work activity. This structured approach is particularly important in confined spaces, where conditions can change rapidly.

In practical terms, this process applies in many real-world scenarios. For example, in infrastructure tunnelling projects, atmospheric monitoring must be continuous due to changing ground conditions. In naval environments, confined compartments require strict permit control due to overlapping activities such as welding and coating. In industrial facilities, tank entry procedures often involve detailed permits and monitoring due to chemical residues.

The Rules of Engagement: Keeping Yourself and Others Safe

1. Wherever possible, avoid carrying out tasks in confined spaces. If entry is unavoidable, it should only take place when appropriate controls are in place.
2. Workers must be aware of the risks present within the confined space and ensure they are fully trained and capable of carrying out the task.
3. Risk assessments must have been conducted, and permits issued where required.
4. There must always be an emergency rescue plan in place, and first aid and resuscitation equipment should be readily available.
5. Standby personnel must always be present and must never leave their post. Their role is critical in maintaining communication and initiating emergency procedures if required.

These principles are particularly important in high-risk environments. In construction sites, work in service ducts or basements must never be undertaken in isolation. In infrastructure works such as manhole or sewer entry, standby personnel are essential due to the high risk of gas exposure. In naval environments, where confined spaces are often complex and interconnected, strict adherence to procedures is vital. Similarly, in industrial facilities, confined space entry procedures are often tightly controlled due to the presence of hazardous substances.

Roles and Responsibilities

Clear roles and responsibilities are essential when working in confined spaces, and everyone involved has a specific duty to ensure safety.

The supervisor should inspect the confined space and is responsible for the permit-to-work system. They should check and supervise the work being undertaken and re-inspect and re-issue permits as required.

To clarify responsibilities in practice:

Supervisor

• Responsible for the permit-to-work system
• Inspects the confined space before entry
• Supervises the work being undertaken
• Re-inspects and re-issues permits as required

The work team must be authorised to enter and should ensure that they have the correct PPE and safety equipment as specified in the risk assessment. Workers should be physically and psychologically fit to carry out the work and must monitor their own condition as well as that of their colleagues.

Work Team

• Must be authorised to enter the confined space
• Must use the correct PPE and safety equipment
• Must be physically and psychologically fit for the task
• Should monitor themselves and co-workers for signs of risk
• Must report accidents, near misses and equipment faults

Standby personnel must remain positioned close to the entry point and must never leave their post. They are responsible for preventing unauthorised entry, maintaining communication and monitoring conditions.

Standby Personnel

• Must remain at the entry point at all times
• Must never leave their post
• Prevent unauthorised access to the confined space
• Maintain constant communication with the work team
• Monitor safety equipment and environmental conditions
• Initiate emergency procedures if required, but never enter

Rescue personnel must be fully trained in rescue techniques, CPR and first aid and should take precautions to ensure their own safety.

Rescue Personnel

• Must be fully trained in rescue procedures
• Must be trained in CPR and first aid
• Must follow the emergency rescue plan
• Must ensure their own safety before undertaking any rescue

In practice, these roles are clearly defined in environments such as naval vessels, infrastructure projects and industrial facilities, where multiple teams may be working simultaneously and responsibilities must be clearly understood at all times.

What Are the Main Hazards in Confined Spaces?

Air may not move freely in confined spaces and gases can build up, creating an oxygen-deficient or toxic atmosphere. This is one of the most significant hazards and is often difficult to detect without proper monitoring equipment.

In addition to atmospheric risks, a wide range of hazards may be present depending on the environment, the activities being carried out and the materials involved. These include:

• Gas build-up and oxygen deficiency, leading to asphyxiation or suffocation
• Activities such as grinding, welding, spray coating, sandblasting and hydroblasting, which can introduce fumes, dust and heat
• Machinery-related hazards, including diesel fumes, sparks, electric shocks, collapse, slips and falling objects
• Flammable or explosive atmospheres, caused by dusts, vapours, gases, chemicals and solvents (with methane explosive between 4% and 17%)
• Respiratory hazards, including breathing difficulties, lung inflammation and occupational asthma
• Engulfment risks, where materials such as sand, flour or dust can cause suffocation
• Liquid hazards, including drowning, and exposure to corrosive or toxic substances
• Biological hazards, particularly in sewage or contaminated environments, including bacteria and infection risks
• Restricted access and egress, meaning escape routes may be limited or easily blocked
• Noise amplification, increasing the risk of hearing damage
• Extreme temperatures, causing dizziness, nausea, exhaustion, confusion or more serious effects

These hazards are encountered across all sectors. In construction, excavation and service spaces create atmospheric risks. In infrastructure, tunnels and shafts can trap gases. In naval environments, enclosed compartments amplify hazards, while in industrial settings, chemical exposure and confined processing equipment present additional challenges.

Oxygen Levels and Atmospheric Monitoring

Understanding oxygen levels is critical when working in confined spaces. Normal air contains approximately 20.9% oxygen. As levels fall, the effects on the body become increasingly severe.

At 19.5%, the heart rate begins to increase. At 16%, breathing becomes difficult. At 14%, fatigue and nausea occur. At 11%, loss of consciousness may occur.

Because these changes may happen without warning, continuous monitoring is essential. The type of monitor required will depend on the specific hazards present. Oxygen monitors are used to detect oxygen levels, while combustible gas monitors detect explosive atmospheres. Additional toxic gas monitors may detect substances such as carbon monoxide or hydrogen sulphide.

Most modern monitors are capable of measuring multiple gases at once, providing real-time information to workers.

If the air becomes contaminated or oxygen levels drop too low, alarms will sound, and workers must leave the confined space immediately.

Health and Safety Equipment for Confined Spaces

Effective equipment plays a key role in reducing risk and ensuring safe access to confined spaces.

A range of equipment is typically used, depending on the nature of the space and the hazards present:

• Access equipment, such as tripods, winches, harnesses, fall arrest systems and davit arms, used to ensure safe entry and exit into confined spaces such as shafts, silos, tanks and manholes
• Respiratory equipment, including air-filtering masks and air-supplying systems; filtering masks remove contaminants, while air-fed systems provide a clean air source where contamination levels are too high
• Monitoring equipment, including oxygen monitors, combustible gas (LEL) monitors and toxic gas detectors (such as CO and H₂S), often combined in multi-gas devices
• Ventilation systems, used to introduce clean air into the space or extract contaminated air, depending on the environment and hazard type

Ventilation is particularly important, as many confined space hazards can be reduced by improving air movement. In some cases, positive pressure ventilation is used to push clean air into the space, while in others extraction may be required to remove contaminants at source.

Every project is different and presents its own challenges. In construction and infrastructure, ventilation is often used to manage dust and fumes. In naval environments, it is critical due to tightly enclosed compartments. In industrial facilities, extraction systems are often essential for managing chemical vapours and maintaining safe working conditions.

Conclusion

Working in confined spaces presents a wide range of risks that require careful planning, assessment and control. From oxygen deficiency and toxic atmospheres to physical hazards and emergency response challenges, each aspect must be addressed through a structured approach.

The principles set out in this guide - risk assessment, permit systems, defined roles and appropriate equipment - form the foundation of safe working practices across construction, infrastructure, naval defence and industrial sectors.

By understanding the hazards and implementing effective controls, organisations can significantly reduce the risks associated with confined space work, protecting both workers and operations.

RVT offers expert guidance and solutions to help identify risks and implement effective confined space safety measures across all project types. Speak with an expert about how we can help keep your next confined space project safe and compliant >