Silent Threat: Understanding and Mitigating Gas Hazards in Confined Spaces

Confined spaces, while vital for numerous industrial processes, harbour hidden perils that demand vigilance and preparedness from workers. Despite stringent regulations and technological advances, accidents and fatalities related to permit-required confined space entry persist. According to Who are IOSH, 35 deaths in the UK are attributed to confined space But what holds the key to mitigating these risks in confined spaces? One crucial aspect is managing gas hazards effectively.

The Common Culprits: Confined Space Gas Levels

Awareness of atmospheric condition within a confined space is paramount to worker safety. While oxygen and combustible gases are primary concerns, carbon monoxide and hydrogen sulphide also pose significant threats. Workers must be well-informed and equipped to guard against these dangers.


HSE specifies “safe levels” for oxygen in confined space. The minimum safe oxygen level is set at 19.5%, while the maximum safe oxygen level stands at 23.5%. Insufficient oxygen is the leading cause of gas-related fatalities in confined spaces.

Accurate and frequent oxygen level measurements are essential. Workers should assess oxygen levels before entering a confined space and continuously monitor them throughout their work. Oxygen levels above 23.5% can lead to combustible gas ignition, while low oxygen levels impair judgment and coordination, causing symptoms like nausea, vomiting, and loss of consciousness.

When oxygen levels are low, it usually indicates that another gas is displacing oxygen. Identifying the gas responsible for displacement is crucial.

Combustible Gases

Gas ignition in confined spaces hinges on oxygen levels. Monitoring a confined space’s oxygen level provides insights into the concentration of combustible gases. Two significant thresholds are considered:

  • Lower Explosive Limit (LEL): This is the lowest concentration of a gas in the air capable of combustion when exposed to an ignition source.
  • Upper Explosive Limit (UEL): This is the highest gas concentration in the air capable of combustion with an ignition source.

For each gas, LEL and UEL values differ. For example, methane has an LEL of 5% vol and a UEL of 15% vol. Methane can ignite when its concentration is at or above 5%, but below 15%. Gas detectors display the gas presence as a percentage of its LEL. An atmosphere free of methane would show 0% LEL, while an atmosphere with 5% methane would display 100% LEL.

Vigilant monitoring of combustible gas levels is essential as they can change over time. Ventilation can dilute gas concentrations, potentially shifting them into the combustible range.

Carbon Monoxide and Hydrogen Sulphide

Multi-gas monitors often include carbon monoxide and hydrogen sulphide, although these gases aren’t always prevalent in confined spaces. Carbon monoxide typically results from incomplete combustion, making it less common unless machinery operates within the space.

While both gases are highly toxic, it’s crucial for confined space workers to understand the specific gas hazards they might encounter. Knowledge of the Lower Explosive Limit/Upper Explosive Limit (LEL/UEL) and the HSE’s permissible exposure limit (PEL) for each gas is essential.

Hydrogen sulphide has a PEL of 20 parts per million (PPM) and an LEL of 4.0%, while carbon monoxide has a PEL of 50 PPM and an LEL of 12.5%.

The Role of Direct-Reading Portable Gas Monitors in Confined Spaces

Gas hazards in confined spaces are unpredictable, making continuous monitoring with direct-reading portable gas monitors essential. These devices not only indicate whether the atmosphere is safe but also provide precise information on safety levels. They empower workers to make informed decisions regarding entry and ongoing work.

While safety regulations require pre-entry testing, workers are typically only required to test upon exiting the confined space, Continuous monitoring through direct-reading personal gas monitors enhances safety by providing real-time data. This proactive approach helps workers recognise potential dangers promptly

Unlike alarm-only monitors with predefined thresholds, direct-reading gas monitors offer workers detailed gas level information, improving their safety awareness.


When it comes to confined spaces, one of the main dangers lie in the invisible, odourless gases that can silently jeopardise worker safety. Understanding and effectively managing gas hazards is not an option but a necessity. By adopting continuous monitoring through direct-reading portable gas monitors and provide workers with the knowledge, organisations can ensure safer confined space operations, reducing the risk of accidents and fatalities. For the safety of your workforce and compliance with regulations, make gas hazard awareness a top priority in your confined space management strategy.

For more information on confined space safety and emergency rescue, contact Triton Risk today.