Shah Field, United Arab Emirates, 3 February, 2009. Three workers are killed and one injured, by an “invisible killer”. This killer could not be seen or heard. Acting silently, victims’ lives were taken without warning or chance of escape. This was not the work of a murderous assailant, however, but rather an audience with hydrogen sulphide (H2S). This is a common hazard in the Oil and Gas, and Petro-Chemical Industries..

This Invisible killer not only attacks workforces and workplaces, however, as evidenced in Bhopal, India, in December 1984. Many thousands of innocent people were killed, with many more made to suffer from severe ill-health, due to the release of a substance known as Methyl Isocyante (MIC), in the form of a deadly chemical gas cloud. This release came from the local pesticide manufacturing plant, and drifted into the unsuspecting homes and businesses of the local towns and villages.

So does that mean whether we work in a place dealing with gas, or next door to it, that there is nothing we can do about it? Of course not. Gas, just like any hazard in the workplace, can be managed. The best way to conquer your enemy, is to know your enemy. In this article, let us discuss some of the hazards and the risks posed. Then, whilst there are many ways we can be safe from gas, we will focus on one of the main weapons in our armoury, gas detection and monitoring.

Hazard basics

So, what is gas exactly? There are many states, or forms, of matter in our world. The most common three are solid materials, such as glass and stone; liquids, such as water and oil; and gasses, such as Methane (CH4) and Carbon Dioxide (CO2). Gasses can naturally occur in the environment around us. Without oxygen in our atmosphere, for example, we would not be able to breath. Gasses can also be as result of something man-made. We can take several gasses, and mix them together, to form Liquefied Petroleum Gas (LPG), which is useful as a fuel for cooking food, or heating homes and businesses. Other examples of uses of gas include various forms of “hot works” and purging (in certain industries).

Whilst this sounds useful, there are several problems that gas can present.

One of the big issues surrounding gas is that it has no fixed shape. In other words, it will fill the space it enters (if there is enough of it). This means if there is a gas leak, it is very difficult to stop it from spreading uncontrollably. Some gasses are also undetectable to humans, presenting major health risks. Various gasses have no taste or smell, and cannot be seen. This means the only way we can know that gas is present is to use special equipment to detect it. Other gasses are extremely flammable, leading to fires and/or explosions, a particularly severe risk when performing hot works (such as welding or grinding.) Some gasses can also be quite dense, which means they are heavier than other gasses. This is an issue as these particular gasses can collect in low-lying areas such as trenches, pits and wells. This is a risk as often they can go undetected, through improperly testing for the gas, or simply not realising it is there – but more on this later.

Even gasses which are critical to our survival can be detrimental if they are present in the wrong quantity. Take oxygen as an example. Without oxygen (oxygen deficiency) we cannot survive, as we mentioned earlier. Too much oxygen, however, (oxygen enrichment) can also be dangerous through increased risks of fire and explosion, and greater potential for severe health effects to occur (such as oxygen toxicity).

Finding solutions

So what do we do about these problems? Well, first we have to see if we have a problem, by performing gas testing, and using gas monitoring systems. What exactly is appropriate depends upon the exact circumstances of the work. We may only need to conduct a test just before starting a job, to see if gas is present. In other circumstances we may need to perform several tests for several different types of gas. This is because gas may be present in the work area constantly, so in this scenario, performing only one test would not be adequate. Gas levels can fluctuate randomly between harmless and dangerous levels. This is why we have measurements such as LELs (Lower Explosive Limits), UELs (Upper Explosive Limits), TWA (Time Weighted Average) and PELs (Personal Exposure Limit), amongst others. You should be able to develop gas testing and monitoring procedures appropriate for your business, through a suitable and sufficient risk assessment for the particular tasks or work area you are in. Again, due to the varied nature and effects of gasses, you must ensure the persons developing these systems are competent. If you are not sure, seek expert advice and assistance.

So what do the terms “gas testing” and “gas monitoring” mean? Gas testing involves using some type of equipment
to test an environment for the presence of gasses, usually before any work has started. Gas monitoring is reactive, usually coming in the form of monitoring devices worn by personnel, and/or detection systems attached to plant, equipment, pipework and so on. Rather than taking a single measurement, these devices will constantly check the environment around the person or equipment, to see if gas is present, and at what level. These systems are usually alarmed in some way, to warn the workforce of the potential danger if gasses reach dangerous levels. As mentioned earlier, it is important to ensure that all persons involved in gas detection and monitoring are competent, as any miscalculation or misunderstanding can lead to disaster.

“persons involved in gas detection and monitoring must be competent, any miscalculation can lead to disaster”


As mentioned earlier, gasses can be present at different levels of an environment. This is a phenomenon called “Stratification” and it can be deadly. Let’s use a confined space as an example. This space may contain hydrogen sulphide. This gas is heavier than air, so will settle at the bottom of the confined space, any other gasses present will sit above it. If our gas tester is not competent, they may only check the top half of the confined space, finding the space is safe to work in, when in fact it is not. A competent gas tester would test the whole space, at all levels of the space, picking up the presence of the hydrogen sulphide and therefore saving lives. This is because we can then purge/vent the space, re-testing it after this is done to ensure all the hydrogen sulphide has been removed before any work starts. The equipment itself can also cause problems. Often, testing and monitoring equipment is very sensitive, and therefore should be calibrated correctly for the job at hand. Incorrect results will mean we make incorrect decisions about the safety of an area, particularly if those results show “safe” levels of gas. Earlier, I mentioned that our gas tester should be competent. This is true however for any of our workers who are likely to work in areas where there may be dangerous gas. A lack of training on
the dangers of gasses means the workers cannot possibly know what the safe levels are, how to interpret information provided by the equipment they are using, and ultimately, what to do in an emergency situation.

Types of gas detectors

As with anything in health and safety, gas detection and monitoring equipment can come in many different forms, and what is right for your business depends upon your exact circumstances.

Single gas

You may only be worried about a relatively small quantity of one particular gas, so a gas tube sampling system may be enough. This is a simple handheld pump, with a glass tube inserted into the end of it. A person holds the pump in one hand, and the pump handle in the other. They take a reading from a space by simply pulling the handle all the way back, drawing sampled air from the space/atmosphere into the tube. These glass tubes contain reactive elements or reagents, that will change colour if a gas is present, also giving an indication of how much gas there is through a measurement scale marked on the tube.

These systems are relatively easy to handle and use when compared with some other equipment, requiring a minimum amount of training. This makes the gas tube system a relatively low cost solution. They also do not present a fire risk, as they are not spark or heat producing, containing no electrical parts or systems. Like anything else, however, they do have limitations. The tubes are specific to one type of gas only, so several different tube types would be required to test for several different gasses. The tubes are also incredibly fragile and break easily. As these systems require a person to hold them and pull a handle to use them, this presents problems if trying to test large spaces, as the person testing for gas can only test places they can physically reach. Sampling tubes also only tell us the amount of gas present at that very moment in time. They cannot be used for continuous air monitoring, as once a sample is taken, the tube has to be replaced with a new one. These systems were first developed back in the 1900s, and whilst technology has greatly improved, they are still manufactured and in use today.

“gas monitors will not prevent the release or presence of gas, only inform us it is there”

Multiple gasses

If you want to test for multiple gasses, or have more complex hazards to deal with, a multi-gas detector is a suitable option. Thousands of manufacturers all around the world make this equipment, suited to specific industries or for more general applications. They test and monitor the air around them, but unlike sampling tubes, they can “analyse” a sample of air for multiple gas types. These testing and detection systems can also be fixed to certain areas of plant or equipment if required, and are able to be used to test varying levels of an atmosphere, without putting the user at risk. For example, a gas tester may be using a device with a long tube attached to it. The tube can be lowered into the atmosphere to be tested, meaning the worker doing the testing can safely do their job from outside the dangerous area.

These machines also have multiple alert systems through audible alarms, lights and visual displays. Usually, these alerts will occur when the gas is at a level where workers are able to “down tools” and escape the environment, before it becomes too dangerous. Another advantage these systems have is that as well as being used for initially testing an atmosphere, they can be used for “real-time” monitoring of the air, meaning we can have knowledge on if gas levels are fluctuating or not. These systems can present issues, however. They can be very complicated to use (although lots of manufacturers are pushing towards one-button operation), so the correct type and level of training in their use is essential. Correct maintenance and calibration are also key to ensuring a “true” reading of the gasses detected. Particularly for environments that present a fire or explosion hazard, these pieces of equipment must be “intrinsically safe”. This means they are designed in such a way that their use will not cause flammable gasses to ignite. This is indicated by the markings on them, such as (EX) or ATEX, meaning explosion proof. The portability and mobility of this equipment can also cause problems, with batteries running out, and equipment being damaged through rough use or mishandling.

Personal monitors

Personal monitors are also a great weapon in the fight against the ‘Invisible Killer’. These machines are designed to monitor the breathing zone of the individual, so are worn somewhere close to the face, such as being clipped to a chest pocket on a set of coveralls. These are particularly useful when gas testing is intermittent, a necessary contingency in case there are issues with other gas testing and monitoring systems. Personal monitors are relatively easy to use, once workers have been trained and know what to look for when reading the visual displays and alarms (usually lights, sounds and vibration).

“correct maintenance and calibration are key to ensuring a “true” reading of the gasses detected”

Just like gas detectors, personal monitors have to be regularly calibrated and maintained to ensure accurate readings are being given. Batteries losing power during the working shift, or being lost altogether are another concern. Accidental or deliberate misuse by the workforce is also an issue that needs addressing. Workers may forget to wear them, or wear them in the incorrect place on their body. Examples of this include monitors worn on hard hats or hip pockets which can easily be knocked off the body, and difficulty in knowing if activation has occurred due to the incorrect placement of the equipment on the body. Like any portable equipment, they can also be damaged through being dropped or miss handled. Using the wrong type of personal monitor could also mean hazardous gasses go undetected, so if multiple gasses are present, a multi-gas personal monitor will be required. Remember, gas monitors will not prevent the release or presence of gas, only inform us it is there. Gas testing can at least be used to give us an indication of any gas being present before work starts, hopefully preventing many gas related incidents and accidents from occurring in the first place.


If we want to keep our workforce safe when working with gasses, and in the environments containing them, it is critical that we conduct gas testing and monitoring correctly. Proper risk assessment of your workplaces and tasks will help you discover if you have any gas-related hazards. This will then provide you with some of the essential information in helping to develop the strategy you are going to use to deal with the hazards you have identified. As well as the gas testing and monitoring we have discussed, consider other control measures. Can you do the work without using gas, therefore totally eliminating the hazard and risk? Can you replace the gas with another that is less hazardous? Is there a time when the gas level is safer than at other times? Provide information, instruction and training to everyone who may be potentially affected by your work with gas, not just your workforce (e.g. provide information to nearby businesses, neighbours and members of the public). Always have emergency arrangements ready to go, just in case the worst should happen. If necessary seek expert help and advice. By taking these steps, you will start down the path of protecting everyone from the dangers of gas, the Invisible Killer.