In February 2019 the UK Health and Safety Executive tightened up the requirement to protect workers from welding fume. The move came in response to new research that identified mild steel welding as a cause of lung and possible kidney cancer.

Welding is a very energetic process that produces a range of compounds, many of which are extremely harmful, such as highly reactive free radicals that can damage any tissue they come into contact with. But the most damaging of all the welding emissions are the metal particulates. Our bodies are adapted to deal with carbon-based dusts such as pollen but they cannot guard against the lasting damage caused by metal dusts, which can permanently disrupt the lungs’ natural protection and leave people wide open to dangerous infections.

HSE’s revised expectations now place a requirement on employers to take special measures to protect workers exposed to welding fume of all types, because general ventilation does not achieve the necessary control.

All welding generates a range of gases and particulates, with different types of welding and different materials generating a variety of potential health hazards.

For instance, gas welding produces nitrogen dioxide, inhalation of which can result in pulmonary oedema, while inhaling the fume from electric arc welding can lead to chronic cough and bronchitis. Welding zinc-coated or galvanised steel can cause zinc fume fever, which is linked to coronary heart disease, while welding stainless steel generates hexavalent chromium, which can result in lung cancer. Asthma is a particular risk when welding polyurethane coated steel or pipes. In addition, many welding applications release neurotoxic metals such as aluminium, lead and manganese. Manganese exposure can cause a disease similar to Parkinson’s, for example.

HSE’s bulletin (STSU1 – 2019) says businesses should take the following action to mitigate the risks posed by welding:

1. Make sure exposure to any welding fume released is adequately controlled using engineering controls, typically local exhaust ventilation (LEV).
2. Make sure suitable controls are provided for all welding activities, regardless of duration. This includes welding outdoors.
3. Where engineering controls alone cannot control exposure, then adequate and suitable respiratory protective equipment (RPE) should be provided to control the risk from any residual fume.
4. Make sure all engineering controls are correctly used, suitably maintained and are subject to thorough examination and test where required.
5. Make sure any RPE is subject to an RPE programme, which encapsulates all the elements of RPE use necessary to ensure that RPE provides effective protection.

Most organic chemical compounds will burn. Burning is a simple chemical reaction in which oxygen from the atmosphere reacts rapidly with a substance, producing heat.

The simplest organic compounds are those known as hydrocarbons, and these are the main constituents of crude oil/gas. These compounds are composed of carbon and hydrogen, the simplest hydrocarbon being methane, each molecule of which consists of one carbon atom and four hydrogen atoms. It is the first compound in the family known as alkanes. The physical properties of alkanes change with increasing number of carbon atoms in the molecule, those with one to four being gases, those with five to ten being volatile liquids, those with 11 to 18 being heavier fuel oils and those with 19 to 40 being lubricating oils. Longer carbon chain hydrocarbons are tars and waxes.

The first ten alkanes are:

CH₄ methane (gas) C₆H₁₄ hexane (liquid)
C₂H₆ ethane (gas) C₇H₁₆ heptane (liquid)
C₃H₈ propane (gas) C₈H₁₈ octane (liquid)
C₄H₁₀ butane (gas) C₉H₂₀ nonane (liquid)
C₅H₁₂ pentane (liquid) C₁₀H₂₂ decane (liquid)

Alkenes are similar but their molecular structure includes double bonds (examples are ethylene and propylene). They have more energy per molecule and so burn hotter. They are also more valuable in the manufacture of other chemicals including plastics. Alkynes contain triple bonds (example is acetylene), used in welding of metals. The above compounds are all known as aliphatics, which means the carbon atoms are all stretched out in a line. Aromatic hydrocarbons such as benzene have a ring molecular structure, hence less hydrogen per carbon atom and thus burn with a smoky flame.

When hydrocarbons burn, they react with oxygen from the atmosphere to produce carbon dioxide and steam, although if the combustion is incomplete because there is insufficient oxygen, carbon monoxide will result aswell.

More complex organic compounds contain elements such as oxygen, nitrogen, sulphur, chlorine, bromine or fluorine and if these burn, the products of combustion will include additional compounds. For example substances containing sulphur such as oil or coal will result in sulphur dioxide whilst those containing chlorine such as methyl chloride or polyvinyl chloride (PVC) will result in hydrogen chloride.

In most industrial environments where there is the risk of explosion or fire because of the presence of flammable gases or vapours, a mixture of compounds is likely to be encountered. In the petrochemical industry the raw materials are a mixture of chemicals, many of which decompose naturally or can be altered by processing. For example, crude oil is separated into many materials using fractionation (or fractional distillation) and ‘cracking’. Fractionation is where highly volatile gases are removed at temperatures where they alone are volatile, then higher temperatures where heavier compounds are volatile then hotter still for larger hydrocarbons. Cracking is where big hydrocarbon molecules are broken up by heat and catalytic action to form smaller hydrocarbon molecules.

Inerting

In order to prevent explosions during shutdown and maintenance operations many industrial processes employ an inerting procedure. Fill a container of hydrocarbon gas with air and at some point, the mixture will become explosive and dangerous. Use a 2-stage process where the hydrocarbon is replaced by nitrogen and then the nitrogen is replaced by air, and at no stage do you risk explosion.  This is called purging a vessel (for example a fuel tanker, or storage tanks on an oil tanker). Purging of hydrocarbons is common practice before carrying out maintenance or repair work. Before entry by personnel, the vessel must be purged with breathable air. Crowcon has special instrumentation to monitor this whole process to ensure efficient inerting and alert operators to the presence of potentially dangerous mixes of air, nitrogen and hydrocarbons during maintenance operations.

Standards defining LEL concentration

Safety procedures are generally concerned with detecting flammable gas before it reaches its lower explosive limit. There are two commonly used standards which define the ‘LEL’ concentration for flammable substances: ISO10156 (also referenced in the superseded standard EN50054), and IEC60079-20-1:2010. The IEC (International Electrotechnical Commission) is a worldwide organization for standardization. Historically, the flammability levels have been determined by a single standard: ISO10156 (Gases and gas mixtures- Determination of the fire potential and oxidizing ability for the selection of cylinder valve outlets).

IEC and EU (European) standards (IEC60079 and EN61779) define LEL concentrations measured using a ‘stirred’ concentration of gas (as oppose to the ‘still’ gas method employed in ISO10156). Some gases/ vapours have been shown to be able to sustain a flame front at lower fuel concentrations when stirred than when still. Small differences in the 100%LEL volume results. It is caused by the average distance of a burning molecule from an unburned molecule being a little less when the gas is being stirred. The resultant LEL’s vary a small amount between the two standards for some gases/vapours.

The table on the following page shows some of the notable differences in LEL values between the two standards. It can clearly be seen that 50% LEL of methane in EN60079 calculates to a 2.2% volume concentration in air, as oppose to 2.5% volume as stated in ISO10156. Therefore, if a detector is calibrated according to EN60079 using a mixture of 50% LEL methane made to ISO 10156, a 13.6% sensitivity error would occur potentially invalidating the calibration. The error could even be greater for non-linear infrared detectors.

The European ATEX Directive (covering the certification and use of equipment in flammable atmospheres), stipulates that manufacturers and users comply with the EN61779 standard. Crowcon’s policy is to apply the new values of LEL in Europe and territories that adhere to European Standards. However, as the old standard is still used in the US and other markets, we will continue to calibrate to ISO 10156 in these territories. ATEX/IECEx certified Crowcon products will be supplied calibrated according to the IEC60079/EN61779 standards (i.e., methane sensors will be calibrated such that 100% LEL = 4.4% volume). UL/CSA certified products will be calibrated according to the ISO10156 standard (i.e., methane sensors will be calibrated such that 100% LEL = 5% volume) unless a customer stipulates otherwise.

Alarm Levels

Flammable gas detection systems are designed to create alarms before gases/vapours reach an explosive concentration. Typically, the first alarm level is set at 20% LEL (although there are industries that prefer 10%LEL; particularly Oil and Gas companies). Second and third alarm levels vary according to the type of industry and application but are commonly set to 40% LEL and 100% LEL respectively.

The evolution of gas detection has changed considerably over the years. New, innovative ideas from canaries to portable monitoring equipment provides workers with continuous precise gas monitoring. Gas detection equipment can be broken down into the monitoring of gas using sensors and gas path technology, the user interface that informs people or equipment of any necessary action, and the supporting power management system that keeps it all charged up and working. To the package we can now add a fourth consideration – communications and recording technology.

Types of sensors

Photo-ionisation detection (PID)

PID technology is generally considered the technology of choice for monitoring exposure to toxic levels of VOCs. The sensors include a lamp as a source of high-energy ultraviolet (UV) light. The UV light’s energy excites the neutrally charged VOC (Volatile Organic Compounds) molecules, by removing an electron to leave it charged. A current then flows between two charged plates within the sensor, and the gas concentration is proportional to that current.

Electrochemical

Electrochemical sensors measure gas which enters through a small hole in the face of the cell, pass through a PTFE moisture and oil filter and onto an electrode via an electrochemical solution. Sensor ranges and sensitivities can be varied in design by using different size holes, with larger holes providing higher sensitivity and resolution, and smaller holes reduced sensitivity and resolution but increased range. The gas type measured is chosen by selection of the electrode material, selection of the electrolyte and sometimes use of filters to block unwanted gas types.

Catalytic Beads (Pellistor)

Pellistor sensors consist of two matched wire coils, each encased in ceramic beads. Current is passed through the resistance coils, heating them to approximately 230˚C. One of the beads contains a catalyst material, so when a mixture of air and flammable gas enters the sensor, it contacts the beads and burns near the one containing the catalyst. This results in a temperature difference between this active and the other ‘reference’ bead.  The temperature difference causes a difference in resistance, which is measured; the amount of gas present is directly proportional to the resistance change, so gas concentration as a percentage of its lower explosive limit (% LEL*) can be accurately determined. Pellistor sensors are widely used throughout industry including on oil rigs, at refineries, and in underground construction environments such as mines, and tunnels.

Infrared Sensors

Infrared emitters within the sensor each generate beams of IR light. Each beam passes through a sample of atmosphere and is measured by a photo-receiver. A “measuring” beam, with a frequency of around 3.3μm, is absorbed by hydrocarbon gas molecules, so the beam intensity is reduced if there is an appropriate concentration of a gas with C-H bonds present. A “reference” beam (usually around 3.0μm) is not absorbed by gas, so arrives at the receiver at full strength. The %LEL of gas present is determined by the ratio of the beams measured by the photo-receiver.

Molecular Property Spectrometer™ (MPS™)

MPS™ sensors represent the new generation of flammable gas detectors. MPS™ can quickly detect many gas types and identify over 15 characterised flammable gases at once. Until recently, anyone who needed to monitor flammable gases had to select either a traditional flammable gas detector containing a pellistor sensor calibrated for a specific gas, or containing an infra-red (IR) sensor which also varies in output according to the flammable gas being measured, and hence needs to be calibrated for each gas. While these remain beneficial solutions, each has environments where they can be used and environments to avoid. For example, both pellistors and infrared sensors require regular calibration and the catalytic pellistor sensors also need frequent bump testing to ensure they have not been damaged by contaminants containing permanent poisons (known as ‘sensor poisoning’ agents) or by harsh conditions. In some environments, sensors must be changed frequently, which is costly in terms of both money and downtime, and product availability. IR technology cannot detect hydrogen – which has no IR signature, and both IR and pellistor detectors sometimes incidentally detect other (i.e., non-calibrated) gases, giving inaccurate readings that may trigger false alarms or concern operators. The solution is the MPS sensor which detects both hydrogen and other flammable gases, identifies them, and applies the right calibration for each gas or constituent gas of any mixture it monitors.

Some instruments use a pump to supply air or gas samples to the sensor.

Types of Detection

Fixed

Fixed gas detectors are permanent fixtures that stay mounted in one location. They can be set up in single-detector configurations, in small and large multiple-detector configurations and in an addressable ‘daisy chained’ loop. Fixed gas detectors are generally installed anywhere there is a risk to plant, buildings or installations, and can detect slow build ups or major leaks to give an early or automated warning of gas leaking from a particular source. They are often set up to trigger other safety measures, so they can open vents, start fans, close valves or even shut processes down automatically once they detect a problem. Quite often they are set up to warn a control room or security personnel of a potentially dangerous gas leak, so executive action can be taken by people. They can also set off alarms to begin an evacuation. On the other hand, fixed gas detectors are usually not designed to prevent a worker coming into contact with the gas, though some systems do have a component of area coverage to their design. Portable gas detectors and the best way to protect individuals at risk of coming into contact with toxic or flammable gas build ups or releases.

Each fixed gas detector must communicate with a control panel. The control panel is the hub of the fixed gas detection system, which compares the quantities of gas with pre-set levels and provides various options for input and output functions. The gas control panels are normally located in a safe area but can be installed in hazardous zones if appropriately housed. They communicate with gas detection sensor heads or transmitters and can be networked to a central point so that multiple control panels/systems can be monitored remotely. There are multiple methods of communicating with fixed gas detectors. The most common is analogue, but there is a growing demand for digital and wireless communications. There are also various features available via the detector to improve efficiency and reduce the time spent by personnel in potentially hazardous locations, thereby reducing risk to people.

Portable

Portable gas detectors are personal protection devices that continuously monitor the user’s breathing zone. Because they are generally small, these handheld, lightweight and robust devices are carried on the person and constructed to be ergonomic and worn unobtrusively. They are also sometimes used to check confined spaces such as tanks where the type of gas risk is known, before someone enters the space. They are intended for monitoring at close range and are usually not suitable for long term continuous monitoring of larger spaces. Portable gas detectors are the safest proven way to protect individual workers as they move around.

Portable detectors store information on gas exposure throughout the duration of a shift, as well as events such as alarms or near misses. This data can be transmitted to a cloud-based portal to allow for numerous benefits such as improved operational efficiency and safety compliance, as well as providing a robust and flexible mechanism to deliver valuable actionable insights. Data solutions offer tangible benefits to all sizes of portable fleet, whether gas detectors are being used onsite, offsite or both. Portable gas detectors typically cost less than fixed systems and most are battery powered. On the other hand, each user must be properly trained to operate their portable detector. In addition, portable detectors are not typically connected directly to other safety systems. If the detector raises an alarm, the user is therefore required to take action on their own to mitigate any risk to themselves or others.

Alarm Levels

It is important to note that whereas portable gas detection instruments measure and alarm at the TWA levels, instantaneous alarms are included to provide early warning of an exposure to dangerous gas concentrations. Workers are often under risk of gas exposure in situations where atmospheres cannot be controlled, such as in confined space entry applications where alarming at TWA values would be inappropriate.

You must perform your own risk assessment to ensure alarms are set to appropriate levels for your application and in accordance with local legislation and practices.

The following data has been extracted from EH40 and EH40 for some common toxic gases:

Workplace Exposure Limits:

*C – Ceiling Limit

Gases and vapours produced, under many circumstances, have harmful effects on workers exposed to them by inhalation, being absorbed through the skin, or swallowed. Many toxic substances are dangerous to health in concentrations as little as 1ppm (parts per million). Given that 10,000ppm is equivalent to 1% volume of any space, it can be seen that an extremely low concentration of some toxic gases can present a hazard to health.

Gaseous toxic substances are especially dangerous because they are often invisible and/or odourless, and are physically harder to avoid than liquids or solids. Their physical behaviour is not always predictable: ambient temperature, pressure and ventilation patterns significantly influence the behaviour of a gas leak. Hydrogen sulphide for example is particularly hazardous; although it has a very distinctive ‘bad egg’ odour at concentrations above 0.1ppm, exposure to concentrations of 50ppm or higher will lead to paralysis of the olfactory nerves rendering the sense of smell inactive. This in turn may result in the assumption that the danger has cleared. Prolonged exposure to concentrations above 50ppm can have other symptoms and in extreme cases result in paralysis and death.

Definitions for maximum exposure concentrations of toxic gases vary according to country. Limits are generally time-weighted as exposure effects are cumulative: the limits stipulate the maximum exposure during a normal working day and for shorter periods down to 15 minutes or less.

UK Health and Safety Executive (HSE) and COSHH Regulations

Chemicals, fumes, dusts and fibres can under many circumstances have harmful effects to workers exposed to them by inhalation, being absorbed through the skin, or swallowed. Persons exposed to harmful substances may develop illnesses (for example, cancer) many years after the first exposure. Many toxic substances are dangerous to health in concentrations as little as 1ppm (parts per million). Given that 10,000ppm is equivalent to 1% volume of any space, it can be seen that an extremely low concentration of some toxic gases can present a hazard to health.

It’s worth noting that most flammable gas hazards can potentially occur when the concentration of gases exceed 10,000ppm (1%) volume in air or higher. Toxic gases typically need detecting in sub-100ppm (0.01%) volume levels to protect personnel and often at concentrations sub 5ppm.

In the UK, under the Control of substances hazardous to health regulations 1999 (COSHH regulations) the Health and Safety Executive (HSE) sets occupational exposure limits (OELs) and publishes these in a document entitled EH40. These lists have legal status and similar legislation exists elsewhere; COSHH takes into account the European Commission Directive 80/ 1107/EEC. COSHH covers all toxic substances except those which have their own legislation (asbestos, lead, radioactive materials and materials present in mines).

The regulations stipulate requirements for employers, and in a few cases employees (failure to comply is subject to the penalties of the Health and Safety at work act 1974). The requirements are:

• Design and operate processes and activities to minimise emission, release and spread of substances hazardous to health.
• Design and operate processes to minimise human interaction within potentially dangerous environments.
• Take into account all relevant routes of exposure, inhalation, skin absorption and ingestion, when developing control measures.
• Control exposure by measures that are proportionate to the health risk.
• Choose the most effective and reliable control options which minimise the escape and spread of substances hazardous to health.
• Where adequate control of exposure cannot be achieved by other means, provide, in combination with other control measures, suitable personal protective equipment.
• Check and review regularly all elements of control measures for their continuing effectiveness.
• Inform and train all employees on the hazards and risks from the substances with which they work, and the use of control measures developed to minimise the risks.
• Ensure that the introduction of control measures does not increase the overall risk to health and safety.

The assessment is performed by the employer with help from the HSE if needed. The best way of controlling a risk is to prevent exposure but if this is not possible, a process may have to be enclosed or ventilation and extraction equipment used, or special handling procedures employed. It should be possible for all people to work in a safe environment day after day and HSE publishes Guidance Note EH40 to help employers to control their processes adequately so that workers are not exposed to levels of toxic materials above the recognised safe levels.

The monitoring aspect of COSHH is particularly relevant to Crowcon’s products where monitoring is required:

• If the failure of control measures would lead to serious health risks
• If it is not certain that exposure limits are not being exceeded
• If it is not clear that control measures are working properly

When monitoring of toxic gas exposure is required employees must be told about potential risks and precautions to be followed. The results of any monitoring and health surveillance should be recorded.

Gaseous toxic substances are especially dangerous because they are often invisible and/or odourless and are harder to physically avoid than liquids or solids. Their physical behaviour is not always predictable: ambient temperature, pressure and ventilation patterns significantly influence the behaviour of a gas leak. Crowcon’s toxic gas detectors and their accessories have been designed with this in mind, and the need for continuous monitoring and recording has led to the development of data logging facilities.

There is increasing emphasis on Environmental Monitoring in the workplace. It is recognised that employee’s health and well‑being may be affected by pollution from industrial processes, traffic fumes and the decay of waste. Levels of NOx (oxides of nitrogen), SOx (oxides of sulphur) and increasingly; CO₂ are being monitored to quantify exposure.

The 2005 issue of EH40 introduced new terminology for defining occupational exposure limits (OELs). The previous system defined OELs as maximum exposure limits (MELs) and occupational exposure standards (OESs). MELs and OESs have been discontinued and replaced by a single type of OEL known as the workplace exposure limit (WEL). The numerical values initially remained the same, but some have since been lowered as new information has become available. The OESs for around 100 substances have been deleted as the substances are now banned, scarcely used or there is evidence to suggest adverse health effects close to the old limit value.

From 1989 to April 2005, Occupational exposure standards were in two categories.

Maximum exposure levels (MELs) were for the more dangerous substances which may cause the most serious health effects (such as cancer or asthma) and exposure to materials with MELs were kept as low as possible and certainly not above their MEL.

Occupational exposure standards were set at a level at which there was no indication of risk to the health of workers and employees exposed by inhalation day after day.

As mentioned above the new workplace exposure limit (WEL) list will combine OELs and MELs using the same numerical values. the list gives long term (8 hour) exposure limits (LTELs) applicable to exposure during a normal working day and short term (15 minute) exposure limits (STELs) applicable to occasional exposure to higher levels. Therefore, WELS are concentrations of toxic substances in the air, averaged over a specified period of time and referred to as the time weighted average (TWA).

WELs can be expressed as parts per million (ppm) and milligrams per cubic metre (mg/m3) if the substance exists as a gas or vapour at normal room temperature and pressure. Compounds that do not form vapours at room temperature and pressure are expressed in mg/m3 only. Refer to the Detector Calibration section of this document for information on converting WELs expressed in PPM to mg/m3.

When mixtures of toxic gases are encountered the effects on health are often additive and this needs to be taken into account (exposure to two gases with similar effects, each at 50% of their WELs may be equivalent to working at a WEL or the two gases together may have an enhanced effect). There is a detailed explanation of Mixed Exposures in EH40/2005.

Gases and vapours produced, under many circumstances, have harmful effects on workers exposed to them by inhalation, being absorbed through the skin, or swallowed. Many toxic substances are dangerous to health in concentrations as little as 1ppm (parts per million). Given that 10,000ppm is equivalent to 1% volume of any space, it can be seen that an extremely low concentration of some toxic gases can present a hazard to health. But what are the characteristics of the gases?

Characteristics of Toxic Gases:

Want to know more about Toxic Gases? Check out our Monitoring for toxic gases or Toxic gas exposure limits and alarm levels articles.

Got questions specific to your industry, application or business? Contact us for more information!

Oxygen monitors usually provide a first-level alarm when the oxygen concentration has dropped to 19% volume. Most people will begin to behave abnormally when the level reaches 17%, and hence a second alarm is usually set at this threshold. Exposure to atmospheres containing between 10% and 13% oxygen can bring about unconsciousness very rapidly; death comes very quickly if the oxygen level drops below 6% volume.

The hazard presented by oxygen deficiency is easily under-estimated; especially as risks can exist in non-industrial environments such as cellars or bars where CO₂ and nitrogen are used. Oxygen depletion due to corrosion or bacterial activities presents a significant risk in confined spaces such as pipes, vessels, sewers and tunnels. Oxygen sensors are often installed in laboratories where inert gases (eg nitrogen) are stored in enclosed areas.

Oxygen Enrichment:

Increased levels of oxygen may dramatically increase the flammability of any combustible matter. If oxygen levels exceed 24% volume, even materials such as clothing which might normally just smoulder may burst into flame.

The risk from oxygen enrichment exists where pure oxygen is stored; for example in hospitals and industrial gas manufacturing and distribution plants. Oxygen sensors with rising alarms set at 23.5% volume are typically used in such environments.

The name gas comes from the word chaos, which neatly summarises the main feature of the simplest state of matter.

A gas is a swarm of particles moving randomly and chaotically, constantly colliding with each other and the walls of any container. The real volume of the particles is minute compared to the total space which they occupy, and this is why gases fill any available volume and are readily compressed. The average speeds of gas molecules are of the order of 100s of metres per second and they collide with each other billions of times per second. This is why gases mix rapidly and why they exert pressure.

This constant motion is easily demonstrated by releasing a small amount of odorous gas into a room. Within seconds the gas can be smelt in all parts of the room. These properties apply to evaporated liquids.

A volume of any gas at the same temperature and pressure contains the same number of molecules irrespective of what the gas is. This means that measuring gas by volume is very convenient. Gas measurements at high concentrations are in % (volume) and at low concentrations parts per million, ppm (volume).

Whilst different gases have different densities, they do not totally separate into layers according to their density. Heavy gases tend to sink and light gases tend to rise, but their constant motion means that there is continuous mixing (i.e., they do not collect together and repel other types as liquids often do).

So, in a room where there is a natural gas (methane) leak, the gas will tend to rise because it is lighter than air, but the constant motion means that there will be a considerable concentration at floor level. This will happen in perfectly still conditions but if there are any air currents, the mixing will be increased.

Air is a mixture of gases, typically:

Nitrogen 77.2 %
Oxygen 20.9 %
Water Vapour 0.9 % (dependent upon temperature)
Argon 0.9 %
Carbon Dioxide 0.04 % and rising at 0.0002% per year
Other Gases 0.07 %

Because its composition is reasonably constant, air with the composition listed above is usually considered as a baseline gas mixture. We measure deviations from this mixture which simplifies the measurement of toxic and flammable gases for safety and health applications.