FIRE’S DEADLY INGREDIENT – TOXIC SMOKE
FIVE TIMES MORE FIRE VICTIMS DIE FROM THE EFFECTS OF SMOKE AND GAS THAN FROM THE FLAMES
Statistics for fire deaths in most countries tell a tragic story. When the bodies of those who lose their lives in a fire are recovered, it is five times more likely that they died not from exposure to the flames but due to the effects of the toxic smoke and gases that were produced4.
Smoke of some description is the inevitable result of combustion, as intense heat melts or burns off various ingredients and releases them to the atmosphere. It follows, therefore, that the toxicity of the smoke produced by a fire is largely dependent on what is burning.
Every combustible product produces a smoke atmosphere that is toxic and in sufficiently high concentrations, over time, may present hazardous conditions to exposed humans. These hazards can cause impaired vision, narcosis from inhalation of asphyxiant gases, and irritation of the upper and/or lower respiratory tracts.
These effects, often occurring simultaneously in a fire, contribute to loss of mental acuity and motor coordination, disorientation, panic and eventually physical incapacity. The resulting delay or prevention of escape may lead to subsequent injury or death from further inhalation of toxic gases and/or the suffering of thermal burns.
A major design requirement for any occupied enclosure such as a building is to ensure that occupants are able to escape safely in case of fire. The main cause of injury and death in fires is exposure to toxic smoke and gases, while the next most important cause is exposure to heat. It is therefore necessary to ensure that the performance of a building and its systems, including any combustible structural products or contents, is such that occupants are able to escape before they are overcome by toxic smoke or heat2.
An asphyxiant is a toxicant causing hypoxia (a decrease in oxygen supplied to, or utilized by, body tissue), resulting in central-nervous-system depression with loss of consciousness and, ultimately, death. Exposure to asphyxiant gases is the main cause of incapacitation and death during and immediately after a fire.
Carbon monoxide (CO) and hydrogen cyanide (HCN) are the two major asphyxiant gases. While the onset of CO intoxication is slow and insidious, HCN intoxication tends to be more rapid. It is, after all, 25 times more toxic than carbon monoxide. A short exposure to a high concentration of hydrogen cyanide is much more hazardous than a longer exposure to a lower concentration.3
Some toxic or physical effects of exposure to combustion products occur almost immediately on exposure, and the severity of the effect is proportional to the concentration of the substance and its potency. For substances such as asphyxiant gases, the effect depends upon the dose inhaled.
These are gases or aerosols that stimulate nerve receptors in the eyes, nose, mouth, throat and respiratory tract, causing varying degrees of discomfort and pain.3 Though irritant gases do not have a direct lethal effect they can affect the ability to escape.
Since hazards from toxic effluent are the main causes of incapacitation and death in fires it may seem sensible to regulate for toxic hazard or toxicity, with respect to both the overall design and performance of the built environment and the individual products used in their structure and contents2.
Materials incapable of combustion due to a very low organic content, or products failing to contribute to flaming combustion, are unlikely to support the formation of a large growing, hazardous fire in the end-use application. Because of this, they are unlikely to contribute a significant amount of smoke or heat to a rapidly developing fire hazard.2
While it is recognised that keeping fires small should be part of the general approach, it should not be the only route to ensure life safety. The only possible way to ensure complete fire safety of a building’s environment, without the consideration of smoke toxicity, would be to prescribe the use of only non-combustible products.
So why isn’t control of smoke toxicity a regulatory requirement in most countries? There are two possible reasons for this:
- Until recently there has not been an appropriate test or calculation model available. This changed with the publication of ISO/TS19700 and ISO 13571.
- It is not possible to assign a toxicity ranking to products directly.
In order to evaluate the fire safety of any built environment, or any combustible material or product used within that environment, it is therefore necessary to consider the overall fire performance of the system – either by performing a full-scale fire test on the end-use system or by modelling the fullscale fire conditions using suitable fire dynamic models run with input data on a range of performance parameters.2 Consequently, smoke toxicity needs to be considered not in isolation but in association with fire safety engineering.
A study7 comparing thermal insulation products has shown that during fire polyurethane (PIR)and polyisocyanurate (PUR)will produce the most toxic smoke due to high yields of hydrogen cyanide while Phenolic Foam and expanded polystyrene (EPS) will produce a moderately toxic smoke. Stone wool and glass wool both showed no significant toxic smoke in the tests. The study concludes that “… the contribution to the fire toxicity for either glass wool or stone wool is negligible compared to that from any of the foam products.”
- D. Purser, “Physiological effects of combustion products; Hazards of combustion products”, 10 —11 November 2008, The Royal Society of London, Interscience Communications.
- ISO 13571-2007 ”Life-threatening components of fire — Guidelines for the estimation of time available for escape using fire data”. ISO.
- D. Purser, “Toxicity Assessment of Combustion Products”, SFPE Handbook 3rd edition.
- Anna A. Stec & T. Richard Hull, “Assessment of the fire toxicity of building insulation materials”, Building and Energy, October 2010.