For these to occur, there is a need for combustible material, oxygen and a source of ignition. The risk of these being present results from the use of high oxygen partial pressures and the use of inflammable anaesthetic agents or other inflammable materials. If the pressure of any gas is increased, heat is liberated. If the gas is oxygen and this comes into contact with something flammable like oil or grease in a confined space, the heat liberated may cause an explosion. Hence oil or grease should be kept well away from pressurised oxygen sources. These include not only oxygen, but pressurised air and pressurised nitrous oxide, which can dissociate into nitrogen and oxygen. Although modern anaesthetic volatile agents are non-flammable, ether and cyclopropane are flammable and may still be used in some parts of the world. Ethyl chloride, used to test sensory perception in local anaesthetic blocks and methyl alcohol for cleaning skin, are also flammable. Ignition sources include sparks from static electricity, or faulty electrical apparatus from the diathermy machine or from mains plugs sparking when disconnected. To prevent static electricity causing ignition, not only should efforts be made to minimise the generation of static electricity, but also to discharge any static slowly to earth. There should therefore be an upper and a lower limit to the electrical resistance between the antistatic floor and earth, of between 5MΩ and 20 kΩ respectively. All equipment capable of generating static electricity should make electrical contact with the floor through a medium made of antistatic (conducting) rubber. Staff footwear should also have antistatic rubber soles and the tubing of breathing systems should also be made of antistatic material. Classification of anaesthetic proof equipment is based on the ignition energy required to ignite the most flammable mixture of ether and air. ‘AP’ standard equipment can be used between 5 and 25 cm from such an inflammable anaesthetic gas mixture escaping from a breathing system; furthermore its temperature should not exceed 200◦C. ‘APG’ standard is a more stringent one for anaesthetic proof equipment; it is based on the ignition energy required to ignite the most flammable mixture of ether and oxygen, which should be less than 1 μJ.
Keywords: AP standard, APG standard, active scavenger systems, atmospheric pollution, explosion risk, fire risk, greenhouse effect, ignition sources, occupational exposure limits, ozone depletion, passive scavenging systems, scavenging systems, subacute combined degeneration of the spinal cord
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