Hidden dangers

22 February 2022

The risk to the safety of people entering and working in confined spaces include flammable and explosive risk, low levels of oxygen and a range of toxic risks from gaseous toxins to solvent build ups. Here, Georgia Pratt looks at the risks and gas detection choices.

HUNDREDS OF people die each year from the effects of accidents in a confined space. A recent report shows 280 fatalities in the USA from inhaling harmful substances in oil storage tanks, mines, caves, tunnels, sewers, manholes, drains, tanker truck interiors and manure pits.

Many of those are unnecessary fatalities, which would not have happened with appropriate gas detection procedures in place. A confined space is a location that is enclosed although not always entirely, and where significant injury can occur from hazardous substances or conditions within the space or nearby such as a lack of oxygen. As they are so dangerous, it must be noted that any entry to confined spaces must be the only and final option to carry out work - Confined Spaces Regulations apply. There is an approved Code of Practice, and regulations and guidance are available for employees that work in Confined Spaces, those who employ or train such people and those who represent them.

Confined space identification

Most confined spaces are easy to identify although, identification is sometimes required as a Confined Space may not necessarily be an enclosed on all sides – some, such as vats, silos and ships’ hold, may have open tops or sides. Nor are confined spaces always small and/or difficult to work in space – some, like grain silos and ships’ holds, can be very large. They may not be that difficult to get in or out of – some have several entrances/exits, others have quite large openings or are apparently easy to escape from. Most are places where people do not regularly work – but as with all things, some confined spaces are used regularly by people in the course of their work. 

There may be instances where a space itself may not be defined as a confined space, however, while work is ongoing, and until the level of oxygen recovers, it is classified as a confined space. Example scenarios are welding that would consume some of the available breathable oxygen, a spray booth during paint spraying; using chemicals for cleaning purposes which can add volatile organic compounds (VOCs) or acidic gases, or an area subjected to significant rust which has reduced available oxygen to dangerous levels.

Testing/ monitoring the atmosphere

Prior to entry, the atmosphere within a confined space should be tested to check the oxygen concentration and for the presence of hazardous gas, fume or vapour. Testing should be carried out where knowledge of the confined space indicates that the atmosphere might be contaminated or to any extent unsafe to breathe, or where any doubt exists as to the condition of the atmosphere. Testing should also be carried out if the atmosphere is having been previously contaminated and was ventilated consequently.

The choice of monitoring and detecting equipment will depend on the circumstances and knowledge of possible contaminants and you may need to take advice from a competent person when deciding on the type that best suits the situation. Monitoring equipment should be in good working order. Testing and calibration may be included in daily operator checks (a response check) where identified as necessary in accordance with our specification. Where there is a potential risk of flammable or explosive atmospheres, equipment specifically designed to measure for these will be required and certified Intrinsically Safe. All such monitoring equipment should be specifically suited for use in potentially flammable or explosive atmospheres. If you are unsure what the zonal flammable gas risk is that your Flammable gas monitors must be certified to work in, then we recommend use of a zone zero flammable gas monitor or multi-gas monitor. Note: it is unsafe and, in some circumstances, illegal to use a monitor designed for use in the wrong zone level. To ensure you have safely got around this problem we recommend use of a zone zero confined space entry portable. There are some pellistors and some IR sensors that are certified for zone zero use, and all MPS sensors are too. It is necessary to ensure the product is calibrated for the different gases or vapours which the risk assessment has identified could be present and these may need alternative calibrations for different confined spaces. This is most important for IR detectors which have a wide range of responsivities for the different gas types. Pellistors have a slightly better consistency – usually about 2:1 but the MPS is a universal calibration, hence that is what we recommend.

Testing should be carried out by people who are competent in the practice and aware of the existing standards for the relevant airborne contaminates being measured and are also instructed and trained in the risks involved in carrying out such testing in a confined space. Those carrying out the testing should also be capable of interpreting the results and taking any necessary action. Records should be kept of the results and findings ensuring that readings are taken in the following order: oxygen, flammable and then toxics. The atmosphere in a confined space can often be tested from the outside, without the need for entry, by drawing samples through a long probe. Where flexible sample tubing is used, ensure that it does not draw water or is not impeded by kinks, blockages, or blocked or restricted nozzles, in-line filters can help with this. During such pre-entry checks, it is important to ensure the temperature of the tube or portable product is above the flash point of any solvent present along its whole length. Failure to do so will result in an unsafe condition where you think there is less gas present than there really is.

Importance of CSE training

Whatever device you are using for gas detection within confined spaces it is crucial to properly train your staff to ensure their ongoing safety, and the safety of the environment. Workers need to be fully aware of the dangers they may encounter and understand any hazards in places where permits are required. Crowcon is passionate about the importance of training and has held many events with safety education at the fore. People attempting to rescue a confined space worker make up 60% of confined space fatalities. These rescuers are frequently untrained and unaware of the hazards. This shocking statistic illuminates the increased need for rigorous education around confined space entry to avoid accidents and fatalities. It is also helpful to point out that CSE training should be undertaken for all staff, not just those routinely involved in confined space working. Training should incorporate the properties of different gases and how they behave differently depending on the space they are within, specifically in confined spaces. Operators should understand that different sensor technologies are optimal under different circumstances, and how these feeds into the design and selection of the most appropriate gas detection equipment. Best CSE practice should also be a point of focus for training packages, as well as understanding the broader demands of mounting a safe and effective rescue.

Gas detection

Flammable gas risk

Natural gas or liquefied petroleum gas (the most common examples) are highly explosive when mixed with air, as well-being invisible to the naked eye and undetectable by our senses. Sometimes a strong odourant is added to aid human detection, but most of the time there is no deliberate odourant present meaning the person entering the confined space must rely on their gas detection instrumentation. 

Sometimes flammable gas can build up during a work cycle either due to general airflow through the work area or leaks from equipment being used, so continuous monitoring is always safer than just relying on an entry test. Changes in pressure and even minor changes in temperature can sometimes be enough to cause pools of solvent to enter the air, and when due to sun on the outside of a tank with a large internal surface area, unexpected build-up of solvents can be extremely fast.

Oxygen depletion or inertisation risk

If there is normally no oxygen present in the confined space or it has undergone a purging process, then the low oxygen risk is obvious, however if there was breathable air but the confined space has been sealed, and there are metal surfaces in the confined space, then oxidation of the metals often leads to lower oxygen concentration in the atmosphere. Iron, for example, continues to rust down to 6% oxygen present, well below the safe level for breathing whilst doing work.

Toxic gas risk

Your risk assessment should inform you of specific chemical risk, for example if the container was previously used to store chemicals prior to your work commencing. However, some toxic gases can build up from biological activity, and these can be just as dangerous. For example, hydrogen sulphide is a by-product of some bacteria respiring and this can build up to a high concentration in an enclosed space. Furthermore, a pre-entry check does not always find it because it can be trapped beneath thick films of oil just waiting for you to break the film by stepping on it, releasing the hydrogen sulphide into the rest of the confined space.

Georgina Pratt is marketing executive at Crowcon. For more information, visit