What Gases Should be Monitored in Confined Spaces?

Confined spaces present some of the highest-risk environments across industries such as utilities, manufacturing, construction, and oil & gas. Limited ventilation, restricted access, and the potential for hazardous atmospheres make effective gas monitoring not just a regulatory requirement, but a critical safeguard for worker safety.

For health & safety managers and operations leaders, understanding what gases should be monitored in confined spaces is essential to building a robust safety strategy, selecting the right equipment, and ensuring compliance with regulations.

This guide outlines the key gases to monitor and how to approach detection in real-world industrial environments.

What is a Confined Space?

A confined space is typically defined as an area that:

• Is fully or partially enclosed
• Has limited ventilation
• Is not designed for continuous occupancy
• Has limited entry and exit points
• Has a foreseeable risk of hazardous atmospheres.

Examples include:

• Tanks and vessels
• Sewers and manholes
• Silos and storage bins
• Utility vaults and pipelines
• Ship holds
• Pipelines and culverts
• Open topped pits or trenches
• Manure pits.

These environments can quickly become dangerous without proper atmospheric monitoring. 

Why Gas Monitoring is Critical in Confined Spaces

The primary risks in confined spaces come from:

• Oxygen deficiency or enrichment
• Toxic gas exposure
• Flammable or explosive atmospheres.

These risks are often invisible and can escalate rapidly. Pre-entry testing and continuous gas monitoring ensures:

• Early detection of hazardous conditions
• Immediate alerts for workers
• Compliance with safety regulations.

The Four Main Gas Hazards to Monitor

In most confined space applications, a four-gas monitoring approach is considered the baseline.

 1. Oxygen (O₂) 

Oxygen levels are critical for life and combustion.

• Safe range: 19.5% – 23.5%
• Below 19.5%: Risk of oxygen deficiency (asphyxiation)
• Above 23.5%: Increased fire and explosion risk.

Oxygen levels can be affected by:

• Displacement by other gases
• Chemical reactions
• Poor ventilation.

Monitoring oxygen is always a priority in confined space entry.

2. Carbon Monoxide (CO)

A highly toxic, colourless, and odourless gas that can accumulate rapidly in confined spaces.

• Produced by incomplete combustion
• Common in environments with engines or fuel-burning equipment.

Risks:

• Binds to haemoglobin, reducing oxygen delivery in the body.
• Can cause sudden dizziness, unconsciousness, and death.

3. Hydrogen Sulphide (H₂S)

A highly flammable and toxic gas often found in:

• Wastewater treatment
• Oil & gas operations
• Decomposing organic matter

Characteristics:

• Smells like rotten eggs
• At low concentrations can cause irritation to the eyes and respiratory tract
• Heavier than air and can travel along the ground or accumulate in low-lying areasCan deaden the sense of smell at higher levels.

Risks:

• Rapidly toxic, even at low concentrations
• Can cause respiratory failure.

4. Flammable Gases (LEL – Lower Explosive Limit)

Flammable gases such as methane, hydrogen, acetylene, or propane are monitored as a percentage of their Lower Explosive Limit (LEL).

• 0–100% LEL scale
• Typically, alarms trigger around 10–20% LEL
• Common sources include sewer gas, decaying matter, fuel leaks or industrial work such as welding.

Risks:

• Explosion or fire if ignition occurs
• Often present in industrial and underground environments.

Additional Gases to Consider

While the four-gas approach covers most scenarios, many industries require monitoring for additional hazards.

Carbon Dioxide (CO₂)

• Common in confined or poorly ventilated areas
• Can displace oxygen and cause drowsiness or unconsciousness.

Volatile Organic Compounds (VOCs)

• Found in chemicals, fuels, and solvents
• Require PID (Photoionisation Detector) technology.

Ammonia (NH₃)

• Used in refrigeration and industrial processes
• Toxic and corrosive.

Chlorine (Cl₂)

• Used in water treatment
• Highly toxic, even at low levels.

Sulphur Dioxide (SO₂)

• Found in combustion processes and industrial emissions.

The specific gases to monitor should always be based on a site survey conducted by a competent safety supplier and a risk assessment of the environment and process.

Real-World Applications by Industry

Utilities & Water Treatment

• H₂S, methane, and oxygen deficiency are common risks
• Confined spaces include sewers, tanks, and pumping stations.

Oil & Gas

• Flammable gases, H₂S, and VOCs are key concernsHigh-risk environments with potential for explosive atmospheres.

Manufacturing

• CO, VOCs, and process-specific gases
• Confined spaces may include vessels, reactors, and storage areas.

Waste & Recycling

• Methane and hydrogen sulphide from decomposition
• Oxygen displacement risks in enclosed areas.

Each industry requires a tailored gas detection approach aligned with operational risks.

Key Decision Criteria

When determining the right approach:

• Mobility requirements → Portable systems
• Continuous monitoring needs → Fixed systems
• Number of personnel potentially working in the area
• Frequency of confined space entry
• Regulatory requirements and risk assessments.

In most cases, portable gas detectors are non-negotiable for confined space work, often supported by fixed systems at the facility level.

Calibration, Testing, and Compliance

Even the most advanced gas detector is only effective if it is properly maintained.

Best Practices:

• Perform bump tests before each use
• Schedule regular calibration according to manufacturer recommendations
• Maintain service records for compliance and audit purposes
• Replace sensors as required.

Failure to maintain equipment can result in:

• False readings
• Undetected hazards
• Increased liability.

Working with a specialist provider ensures your equipment remains accurate and compliant.

Choosing the Right Gas Detection Strategy

For decision-makers, selecting the right solution goes beyond device specifications.

Consider:

• Site-specific risk assessments
• Type and number of gases present
• Environmental conditions
• Workforce size and exposure risk
• Integration with safety procedures.

A tailored approach ensures both safety and operational efficiency.

How Shawcity Supports Confined Space Safety

Shawcity works with organisations across the UK and Ireland to deliver complete gas detection solutions, including:

• Portable multi-gas detectors for confined space entry
• Fixed gas detection systems for continuous monitoring
• Calibration, servicing and maintenance support
• Technical guidance tailored to your industry.

Whether you require short-term portable hires or a fully integrated fixed detection system, having expert support ensures you deploy the right solution for your environment.

Conclusion

Understanding what gases should be monitored in confined spaces is fundamental to protecting workers and maintaining compliance.

While oxygen, flammable gases, carbon monoxide, and hydrogen sulphide form the core monitoring requirements, many environments demand a more tailored approach. Combining the right detection technology with proper maintenance and expert guidance is key to reducing risk.