Toxic gases pose a significant threat to health and safety across many UK industries, from manufacturing and wastewater treatment to oil and gas operations. Often invisible and odourless, these gases can cause serious harm even at low concentrations, making early detection and proper safety measures essential. In this blog, we’ll explore the key characteristics of the most common toxic gases found in the UK, providing the knowledge needed to recognise risks and implement effective control measures.
| Gas | Formula | Characteristics | |
|---|---|---|---|
| Ammonia | NH₃ | The only common alkaline gas, ammonia is about half as dense as air and has a distinctive smell. Its maximum safe level is 25ppm. Its alkalinity makes it highly reactive with acidic gases and chlorine, often masking its presence in mixed-gas atmospheres. For example, equal concentrations of ammonia and chlorine produce ammonium chloride, removing both gases from the air. Ammonia is flammable (LEL: 15%) and produced globally for fertilisers, resins, explosives, and nylon. It is also used as a refrigerant, an application increasing as CFCs are phased out and to maintain water sterility after chlorine and sulphur dioxide treatment. Single Gas: Crowcon Gasman Detector | Multi Gas Monitor | Rockall Safety | |
| Arsine | AsH₃ | A colourless, flammable, highly toxic gas with a garlic or fishy smell detectable at 0.5ppm. Arsine causes no irritation or immediate symptoms, so dangerous exposure may go unnoticed. Transported in cylinders as a liquefied compressed gas, it forms when arsenic-containing metals or ores react with acids. It is used in semiconductor manufacturing for arsenic deposition on microchips. Fixed Gas: Crowcon Xgard Fixed Multi Gas System | Rockall Safety | |
| Bromine | Br₂ | Used to produce industrial and agricultural compounds, bromine appears in fumigants, flame retardants, water purifiers, dyes, medicines, and photographic bromides. It also forms brominated vegetable oil for use as an emulsifier in citrus soft drinks. Elemental bromine is a strong irritant that can cause painful skin blisters. Even 10ppm can affect breathing, and high inhalation levels may severely damage the respiratory system. | |
| Carbon Dioxide | CO₂ | Naturally present in the atmosphere (400ppm) and exhaled during breathing. The maximum safe level is 5,000ppm (0.5%). Produced during combustion, brewing, distillation, and fermentation, CO₂ is also a major component of landfill and digester gases. In breweries, its heavier-than-air nature allows it to collect at low levels, posing a hazard. CO₂ also increases plant growth in greenhouses. It is colourless, odourless, and usually detected using infrared absorption. Single Gas: Dräger PAC 8000 Gas Detector | Rockall Safety | |
| Carbon Monoxide | CO | An odourless, colourless gas similar in density to air, carbon monoxide is the most common toxic gas and mixes readily in the atmosphere. Known as the “silent killer,” it results from incomplete combustion common in engines, boilers, and fires. In industry, CO is used as a reducing agent (e.g., steel production) and in methanol production. Single Gas: BW Clip 2 Yr Gas Detector | O2, H2S, CO & SO2 | Rockall Safety | |
| Chlorine | Cl₂ | A green-yellow, pungent, corrosive gas used mainly for water purification, PVC production, and bleaching. Single Gas: Crowcon Gasman Detector | Multi Gas Monitor | Rockall Safety | |
| Chlorine Dioxide | ClO₂ | A reddish-yellow gas prone to explosive decomposition into chlorine and oxygen. Mainly used in pulp bleaching, it also disinfects water, sometimes alongside ozone to reduce bromate formation. In industry, it serves as a biocide for cooling towers, process water, and food processing. Inhalation irritates the nose, throat, and lungs. Single Gas: BW Solo Single Gas Detector | Honeywell Gas Detectors | |
| Diborane | B₂H₆ | Has an odour threshold of 2–4ppm but an exposure limit of just 0.1ppm. Long-term low exposure can cause olfactory fatigue, reducing smell detection. Lighter than air, diborane can irritate skin, eyes, and the respiratory system in enclosed areas. It is used in rocket fuel, rubber vulcanisation, hydrocarbon polymerisation, flame-speed acceleration, and semiconductor doping. | |
| Ethylene Oxide (ETO) | C₂H₄O | A colourless, flammable gas with a faintly sweet smell. Used to make ethylene glycol (antifreeze) and to sterilise food and medical equipment, ETO kills bacteria, mould, and fungi without heat. Inhalation can cause headaches, dizziness, seizures, or pulmonary oedema (fluid in lungs). | |
| Fluorine | F₂ | Used in semiconductor etching and water fluoridation (1ppm) to prevent tooth decay. Fluorine compounds also appear in toothpaste. Highly toxic, fluorine must be handled with extreme care. As a strong oxidiser, it can ignite flammable materials on contact. | |
| Germane | GeH₄ | Flammable, toxic, and an asphyxiant, germane burns to form GeO₂ and water. It is used in semiconductor manufacturing for germanium layer growth. | |
| Hydrogen Cyanide | HCN | A colourless gas with a sweet smell, hydrogen cyanide is a well-known poison. The safe limit for 15 minutes is 10ppm. It is mainly used in gold refining. | |
| Hydrogen Chloride | HCl | A toxic, corrosive gas forming white fumes of hydrochloric acid upon contact with moisture. Used in pharmaceuticals, semiconductors, and rubber treatment, HCl can cause severe respiratory damage. Emissions can result from burning PVC. Single Gas: Dräger X-am 5100 Gas Detector | Rockall Safety | |
| Hydrogen Fluoride | HF | Used in refining, glassmaking, aluminium production, and metal finishing. HF irritates skin, eyes, and the respiratory system. High exposure can damage lungs, heart, and kidneys, and can be fatal. | |
| Hydrogen Sulphide | H₂S | Known for its “rotten egg” smell detectable below 0.1ppm, but high levels (>60ppm) cause smell paralysis. Slightly heavier than air, it is found in oil extraction, sewage treatment, and decaying organic matter. Single Gas: BW Solo Single Gas Detector | Honeywell Gas Detectors | |
| Methyl Mercaptan | CH₃SH | Added to natural gas to aid leak detection, it smells like rotten cabbage. Also used in fuels, pharmaceuticals, and pesticides. Less toxic than H₂S, with exposure limits from 0.5ppm (NIOSH) to 10ppm (OSHA). No UK workplace limit set. | |
| Nitrous Oxide / Nitric Oxide / Nitrogen Dioxide | N₂O, NO, NO₂ | N₂O (“laughing gas”) has an LTEL of 100ppm and is used as an anaesthetic and analgesic. NO and NO₂ (collectively NOx) contribute to acid rain and result mainly from fuel combustion. NO is colourless; NO₂ is a pungent brown gas. | |
| Ozone | O₃ | An unstable gas often generated for water treatment as a chlorine alternative. Detectable at low ppm using electrochemical sensors. | |
| Phosgene | COCl₂ | An industrial chemical for plastics, dyes, and pesticides. Heavier than air, phosgene smells like cut hay at low levels but is not a reliable warning. It can damage skin, eyes, and lungs. | |
| Phosphine | PH₃ | Highly toxic, with a STEL of 0.3ppm. Used for fumigation and in semiconductor manufacturing. | |
| Silane | SiH₄ | Pyrophoric (self-igniting in air) gas used in waterproofing, adhesives, and semiconductor production. Exposure can cause headaches, nausea, and severe irritation. | |
| Sulphur Dioxide | SO₂ | Colourless, with a choking smell. Produced by burning sulphur-containing fuels and a cause of acid rain. Found in industrial areas and used in water treatment and food processing. Twice as heavy as air. | |
| Sulphur Hexafluoride | SF₆ | Used as an insulating gas in electrical equipment. While stable, electrical arcing can produce toxic by-products like S₂F₁₀. SF₆ is also used in semiconductors and magnesium production. The UK WEL is 1,000ppm (8 hours). | |
| Volatile Organic Compounds (VOCs) | Various | VOCs are emitted from certain solids or liquids, including cleaning products, fuels, building materials, and office equipment. Exposure can irritate eyes, nose, and throat, cause headaches, and damage internal organs. Common VOCs include acetone, benzene, and toluene. Detectable using PID or electrochemical sensors. | |
| Freons | Various | Carbon compounds with chlorine, fluorine, or bromine, used as refrigerants, propellants, and solvents. Many Freons are CFCs, which persist in the atmosphere and deplete the ozone layer. Detectable with semiconductor or infrared sensors.
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