In a motor vehicle or aircraft fire, flames are not just the only danger. We look at ten of the more common toxic gases associated with such fires.
When someone rescues a driver and/or passengers from a burning vehicle, risking their own safety in the process, we must truly acknowledge that person’s selfless act of courage, not just from the flames but also from the toxic gases created by the fire. 1. Carbon Monoxide (CO): Carbon monoxide is the main toxic gas produced from the combustion of polyethylene and other organic materials that are made up of carbon and hydrogen atoms. Carbon monoxide is produced as a result of incomplete combustion of materials containing carbon and is present in large quantities at most fires. Carbon monoxide that is inhaled causes asphyxiation by combining with haemoglobin in a reversible reaction to form arboxyhaemoglobin. Its formation at the expense of oxyhaemoglobin reduces the availability of oxygen for the cellular systems of the body. Anoxaemia induced by carbon monoxide does not, as with simple asphyxiants, cease as soon as fresh air is inhaled. After even moderate degrees of gassing, only about 50 percent of the carbon monoxide is eliminated in the first hour under ordinary circumstances; complete elimination under the action of fresh air is not effected for many hours. The highest concentration of CO to which man may be exposed day after day without adverse effect is 50 ppm. Above this level, symptoms such as headache, fatigue and dizziness appear in healthy individuals.2. Carbon Dioxide (CO2): Carbon dioxide is produced in quantity at most building fires. Inhalation of carbon dioxide stimulates respiration and this in turn increases inhalation of both oxygen and possible toxic gases and vapours produced by fires. Stimulation is pronounced at 5 percent (50 000 ppm) concentration, and 30 minute exposure produces signs of intoxication. Above 70 000 ppm, unconsciousness results in a few minutes. The threshold limit for CO2, that is the concentration that can be tolerated by workers day after day without adverse effect, is 5 000 ppm.
3. Hydrogen cyanide (HCN): Hydrogen cyanide is produced when materials that contain nitrogen in their structure, e.g. orlon, nylon, wool, polyurethane, urea-formaldehyde and ABS (acrylonitrile-butadiene-styrene) are involved in fire. Hydrogen cyanide and other cyanogen compounds arrest the activity of all forms of living matter and they exert an inhibiting action on the use of oxygen by the living cells of the body tissues.
4. Hydrochloric acid (HCl): Hydrogen chloride is produced by the thermal breakdown of polyvinyl chloride (PVC) or vinyl chloride. Hydrogen chloride is more toxic than CO and may be produced in greater quantity than CO when PVC burned. If inhaled, HCl will damage the upper respiratory tract and lead to asphyxiation or death.
5. Nitrogen Dioxide (NO2 and N2O4): There are three common oxides of nitrogen: nitrous oxide (N2O), nitric oxide (NO), and the two forms of the dioxide (NO2 and N2O4). Nitrogen dioxide, which is very toxic, can be produced from the combustion of cellulose nitrate (a compound used as a film base for photographic purposes). Nitric oxide does not exist in atmospheric air because it is converted into dioxide in the presence of oxygen. These compounds are strong irritants, particularly to mucous membranes and thus when inhaled will damage tissues in the respiratory tract by reacting with moisture to produce nitrous and nitric acids.
6. Styrene (C8H8/ C6H5CH=CH2): Styrene is widely used in a variety of products including synthetic rubber, paint and plastics. It is also used to make polystyrene, a polymer used to make materials such egg cartons, plastic CD jewel cases, computer covers, etc. When polystyrene is decomposed by heat, the major products are carbon monoxide and styrene, the compound from which it was produced. Styrene is broken down into smaller molecules that react with oxygen to form the usual combustion products. However, styrene will be present in smaller quantities.
7. Phosgene (COCl2): Phosgene is a highly reactive colourless gas at room temperature and ambient pressure, and has a suffocating odour similar to mouldy hay. The odour may be detected between 1.6 and 6 mg/m3. Phosgene is produced by the thermal degradation of some chlorinated solvents and chlorinated polymers such as PVC. However, a significant source of phosgene is the photochemical oxidation of chloroethylenes such a tri- and tetraethylene.
8. Isocyantes: Isocyanates are the raw materials from which all polyurethane products are made. Common materials produced from isocyanates include polyurethane foam, insulation materials, surface coatings, car seats, furniture, foam mattresses, under-carpet padding, packaging materials, shoes, laminated fabrics, polyurethane rubber, adhesives, and other polyurethane products. Two important isocyanates that are commonly used are toluene diisocyante(TDI) or methylene bis-phenylisocyanate(MDI). TDI is used to make soft, flexible foams for padding or insulation while MDI is used mainly to make hard, rigid foams for insulation in buildings, vehicles, refrigeration equipment, and industrial equipment. TDI is especially hazardous because it can evaporate quickly and its vapors are heavier than air. TDI thermally decomposes into toxic fumes of carbon monoxide, hydrogen cyanide, carbon dioxide, and nitrogen oxides are produced during combustion. Isocyanates chemically react to form a solid polyurethane foam or a plastic coating. The finished product is almost non-toxic, unless it is burned or caused to generate a dust. Any polyurethane material will give off isocyanates and other highly toxic substances if it is burned or welded.
9. Perfluoroisobutylene (C4F8): Perfluoroisobutylene is a colorless gas that is produced during the thermal degradation of polytetrafluoroethylene (PTFE), better known by the trade name Teflon®. PTFE is used to make non-stick cooking pans, and anything else that needs to be slippery or non-stick. PTFE is also used to treat carpets and fabrics to make them stain resistant. What’s more, it’s also very useful in medical applications. Because human bodies rarely reject it, it can be used for making artificial body parts.
10. Acetaldehyde (C2H4O/CH3CHO): The substance may polymerize under the influence of acids, alkaline materials, such as sodium hydroxide, in the presence of trace metals (iron) with fire or explosion hazard. Vehicle tyres
Tyres are a mixture of vulcanized or cross-linked polymers, carbon black, dispersing oil, sulphur, synthetic fibers, pigments, processing chemicals, and steel or fiberglass. Tyre manufacturers use a variety of formulation recipes when producing tyres. Nonetheless, Table 1 (below) lists the typical tyre composition of a standard passenger car tyre.
Table 1: Typical tyre composition of a passenger tyre
Material Percentage (%)
Styrene butadiene 46.78%
Carbon black 45.49%
Aromatic oil 1.74%
Zinc oxide 1.40%
Stearic acid 0.94%
Antioxidant 6C 1.40%
Wax 0.23%
Sulphur 1.17%
Accelerator CZ 0.75%
• Kaplan, H. L., et al., Combustion Gases in Postcrash Aircraft Fires (DOT/FAA/CT-84/16), Federal Aviation Administration, Atlantic City (1984).
