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Characteristics of Toxic Gases

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:
Ammonia 0.59 NH3 Ammonia is the only common alkaline gas. Its density is about half that of air and it has a characteristic smell. Its maximum safe level is 25ppm but its alkalinity makes it highly reactive with acid gases and chlorine and its presence in atmospheres containing other gases is often masked by this. For instance, if ammonia and chlorine are present in equal concentrations, the result is a cloud of ammonium chloride and neither of the two gases.

Ammonia is flammable with an LEL of 15%. It is produced in vast quantities all over the world to provide fertilisers, urea for resins, explosives and fibres such as nylon. It is also used as a refrigerant gas: this application is increasing with the demise of CFCs. Another application is to maintain the sterility of water supplies after treatment with chlorine and sulphur dioxide.


Arsine 2.7 ASH3 Arsine is a colourless, flammable, and highly toxic gas. It has a garlic-like or fishy odour that can be detected at concentrations of 0.5ppm and above. Because arsine is not irritating and produces no immediate symptoms, persons exposed to hazardous levels may be unaware of its presence.  It is generally shipped in cylinders as a liquefied compressed gas. Arsine gas is generated when metals or crude ores containing arsenic impurities are treated with acid. Arsine gas is also used in the semiconductor industry when depositing arsenic on microchips.


Bromine 5.5 Br2 Bromine is used to manufacture a wide variety compounds used in industry and agriculture. Bromine is also used in the manufacture of fumigants, flame-proofing agents, water purification compounds, dyes, medicines, sanitizers, inorganic bromides for photography, etc. It is also used to form intermediates in organic synthesis, where it is preferred to iodine due to its much lower cost.

Bromine is used to make brominated vegetable oil, which is used as an emulsifier in many citrus flavoured soft drinks.

Elemental bromine is a strong irritant and, in concentrated form, will produce painful blisters on exposed skin and especially mucous membranes. Even low concentrations of bromine vapour (from 10 ppm) can affect breathing, and inhalation of significant amounts of bromine can seriously damage the respiratory system.

Carbon Dioxide 1.53 CO2 Despite the fact that we breathe out carbon dioxide and that it is present in the atmosphere to the extent of about 400ppm, its maximum safe level is 5000ppm (0.5%). It is produced during combustion and in brewing, distillation and other fermentation processes, and is one of the main constituents, with methane, of landfill gas and sewage treatment digester gas. CO2 presents a significant hazard in the brewing industry, particularly as the gas is heavier than air and collects at low levels. There is some degree of risk in crowded, badly ventilated places, and this problem is often worsened by oxygen deficiency. CO2 is also used to increase plant growth by elevating normal levels in greenhouses etc.

It is odourless and colourless and difficult to measure in ppm levels. Infrared absorption is the usual detection technique adopted.

Carbon Monoxide 0.97 CO Carbon monoxide, which is odourless and colourless, is the most abundant toxic gas. Having a similar density to air, it mixes easily and is readily inhaled. It is a renowned “silent killer” in domestic environments.

Any process where there is incomplete combustion of carbon fuel is likely to produce carbon monoxide. For example: petrol and diesel engines, coal, gas and oil boilers, and even smoking. Its presence in mines is due to the slow combustion of coal.


It is also used in enormous quantities as a cheap chemical reducing agent, for example in steel production and other metal refining and heat treatment processes,  and in the production of methanol by reaction with hydrogen.

Chlorine 2.5 Cl2 Chlorine is a pungent smelling, corrosive, green/yellow gas. The best known use is in water purification for domestic supplies and in swimming pools. It is used to make chlorinated compounds such as PVC, and for bleaching paper and fabrics. It is a very heavy gas and is readily absorbed by most materials.

The behaviour of chlorine makes it a very difficult gas to detect (so difficult that even calibration requires special techniques).

Use of the Crowcon Environmental Sampling Unit is an effective way to detect it in chlorine stores successfully and this minimises the number of detectors required.


Chlorine Dioxide 2.3 ClO2 Chlorine dioxide is a reddish-yellow gas which is one of several known oxides of chlorine. It can spontaneously and explosively decompose into chlorine and oxygen. Chlorine dioxide is used primarily for bleaching of wood pulp, but is also used for the bleaching of flour and for the disinfection for water.

Chlorine dioxide is also used in conjunction with ozone for disinfection of water to reduce the formation of bromates which are regulated carcinogens.

Chlorine dioxide is used in many industrial water treatment applications as a biocide including cooling towers, process water and food processing. If inhaled, Chlorine dioxide gas causes irritation in your nose, throat, and lungs.


Diborane 0.96 B2H6 Diborane has an odour threshold between 2 and 4ppm, which is significantly higher than the exposure limit of 0.1ppm. Prolonged low-level exposures, such as those that occur in the workplace, can lead to olfactory fatigue and tolerance of diborane’s irritant effects.

As with all toxic gases the odour does not provide adequate warning of hazardous concentrations.

Diborane is lighter than air and exposure may result in skin, respiratory, and eye irritation in poorly ventilated, enclosed, or low-lying areas. Diborane is used in rocket propellants, as a rubber vulcanizer, as a catalyst for hydrocarbon polymerization, as a flame-speed accelerator, and as a doping agent for the production of semiconductors.

Ethylene Oxide (ETO) 1.52 C2H4O Ethylene oxide is used as an intermediate in the production of ethylene glycol; which is widely used as an automotive coolant and antifreeze. It is also used to sterilize foodstuffs and medical supplies. It is a colourless flammable gas or refrigerated liquid with a faintly sweet odour. Ethylene oxide gas kills bacteria, mould, and fungi, and can be used to sterilize substances that would be damaged by sterilizing techniques such as pasteurization that rely on heat. Additionally, ethylene oxide is widely used to sterilize medical supplies such as bandages, sutures, and surgical implements.

Ethylene oxide is toxic by inhalation. Symptoms of overexposure include headache and dizziness, progressing with increasing exposure to convulsions, seizure and coma. Inhalation may cause the lungs to fill with water several hours after exposure.

Fluorine 1.3 F2 Atomic fluorine and molecular fluorine are used for plasma etching in semiconductor manufacturing and flat panel display production. Fluorine is added to some city water supplies in the proportion of about one part per million to help prevent tooth decay.

Compounds of fluorine, including sodium fluoride, are used in toothpaste to prevent dental cavities. Fluorine is highly toxic and must be handled with great care and any contact with skin and eyes should be strictly avoided.

Fluorine is a powerful oxidizer which can cause organic material, combustibles, or other flammable materials to ignite.


Germane 2.65 GeH4 Germane burns in air to produce GeO2 and water.

Germane is used in the semiconductor industry for the epitaxial growth of germanium by MOVPE or chemical beam epitaxy. Germane is flammable, toxic and is an asphyxiant.


Hydrogen Cyanide 0.94 HCN Hydrogen Cyanide is a well-known poison that is a colourless, sweet smelling gas with a 15 minute maximum safe level of 10ppm. The main industrial application is in gold refining.


Hydrogen Chloride 1.3 HCL Hydrogen chloride is a highly corrosive and toxic colourless gas that forms white fumes on contact with moisture. These fumes consist of hydrochloric acid which forms when hydrogen chloride dissolves in water. Hydrogen chloride gas as well as hydrochloric acid are important in industry, especially pharmaceuticals, semiconductors, treatment of rubber and cotton. It is also emitted from waste incinerators in which PVC is burnt. Inhalation of the fumes can cause coughing, choking, inflammation of the nose, throat, and upper respiratory tract, and in severe cases, death.


Hydrogen Fluoride 0.92 HF Hydrogen Fluoride is used for petroleum refining, glassmaking, aluminium manufacturing, titanium pickling, quartz purification, and metal finishing.

Hydrogen fluoride causes eye, nose and skin irritation. Breathing in a large amount of HF also can harm the lungs, heart and kidneys and ultimately can cause death. It can also burn the eyes and skin.


Hydrogen Sulphide 1.2 H2S Hydrogen sulphide is well known because of its bad egg odour, which can be smelt down to less than 0.1ppm. High concentrations (>60ppm) cannot be smelt due to paralysis of the olfactory glands, and exposure can lead to instant paralysis. H2S is slightly heavier than air, and thus fixed detectors are usually mounted 1 to 1.5 metres from the ground, or near potential sources of leaks.

H2S is produced during the decay of organic materials, extracted with oil (when the oil is said to be sour) and is often found underground during tunnelling and in sewers. It is a constituent of biogas and found in large quantities in sewage treatment works, pumping stations, press houses, boiler houses, and virtually anywhere where sewage is being treated. It has some industrial uses and is produced as a by-product in others (e.g. fibre manufacture).

Methyl Mercaptan 1.66 CH3SH Mercaptan is added to natural gas (methane) to make it easier to detect in case of a leak: natural gas in its native state is colourless and odourless. Mercaptan contains sulphur, and has a strong odour similar to rotten cabbage or bad eggs.  By adding mercaptan to natural gas, any leaks from boilers, furnaces and hot water heaters are easily detected without needing expensive equipment.

Other uses for mercaptan in industry include jet fuel, pharmaceuticals, livestock feed additives, chemical plants, the plastics industry and pesticides. It is a natural substance found in the blood, brain, and other tissues of people and animals. It is released from animal faeces. It occurs naturally in certain foods, such as some nuts and cheese. Mercaptan is less corrosive and less toxic than similar sulphur compounds (H2S).

Maximum recommended exposure levels inNorth Americarange from 0.5ppm (NIOSH 15 minute limit) to 10ppm (OSHA Permissible Exposure Limit).

The UK Heath and Safety Executive has not set a value for a workplace exposure limit.

Nitrous Oxide

Nitric Oxide

Nitrogen Dioxide







There are three oxides of nitrogen. Nitrous oxide (or Laughing Gas) has an LTEL (according to document EH40) of 100ppm. It does not have aSTEL value. Death can result if it is inhaled in such a way that not enough oxygen is breathed. Inhaling industrial-grade nitrous oxide is also dangerous, as it contains many impurities and is not intended for use on humans. Nitrous oxide is a weak general anaesthetic, and is generally not used alone in anaesthesia. However, as it has a very low short-term toxicity and is an excellent analgesic, so a 50/50 mixture of nitrous oxide and oxygen is commonly used during childbirth, for dental procedures, and in emergency medicine.

Nitric oxide (modern name nitrogen monoxide) and nitrogen dioxide are the constituents of so called NOx which with sulphur dioxide causes acid rain. The main causes of these gases in the atmosphere is from combustion of fossil fuels in vehicle engines and power stations. At the point of exhaust, nitric oxide accounts for about 90% of NOx. However it reacts spontaneously with oxygen in the open atmosphere to produce nitrogen dioxide. Nitric oxide is a colourless gas but nitrogen dioxide is an acid, pungent smelling, brown gas.


Ozone 1.6 O3 Ozone is an unstable gas, and is generated as it is required. It is increasingly used instead of chlorine for the treatment of water. It can be detected at low ppm levels electrochemically.


Phosgene 3.48 COCL2 Phosgene is a major industrial chemical used in the production of plastics, dye and pesticides. It is also used in the pharmaceutical industry. Phosgene gas may appear colourless or as a white to pale yellow cloud. At low concentrations, it has a pleasant odour of newly mown hay or green corn, but its odour may not be noticed by all people exposed. At high concentrations, the odour may be strong and unpleasant.

As with all toxic gases the odour does not provide adequate warning of hazardous concentrations.

Phosgene gas is heavier than air, so it would be more likely found in low-lying areas.

Phosgene gas can damage the skin, eyes, nose, throat, and lungs.

Phosphine 1.2 PH3 Phosphine is highly toxic, and hence it has anSTEL of only 0.3ppm. Phosphine gas is used for pest control by fumigation. Phosphine is also used for in the semiconductor industry.


Silane 1.3 SiH4 Silane at room temperature  is a gas, and is pyrophoric which means it undergoes spontaneous combustion in air, without the need for external ignition.

Several industrial and medical applications exist for silanes. For instance, silanes are used as coupling agents to adhere glass fibres to a polymer matrix, stabilizing the composite material. Applications include water repellents, masonry/ concrete sealant and protection, control of graffiti, and applying polycrystalline silicon layers on silicon wafers when manufacturing semiconductors, and sealants. Health effects include headache, nausea, and major skin, eye and respiratory tract irritation.


SulphurDioxide 2.25 SO2 Sulphur Dioxide is colourless and has a characteristic choking smell. It is formed when burning sulphur, and materials containing sulphur such as oil and coal. It is highly acidic, forming sulphuric acid when dissolved in water. Along with the oxides of nitrogen, it is a cause of acid rain.

SO2 is found in industrial areas and near power stations, and it is a raw material for many processes. It has a use in the treatment of water to displace excess chlorine and because of its sterilising properties it is used in food processing. It is twice as heavy as air and it tends to fall to ground level so a Crowcon Environmental Sampling Unit sited near the ground ensures rapid detection in the event of a leak.

Note: Sulphur trioxide S03 is encountered in the exhaust from power stations. It is not a gas but a solid, which readily sublimes (i.e. it goes from the solid state to the gas state on heating).


Sulphur Hexa-fluoride 5 SF6


SF6 is used in the electricity industry as a gaseous insulating medium, highly resistant to electric current, for high-voltage circuit breakers, switchgear, and other electrical equipment. SF6 gas under pressure is used as an insulator in gas insulated switchgear (GIS) because it has a much higher dielectric strength than air or dry nitrogen. Although most of the decomposition products tend to quickly re-form SF6, arcing or corona can produce disulfur decafluoride (S2F10), a highly toxic gas, with toxicity similar to phosgene.

SF6 plasma is also used in the semiconductor industry as an etchant, and in the magnesium industry. It has been used successfully as a tracer in oceanography to study diapycnal mixing and air-sea gas exchange. It is also emitted during the aluminium smelting process.

When SF6 is inhaled, the pitch of a person’s voice decreases dramatically because the speed of sound in SF6 is considerably less than it is in air. This is a similar effect to that of Nitrous Oxide. As SF6 is five times heavier than air it displaces the oxygen needed for breathing. Trace amounts of toxic sulphur tetrafluoride, might have severe health effects.

The UK Heath and Safety Executive has set a value for the 8 hour workplace exposure limit (WEL) of 1000ppm

Volatile Organic Compounds n/a VOC’s Volatile organic compounds (VOCs) are emitted as gases from certain solids or liquids. VOCs include a variety of chemicals, some of which may have short and long-term adverse health effects. VOCs may be encountered in domestic or commercial indoor environments due to emissions from household cleaning products, pesticides, building materials, office equipment such as copiers and printers, graphics and craft materials including glues and adhesives, permanent markers, and photographic solutions.

Fuels are made up of organic chemicals, and can release organic compounds while in use and, to some degree, when they are stored.

Health effects include eye, nose, and throat irritation, headaches, loss of coordination, nausea, damage to liver, kidney, and central nervous system. Key signs or symptoms associated with exposure to VOCs include nose and throat discomfort, headache, and skin reaction.

As with other pollutants, the extent and nature of the health effect will depend on many factors including level of exposure and length of time exposed.

Common VOCs are Acetaldehyde, Butadiene, Carbon disulphide, Dimethyl sulphide, Ethanol, Ethylene, Methanol, Methyl mercaptan, Toluene, Vinyl acetate, acetone, benzene, ethyl acetate, methylamine, methyl ethyl ketone, tetrachloroethylene and vinyl chloride. VOCs can be detected using PID sensors or in some cases electrochemical sensors.



In general, Freons are carbon compounds containing chlorine, fluorine and/or bromine. Freons are used widely in industry because of their high densities, low boiling points, low viscosity, and low surface tension. In addition, they are easily liquefied making them ideal for use as refrigerants and solvents.  Freons are widely used as solvents, propellants, fire extinguishers and blowing agents.

Freon gases are classified using ‘R’ numbers. For example R125 is Pentafluoroethane (CHF2-CF3).

Freon compounds include chlorofluorocarbons or CFCs. The property of inertness which makes CFCs so useful in industry has proved to be the factor that makes them so dangerous to the planet. CFCs do not naturally biodegrade and as a result, once emitted, they persist in the atmosphere contributing to the depletion of the ozone layer. Freons can be detected using semiconductors or IR sensors.

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

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