Fire toxicity of Building products
From a report by Prof. Anna Stec, summarised by Chris Brookman
In this article we discuss the effect of fire on synthetic insulation materials rather than natural fibres such as wood fibre, as the impact of these products is a relatively recent phenomenon. Natural fibres tend to burn very slowly with by-products that would be found in any wood burning stove or fireplace, products that are fairly well understood and not generally regarded as hazardous.
The common view of whether a building product is ‘fire safe’ is generally based on whether the product burns or not. However, fire smoke actually causes more deaths than the fire itself and it is an area which is completely unregulated in construction. Most research focusses on preventing ignition and flame growth without any assessment of products released in the fire.
The use of combustible materials increases growth and severity of fires as well as producing higher concentrations of toxic substances. The use of plastics in construction has only be made possible by heavy dependence on flame retardants, a policy which is now coming under scrutiny.
UK Building control fire regulations do not provide specific guidance on what materials can and cannot be used and the onus is put on the designer to provide ‘appropriate means of escape’ and to provide a place of safety externally which can be ‘safely and effectively’ used at all times. However, the gases released by building materials have an enormous influence on whether occupants are still conscious and able to escape and therefore survive, whether the means of escape are appropriate or not.
Toxic Product Yields
There are two main factors which affect toxic product yields which are ventilation of the fire and the chemical composition of the materials involved. Generally, in fires within buildings there is limited oxygen available but also the heat is trapped within rooms, giving rise to temperatures very much higher than would typically be found in an open fire. It is these types of fires that we are discussing when looking at the toxic products released from the various products below.
Fire toxicity of insulation materials plays a pivotal role in fire safety as they are one of the most prevalent materials within a modern house. Materials such as Rigid Polyurethanes, Polyisocyanurates and Phenolic foams all contain Nitrogen, which can create Hydrogen Cyanide gas and Polystyrenes can release Benzene, a potent carcinogen. Glass wool and mineral wool are not immune from this issue and because of materials mixed with the fibres, still release toxic gases, albeit at much lower levels.
In a fire, these insulation materials can release irritant gases, such as Hydrochloric, Hydrobromic and Hydrofluoric acids, various NOx gases as well as Formaldehyde and Acrolein. Depending on the concentration they cause pain and burning in the eyes and nose as well as throat and lungs, all of which impede escape. In higher doses these may be fatal some hours after exposure due to lung inflammation.
As well as irritant gases, these products can produce asphyxiant gases, such as Hydrogen Cyanide and Carbon Monoxide. Above a threshold these gases rapidly cause confusion, loss of consciousness and death by asphyxiation. Hydrogen Cyanide can be considered a major cause of incapacitation in fires due to it’s potency. A concentration of 200 ppm can cause a loss of consciousness in around 1 minute whereas fifty times more Carbon Monoxide is required or 10,000 ppm, to have a similar effect.
Toxicity of common plastics
Common plastics found inside homes, such as acrylics, polystyrene and polythene all produce significant amounts of Carbon Monoxide, particularly in under ventilated fires such as those found within closed rooms. However, only the larger fires tend to produce lethal concentrations of this gas. PVC, which is used to make u-PVC windows and vinyl flooring amongst others, can give off lethal quantities of Hydrochloric Acid gas along with Carbon Monoxide, even in relatively small fires.
Some of the most significant levels of toxic gases are released by a product called Polyamide. This is commonly known as Nylon and is found in lots of clothing, such as heavy duty rain clothes and some stretchy fabrics used in sports wear. The levels of Hydrogen Cyanide gas given off by these products, even in a small fire, can be five to six times the lethal dose required.
The reason for Hydrogen Cyanide production is that these materials contain Nitrogen and at high temperatures, with limited oxygen, this gas forms. The flame retardants used preventing flaming and therefore burning off of the Hydrogen Cyanide gas that is produced in these high temperatures. However, not all flame retardants produce the same levels of these gases, with Brominated polystyrene and Antimony Trioxide produce by far the highest levels of these asphyxiant gases.
Similarly, Nitrogen containing Polyisocyanurate (PIR), Polyurethane (PUR) and Phenolic rigid foams which are widely used as insulation materials also produce levels of Hydrogen Cyanide gas in quantities well above the dose required to be lethal. Polystyrene insulants do not contain Nitrogen and so do not produce Hydrogen Cyanide but will still produce lethal levels of carbon monoxide and give off carcinogens such as Benzene.
Long Term Toxicity
In addition to the effects felt during or immediately after a fire there is the issue of long term toxicity and the impact on the body. Benzene and Benzo(a)pyrene, along with a long list of other similar chemicals, are classed as carcinogens and are produced in large quantities by building fires with these products in them.
A recent study of fire fighter’s has shown alarmingly high levels of these products are absorbed into the clothes and then skin as well as inhaled. Procedures are only now being put in to place to try and limit the exposure and spread of these carcinogens from the site of fires, back to the stations and homes of the fire fighters in a bid to reduce the already high incidences of cancer among them.
Where present, non-combustible fibres such as glass fibres are released by fires and are often coated with other toxic materials, such as Aluminium and Antimony from flame retardants. These fine fibres pass into the lungs, introducing carcinogens directly in to the body. This dust along with soot released by the fire is a hazard to everyone in the vicinity.
Fire toxicity is the biggest cause of death and injury in fires, rather than the fire itself, but as yet, there is no regulation to restrict the use of potentially toxic substances.
The composition of materials makes a big difference to the levels and types of toxic products produced by a fire. With the increase of products containing Chlorine (PVC) and Nitrogen (PIR, PUR) the number of fires with lethal levels of Hydrogen Cyanide and Hydrogen Chloride gases, as well as Carbon Monoxide has increased.
There is no safety advantage in adding halogenated flame retardants (flame retardants containing Bromine, Chlorine, Iodine, etc.) as the reduction in fire growth rate is compensated by a large increase in the release of toxic products.
The drive towards improving the thermal efficiency of buildings and the increases in quantities of insulation has not been matched by fire safety with the Building Regulations, which are now inadequate.
Finally, it is actually quite easy to assess fire toxicity, given the above information, and this should become an essential part of general fire hazard assessment.
Hopefully this article highlights how important material choices are, both within the rooms and the structure of the building. The rise of the use of plastics in construction, as with all other aspects of life, appears to reduce financial cost upfront but may come at a significant cost in a fire.
Natural fibre materials do not off-gas and can be used to improve indoor air quality. Natural fibre insulations tend to burn slowly, generally do not produce the levels of asphyxiant gases generated by plastics with flame retardants and therefore allow escape from fire by the normal means. Therefore, not only do they make better, more comfortable buildings but they are a healthier, safer alternative to many synthetic insulations.
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