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Fire prevention and occupational safety How can of hypoxia affect humans in active fire prevention and what are the implications for occupational safety? Following on from last issue’s article, Barry Bell outlines the facts about under exposure to oxygen.
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n the previous edition of MENA Infrastructure, we talked about the concept of fi re prevention that involves a permanent reduction of oxygen within a specific enclosure. Having described the concept of fi re prevention and made comparison to the more conventional, fi re extinguishing methodology that employs gaseous suppression systems, we began to look more closely at the effects of lowering oxygen levels on the human body, better known as hypoxia. In this edition, we continue the topic from where we left off in April. Hypoxia means a shortage of oxygen — as opposed to anoxia, which means a total lack of it. In common with other mammals, humans have evolved with a system of breathing and blood circulation, which allows intake of oxygen from the air and its transport throughout the body. Our body tissues need to extract oxygen constantly from the blood, at a basic rate for their metabolism, as well as the additional oxygen needed for work and exercise. Our bodies are accustomed to a certain level of oxygen being available in the immediate environment. The body can however compensate to some extent for a decreased level of oxygen supply, but life remains dependant on maintaining an adequate supply of oxygen. Different organs and tissues can survive lack of oxygen for varying lengths of time: of all our organs, the brain is the most rapidly and irrevocably damaged by severe oxygen starvation. Because the brain regulates both our breathing and blood circulation – the means by which oxygen is supplied to the whole body – deprivation of oxygen to the brain will prevent restoration of the oxygen supply as a whole; a potentially lethal vicious circle. Hypoxia can occur for a variety of reasons, such as when there is less than the normal amount of oxygen in the air inhaled; when breathing is obstructed, is inadequate, or stops; when oxygen is not transferred normally from the lungs to the blood; when the blood cannot carry its normal quota of oxygen; or when the flow of blood is inadequate, or stops. The air inhaled may provide insufficient oxygen either because the atmospheric pressure is low (at high altitude) or when the supply of fresh air is restricted. At high altitude the air is ‘thinner’ in that every molecule of the gas occupies a larger volume. The blood leaves the lungs carrying
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less oxygen than normal, therefore the tissues are exposed to a lower oxygen level. If this is not too profound, the tissues can still obtain oxygen at the required rate, at least for resting metabolism, because the rate of flow of blood can increase. The tissues are living at a lower oxygen level but are still getting a sufficient oxygen supply. When the supply of fresh air is restricted, for example by placing a bag over the head, or through confi nement in a small enclosed space, or even in a larger enclosed space crowded with people, oxygen is progressively depleted and exhaled carbon dioxide accumulates. In some circumstances there may be displacement of air by other gases, and the effect of irritant or toxic gases, such as smoke, chlorine, or sulphur dioxide can complicate the effects of displacement of oxygen. Th is can occur for example, when a person remains in an enclosed space that is subjected to the oxygen depletion effects of a gaseous fi re suppression system, while at the same time being exposed to the smoke and toxic gases produced by the fi re prior to suppression taking place. ■
Barry Bell is the Managing Director of Wagner Fire Safety Management Consultants, Middle East, and a licensed Fire Engineering Consultant in the United Arab Emirates.
If you are interested to read more, log on to the on-line version of this editorial at www.menainfra.com
Hypoxia is often experienced by those at high altitude and in firefighting situations
16/09/2010 10:22