Gas detection is an important part of health and safety. This article discusses the importance of gas detection in relation to identifying the nature and scale of any gas release, and how this applies to chemical incidents. It will look at the various hazards of gases such as asphyxiation, flammability and toxicity; and explain, through real life examples, why gas detection is necessary to avoid incidents or, at the very least, contain them.

Hazardous nature of gases

Firstly, it is important to understand which gases need to be considered during an incident and when they become dangerous – not all gases and vapours are equally hazardous. For instance, the air around us is composed of almost 80% nitrogen, which we breathe in every day, whereas we are always cautious around liquid nitrogen – so why the discrepancy between the two states? First, we should look at the chemical classification of these materials.

The Globally Harmonised System (GHS) of Classification and Labelling of Chemicals provides a great level of consistency in classifying chemical hazards across the world. In the European Union, GHS has been adopted under European Regulation (EC) No 1272/2008 on classification, labelling and packaging of substances and mixtures (the CLP Regulation). NCEC’s emergency responders would tell you that, under CLP, the nitrogen present in air is not hazardous. Liquid nitrogen, on the other hand, which is stored at -196°C, could be dangerous.

There have been instances where the dangers of liquid nitrogen were not understood correctly, leading to people suffering serious injuries. For example, NCEC often sees cases where bar staff have been making cocktails using liquid nitrogen and suffering injuries because they are unaware of the hazards of liquid nitrogen kept at extremely low temperatures.

There is a visible cloud when liquid nitrogen is poured out, which is actually the water vapour in the surrounding air freezing and not the nitrogen itself. Nitrogen is a clear, colourless gas and the nitrogen gas cloud could be much further away than the visible cloud produced due to the water vapour freezing. This holds true for all gases, especially those used as refrigerants or for cryogenic purposes, where there will be a visible cloud due to the temperature at which the gas is stored.

Asphyxiant nature of gases

An example of the hazards of asphyxiant gases is highlighted by an incident at an orchard. Apples were being stored in a nitrogen chamber with an oxygen level of less than 1%, which keeps the apples in the best possible condition. Two members of staff were asked to retrieve some of the best apples from the back of the chamber. However, the first person collapsed while inside, as did his colleague when he tried to recover him. Unfortunately, both lost their lives. The company was fined and found guilty of breaching the Health and Safety at Work Act. Many gas manufacturers state that atmospheres under 10% oxygen can rapidly overwhelm individuals, resulting in unconsciousness without warning.

This highlights the importance of having gas detection equipment on hand where oxygen levels are expected or have the potential to be low, ensuring the staff understand the hazards even when no hazard labels are apparent, and have appropriate personal protective equipment (PPE) available.

Flammable nature of gases

Vapours can be more hazardous than gases especially when they arise from flammable liquids. Often, containers that hold flammable liquids continue to hold small amounts of the liquid or vapour after being emptied and should be cleaned and ventilated thoroughly before being used again. There have been several reports where people cut into steel drums that have held flammable liquids and fires erupt or, even worse, explosions occur. If there is only a small amount of liquid left in the base of a drum, why does this happen?

All flammable liquids and gases have an explosive or flammable range – this is between the lower explosive limit (LEL) and the upper explosive limit (UEL). The minimum concentration of gas in air at which combustion could occur is the LEL. Below the LEL the mixture is too lean to burn. The maximum concentration of gas in air at which combustion could occur is the UEL. Above the UEL, the gas/air mixture is too rich to burn. The LEL and UEL of ethanol are 3.3% by volume and 19% by volume respectively, and for petrol the LEL and UEL are 1.4% by volume and 7.6% by volume respectively. The flammable range for ethanol and petrol are quite small and they require a larger ratio of air vs vapour to ignite. This is often the case with many flammable vapours, but not so much with gases. The difference between vapours and gases is that gases cannot be liquified by the application of pressure alone. Carbon monoxide (LEL 12.5% by volume and UEL 74% by volume) and acetylene (LEL 2.5% by volume and UEL 100% by volume) are both flammable and have large flammable ranges. Acetylene will ignite in almost every gas-to-air ratio, which in combination with oxygen makes it very useful in welding, but it is always sensible to understand the advice provided by the supplier to ensure safe practice can be undertaken.

The increasing use of flammable gases including liquified petroleum gas (LPG) and compressed natural gas (CNG) means that we all need to better understand the hazards when they are being transported. Many vehicles are being converted to run on these gases and emergency services are learning to deal with these new technologies and the associated risks. For example, there have been cases where gas has filled the cabin of a vehicle due to faulty fuel tanks. The risk to the driver and passengers could be mitigated by installing a simple gas detector that would make the driver aware of the situation and enable appropriate action to be taken to deal with the gas. Unfortunately, we have recently had stark reminders of the power these flammable gases possess. For example, an LPG tanker travelling in Italy collided with a second truck that contained flammable solvents. The fire caused by the collision involving the flammable solvents started to heat the LPG tank and after around 8 minutes the LPG tank underwent a boiling liquid expanding vapour explosion (BLEVE). This BLEVE resulted in a section of the elevated motorway collapsing and significant damage to surrounding buildings. Unfortunately, two people were killed in the incident and over 130 injured, including 13 police officers.

Other incidents involving LPG have occurred around the globe and these can often be attributed to poor knowledge of the hazards and unsafe workplace procedures. Many facilities handle these flammable gases in the correct manner without incident, but accidents happen. Understanding how to manage an incident is a critical part of working with hazardous materials. When such incidents do occur, well prepared sites with the correct equipment and well-trained staff know the correct procedures to follow. In one particular incident, a trailer park had 17 of its buildings burnt down due to an explosion at a nearby propane warehouse. Most of the staff at the warehouse had gone home and only the plant manager was left conducting an inventory when the fire started. Investigations into this case are still ongoing, but this highlights the need for emergency planning as required under the UK’s Control of Major Accident Hazards (COMAH) or good risk management strategies such as the Control of Substances Hazardous to Health (COSHH) risk assessments. Please also note that when working with hazardous materials, working alone is not recommended especially when there are over 10,000 propane cylinders on site. In the UK, a site of this scale would be classified as a COMAH site (Seveso in Europe), which would legally require the operators to implement and, critically, test their emergency response and crisis management processes and policies. Even if a chemical or manufacturing site is not classified as a COMAH site, NCEC strongly advocates robust incident management guidelines and procedures are put in place and are well tested. Those nominated to lead responses to incidents should also be properly trained.

Thankfully, most calls to NCEC are resolved in a timely manner. Our chemical experts are regularly contacted for chemical advice to help emergency services, as well as other users of chemicals during incidents including spills, fires and exposures. However, there are instances when large-scale incidents require advice for prolonged periods. In December 2005, we supported the fire and rescue service with advice during the Buncefield fire. This incident was caused by three separate automated systems failing one after another after staff had left the facility in, what they believed to be, a safe state. Buncefield was one of the main oil depots in the United Kingdom and one of the pipelines was left filling automatically, which filled a tank (number 912) at a rate of 550 cubic metres per hour.

“hydrogen sulphide can overcome olfactory senses at high concentrations meaning it cannot be smelt”

The staff had no knowledge about this because the filling gauge showed no change therefore not tripping the automated alarm system. Around 3am, when it was still being filled, a switch at the top of the tank was meant to shut off the feed when the level of fuel reached a certain mark, but this system failed as well. The failure of this switch should have also caused an alarm, but this failed to operate, and the fuel flowed out of the tank and spilt into the bunding, which is there to capture any unintended leaks. Vapour eventually found an ignition source and a huge explosion took place.

This incident highlights the importance of maintenance of all automated systems and gas detection equipment since installing them and not checking them regularly is not acceptable. In the case of Buncefield, maintenance was undertaken and records showed that tank 912 had been serviced in August 2005, but the fuel gauge had kept being intermittently stuck during the months prior to the fire. At the time, this was not raised as a significant concern. The shut-off switch was missing a critical padlock for it to function, meaning it was rendered ineffective. This shows how ‘complacency creep’ can slowly filter into an organisation meaning that equipment, including gas detectors, can degrade. For instance, gas detectors can be ‘poisoned’ with silicone or sulphur-based products, or inhibited by halogenated compounds such as chlorine, fluorine or bromine and this leads us into our final section on toxic gases.

Toxic nature of gases

Many incidents caused by toxic gases have a common thread – casualties occurring in confined spaces where people don’t realise the hazards all around them. Gases such as hydrogen sulphide, ammonia and nitrogen dioxide are toxic in nature. Unfortunately, many individuals do not have access to gas monitoring equipment and there have been instances where septic tanks or holds on fish trawlers that contain these gases have resulted in fatalities. There is a lot of information describing odours and how these can be used to identify gases but relying on a sense of smell is not an acceptable method of identifying a gas leak. Not every individual can smell the sulphur-containing gases such as hydrogen sulphide (a highly toxic gas with a rotten egg smell) or methyl mercaptan (gas that produces toxic vapours and smells like rotten cabbage). In particular, hydrogen sulphide can overcome olfactory senses at high concentrations meaning it cannot be smelt. At low concentrations, it has a rotten egg smell and the literature states that at 100 ppm, this loss of smell can occur after two to 15 minutes and is described as olfactory fatigue or paralysis.

Back in 2013, in what is now colloquially referred to as ‘Le Pong’, gas companies and emergency services in the United Kingdom received over 80,000 calls in a couple of days due to a leak of methyl mercaptan – which is added to natural gas to add smell to an otherwise unscented gas – in the north of France. The windy weather caused the gas to drift across the channel and many people in Kent and Sussex reported gas leaks since they could smell very small concentrations of the chemical. At these concentrations there was no hazard to the public or environment, but several agencies including NCEC were contacted for assistance in finding out what had occurred. Our links with other chemical emergency centres in Europe helped us to identify the source and pass details back to the relevant agencies.

Fortunately, large scale incidents of this nature do not occur on a regular basis, but this shows that good communication and emergency management help to mitigate any scale of incident.

“being prepared with equipment, training and exercising helps staff react to any incident safely, efficiently and cost-effectively”

Phosphine is another highly toxic gas. It is said to smell of garlic and can be formed when aluminium phosphide, a professional rodenticide, reacts with water or moisture. In one instance, some rodenticide tablets were placed under a mobile home to kill off the local vermin. Unfortunately, someone tried to wash away the rodenticide using water. Several people were overcome by the fumes, became disoriented and started feeling sick. Six people from one family were rushed to hospital. Unfortunately, four of them, all children, died from the exposure. There are also cases of accident and emergency units being shut down due to phosphine gas off-gassing from people who have deliberately exposed themselves to the rodenticide. This shows why gas detection is important to the emergency services in establishing what has occurred, and to ensure that they are able to appropriately protect those involved in an incident – and themselves. Had they entered the mobile home without analysing the environment or wearing breathing apparatus, they could have added to the casualty count.

The final gas we will discuss in this article with respect to toxicity is chlorine. Chlorine was used as a chemical weapon in World War I. Nowadays, it is often used in swimming pools and cleaning products to disinfect. However, incidents still occur when an end-user mixes bleach, which is hypochlorite solution, with an acid-based drain cleaner or a spill occurs during swimming pool dosing. Chlorine exposures are made more complicated as, when inhaled in large concentrations, the effects can be delayed for up to 72 hours. Chlorine gas is not a disinfectant by itself, but when dissolved in water it forms hypochlorous acid and this is the chemical that keeps toilets germ free. This should not be confused with hydrogen chloride, which, when dissolved in water, forms hydrochloric acid. Hydrogen chloride is another toxic gas that can be released from high concentrations of hydrochloric acid. Therefore, care must be taken to ensure that lids on containers are replaced correctly. There have been incidents where this has not happened, and industrial units have become filled with acidic hazes causing breathing difficulties for staff.

In conclusion, human history is littered with incidents involving chemicals, and gases and vapours, in particular, pose a unique issue. Liquids and solids can be contained if a release occurs, but stopping a gas leak is far more difficult unless the gas is stopped right at the source. Therefore, early warning is paramount so that staff can take action, shelter or evacuate depending on the situation in which they find themselves. What is important to understand is that it isn’t if an incident is going happen but when. There may only be a slight risk involved during end use, but being prepared with equipment, training and exercising helps staff react to any incident safely, efficiently and cost-effectively.

Fortunately, there is a lot of help available to those who manufacture and those who use gaseous materials and materials that can emit hazardous gases. NCEC is experienced in supporting organisations by providing a wide range of training and preparedness measures – from tactical chemical hazard awareness courses, through to leadershipfocused incident management training and exercising plans. If it does go wrong, our emergency responders are on hand to provide actionable advice over the phone to those dealing with an incident to help keep them safe, and to safely and quickly resolve the incident.

At NCEC, we often receive calls from the emergency services seeking guidance during chemical emergencies including advice on gas detection and analysis. NCEC has been providing this service since 1973. We also provide telephonebased chemical emergency advice to, and on behalf of, hundreds of companies across the world, including many of the top global chemical companies.