Characteristics of Toxic 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.

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.

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

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.

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 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.

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 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.

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 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 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.

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).

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.

Oxyde nitrique

Dioxyde d'azote

1.04

1.60

NO

NO₂

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.

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.

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.

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).

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

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.

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.