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In the year 2019/20 there were 201 non-fatal incidents relating to exposure to harmful gases. More concerning is the fact that there were eight deaths, making it clear that gases in occupational settings are a serious hazard which need to be managed. It is clear that some gases are dangerous, but we need to understand how to detect these gases to protect workforces and people who may interact with services or products which use or generate various gases.
In the construction industry, gases pose a variety of risks, mostly due to their source. These gases could be naturally occurring, which are found trapped in the ground, or they could be used for certain processes or certain trades to carry out tasks effectively. Understanding where gases come from, and how they can be detected, is key for the construction industry to protect its workers. This becomes especially relevant where there is work involving confined spaces, enclosed areas or in specific areas, as there is no quick means of escape for workers.
Encountering gases in the construction industry is something which has become part and parcel of everyday life on site, especially where working in confined spaces, which means that workers can easily become desensitised to the controls which are put into place. It is therefore more important than ever to ensure that controls are as easy to use as possible. Long gone are the days of keeping a canary in a cage, now there are a whole plethora of options which construction companies can make use of to detect and deal with gas.
These gases need to be detected at the earliest possible opportunity to protect workers. Below we explore the ways that gas detection takes place in the construction industry.
The Pre-Construction Phase
Gas detection should not be left until the construction phase before it is thought about. As part of the Construction (Design and Management) Regulations 2015, the Principal Designer (the appointed duty holder in control of the pre-construction phase) must give consideration to hazards which will become apparent before, during, and after the construction phase. This is a key part of the process of construction work and is a part which is largely overlooked or given the due care and attention it needs.
A good Principal Designer will be able to apply the requirements of CDM15 (as well as other health and safety legislation) to a project. Specifically in relation to gas detection, the Principal Designer must take reasonable steps to identify these gases early on and pass this information on to the Principal Contractor. This flow of information forms a vital link in the safe management of the construction project, and this pre-construction information will help to inform the Principal Contractor of the hazards that their workforce are likely to encounter, enabling them to properly control these hazards and record this in their Construction Phase Plan.
Ground Gas Detection
Working on or near to brownfield sites, mines, or land where degradable materials have been disposed of, can be an indicator of increased levels of methane and carbon dioxide in the ground. Where an initial survey detects high levels of these gases, bore holes will be dug and backfilled with granular material. Gas taps are installed and then sealed with bentonite (a kind of absorbent clay formed by a breakdown of volcanic ash) which allow for ground monitoring. It is from these taps that the gas monitors can then be used to detect the presence of gases. Readings must be taken carefully however, as they can be affected by weather conditions. The monitoring should be carried out during periods of low and falling barometric pressure. This information can then help to inform the risk posed to construction activity on those sites. Where potential gas is identified, there should be a minimum of three wells/taps put into place, which should have three separate visits for readings. Where these readings record gases in any kind of quantity, further monitoring should be carried out. Deep excavation work, or work in other confined spaces may need to be modified to ensure the safety of those involved. This information should then be taken by the Principal Designer and work processes should be reviewed to ensure their safety. This information, along with the ground gas readings, should then be passed on to the Principal Contractor.
Buried Services
Sticking with the pre-construction phase, as part of the planning works for a project, the location of buried services must be considered. Groundworkers need to be given the location of the service as well as its depth. This information is essential to understanding where the services are in order to put a plan into place to carry out the work safely. As with the above, this information must be passed on to the Principal Contractor to allow them to plan their work safely and put the necessary steps into place to detect gases should they be released. This is particularly important where any amount of excavation is required, but should also be considered for all construction activity as a matter of course.
Any significant weight on top of a buried service could cause it to rupture. Furthermore, if a building is placed on top of a buried service and the service ruptures and requires maintenance, gaining access to the service becomes complicated and may require parts of the structure to be removed. From here, the planning of the project must consider these services (including their location and depth) or make plans to divert them. The use of heavy plant, demolition, or earth moving could cause these pipes to become damaged, which may increase the risk of exposure.
The above are steps to be taken before anyone even breaks ground on site and allow for the flow of communication, giving the Principal Contractor the best possible chance of identifying gases and keeping workers safe.
The Construction Phase
With all planning completed for the work, the focus now shifts to the Principal Contractor and the steps that they need to take to identify gases. From the work that the Principal Designer has undertaken, and from the pre-construction information that they have provided, the Principal Contractor should have a good idea where they are likely to encounter gases. This doesn’t remove the need for caution however, and there are certain construction activities which are likely to require some form of monitoring to detect gases to keep workers safe as they undertake the construction project.
Below we look at some construction activity where the risk posed by gases is increased. This may mean that workers are more likely to be exposed to potentially harmful levels of gases, or that the conditions make the gas more harmful.
Underground Construction Activity
Underground construction sites (for example mining or tunnel boring) are innately at risk as it is nearly impossible to provide immediate and effective ventilation. This means that any gases that are released will easily build up and could become toxic.
The Working Environment Fund of the Confederation of Norwegian Business and Industry undertook a study which contains a lot of detail about underground working2. Interestingly, they raise the point that the workers are most at risk from gases produced by the machinery being used and the processes being undertaken, but that there are other naturally occurring gases which need consideration too. Below is a list of the gases that workers may be exposed to:
• Nitrogen Monoxide (NO)
• Nitrogen Dioxide (NO2)
• Carbon Monoxide (CO)
• Carbon Dioxide (CO2)
• Hydrocarbons (HC)
• Strata Gases (such as methane, CH4, and hydrogen sulphide, H2S)
It goes on to detail that these gases come from the use of diesel plant, cutting, welding and burning. The lack of ventilation allows these gases to build up and potentially harmful levels of these gases can cause nausea, vomiting, dizziness, headaches, loss of consciousness or asphyxiation. Even at the lower end of the scale, due to the nature of the work being undertaken, this can pose a serious risk to life.
“workers are most at risk from gases produced by the machinery being used”
There are many controls which are put into place, but the one that is of interest to this article is monitoring. This process is regularly undertaken and is essential in underground construction environments as it will give an indication of the effectiveness of controls, such as engineered ventilation. There are different types of sampling which can be undertaken:
Personal Sampling: A worker is fitted with a pump which draws a known volume of air through specific filters. These filters can then be sent to a laboratory for analysis and the substances and their density can be measured. This is a good way to measure specific areas of the site and how they may be affected by differing work activity, but the draw back here is that they are not fast. The sample needs to be sent off and analysed, so this method should be used alongside other methods which may give an immediate indication of a build-up of a potentially harmful gas.
Static Air Sampling: These stations are set up in the same area and continually measure the air quality. It is important that they look for the most harmful and odourless gases (such as nitrogen monoxide – NO). Generally speaking, these are the first port of call as they give an immediate reading, but they do have their limitations. These stations need to be properly calibrated and checked as they can be affected by other gases such as hydrogen, so the setup is very important to ensure that accurate results are obtained.
Pumped Samples: These are the most accurate methods of obtaining air samples. They work in the same way as a personal sample (see above) but are static in one location and measure a known volume of air. Again, they suffer from the same limitations as the personal sampling – the laboratory results can take some time to come through which means that although they are accurate, they are not as regularly used as static air sampling.
Regardless of the method used, the results are compared to EH403 to give an indication of what action needs to be taken. This may include a need for more engineered ventilation, Respiratory Protective Equipment (RPE) or work processes to help reduce these harmful gases.
Excavation
Excavations are a necessary but innately hazardous part of construction work. Presenting a whole host of hazards from collapse to entrapment, to falls from height, they are a complex hazard to work with and control.
From a gas detection standpoint, there are a number of things to consider. Before any ground is broken it is essential that the pre-construction information is there and that all the available information is to hand. This will allow the groundworkers who will be digging the excavation (regardless of its purpose) to do so as safely as possible.
“gases like H2S are heavier than air, if they seep into an excavation they will settle at the bottom of the trench”
There are no hard and fast rules regarding gas detection in an excavation, and there is certainly some cross over here with confined spaces. As a rule of thumb, if the excavation requires any worker to have their head completely within the excavation (i.e. beneath the ground line), it would be sensible to carry out air monitoring to ensure that there are no harmful gases.
Below are some steps which should be undertaken by the Principal Contractor (or their employee) to assist in the detection of gases when excavating:
Ground Surveys: As detailed above, the location of buried services must be given to the Principal Contractor, however, these are not enough on their own. As part of a safe system of work, the Principal Contractor should ensure that ground surveys are carried out. There are a number of methods that can be used, but the safest way to start this is by use of a Cable Avoidance Tool (CAT). While this only detects the location of live electrical services, it will give an indication of the accuracy of the information that the Principal Contractor has. Where this appears to be inaccurate, trial holes could be dug to help locate the services. While this gives no quantifiable measurement of the volumes of potential harmful gases, a lack of the service could give an indication that it is unlikely to be a hazard. Although, if the pre-construction information indicates ground gases are present other controls should still be used.
Ground / Soil Reports (WAC tests): Although these are typically used to assist companies in the removal and tipping of spoil from an excavation, the ground report can provide really important information which helps to identify the presence (or rather potential presence) of gas in an excavation. The soil report should look for substances like chalk and limestone. When these substances come into contact with groundwater, they can release carbon dioxide which is toxic if breathed in.
Gas Detectors: Harmful gases like hydrogen sulphide are heavier than air, this means that if they seep into an excavation they will settle at the bottom of the trench. Gas detectors are the most effective way to monitor for these gases. However, the gas detectors must be properly calibrated and the person using the monitor must understand how to operate it effectively and understand the output of the alarm. While they are all much the same, always refer to the manufacturers guidance. You should supplement this with bump testing to help ensure the accuracy of the monitor.
Confined Spaces
While there is a lot of crossover between a confined space and an excavation, there is an argument that a confined space could be enclosed on all sides (it may also be referred to as an “enclosed” space). This presents a new challenge as, unlike an excavation, it is much harder to escape a confined space when the presence of a harmful gas is detected.
Gas monitors are the best way to get an accurate reading but these should be used in a safe and controlled way. Rather than just sending someone in with a gas monitor, the following steps should be carried out:
Prepare the equipment: ensure that the equipment has been calibrated, is working properly and has been bump tested. This will ensure the most accurate readings.
Prepare the people: Ensure that workers are trained to enter the confined space and that they understand how to use the gas monitors properly.
Carry out sampling without entry: Before entering, use a tube to sample the air within the confined space. While the reading won’t be as accurate as having the gas monitor in the confined space, it will give an indication of the levels of harmful gases.
Testing methods: When testing, it is important that the head of the person with the initial monitor is not above the monitor. This may mean that the person is breathing in harmful gases before the monitor detects them. Always lead with the monitor.
Summary
The above are some over-arching themes which can be considered, and some methods of gas detection in the construction industry. There is plenty of additional guidance from the HSE, as well as service providers, about working with gases. They include best practice and ways to ensure the safety of the workforce.
The detection of a hazardous gas is only the start of the process. Once the presence of the gas has been identified, controls need to be put into place to protect the workforce, and procedures to safely evacuate a confined/enclosed space also need to be considered. While eight deaths don’t sound like a lot, it is still eight deaths too many. Gas detection is the first step on the road to protecting people from death, or acute and chronic illnesses due to breathing in harmful gases.
References
1. https://www.hse.gov.uk/statistics/tables/ridgas.xlsx
2. https://erj.ersjournals.com/content/17/3/416.figures-only
3. https://www.hse.gov.uk/pubns/books/eh40.htm
Author
Ashley is one of the Directors for Landmark Training & Consultancy Services, a family run business based in Suffolk. Specialising in the construction industry, Ashley provides advice to construction companies who undertake projects across the country. Construction isn’t the only field that Ashley operates in however, clients include geology companies, plant hire companies, and even a professional wrestling promotion to name a few others. His unique and refreshing approach to health and safety has allowed his client base to work effectively, safe in the knowledge that they have a competent and hands-on consultant ready to assist them.
Having worked in health and safety for the past eight years, Ashley is a Chartered member of the Institution of Occupational Safety and Health, as well as being a member of the Institute of Industrial Accident Investigators, and has a wealth of experience of working within health and safety at various levels starting as a representative of employees’ safety, and moving into a health, safety and training directorship.
Landmark Training and Consultancy Services is always open to taking new clients on.
Please call 01473 744 404 or email [email protected]
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