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IGNITION BY ELECTROSTATIC SPARKS IN HYPERBARIC OXYGEN
1405 hair. In dead pigs such flash-fires by themselves liberate sufficient energy to blister the skin. There is no acceptable way of reproducing this experiment in man but one must assume that this is at least equally true for him. Studies with brass dummies coated with polyninyl chloride and with dead pigs convince us that igniting clothing in such environment will lead, if left uncontrolled, to fatal damage within five to twenty seconds. Ordinary methods of fire extinction cannot control such fires. For example, they will persist for many minutes beneath an overhead sprinkler array that provides five times the spray density known to control similar fires in air. A dense water spray can extinguish clothing fires in oxygen provided that the density and distribution meet the standards (see below). Any method of extinction must detect and suppress a fire within five seconds of its onset if fatal damage is to be avoided. Normal detection and sprinkler systems cannot satisfy this requirement but a correctly designed one can." In oxygen-filled chambers occupied by more than one person, if one inhabitant catches light the fire is of so vigorous a character that it is extremely likely the others will also catch fire.
Compressed Air at Absolute Pressures not Exceeding 5 Atmospheres The risks of igniting clothing rise moderately with pressure; but although the fire seems to be similar in character to that in air at atmosphere, the burning-rate increases moderately with pressure. The nap of clean denim material or the body hair of dead pigs will not propagate fire. Such fires can certainly be controlled by a dense water spray (of the type recommended later). In a single anaesthetised dog ventilated on pure oxygen while lying in compressed air at 3 atmospheres, clothed in unproofed denim material, ignition of its clothing revealed no gross difference due to oxygenation.
from the sprinkler supply, to douse those areas such as the crutch and axillae inevitably shielded from the general spray. SUMMARY
Little is known of the fire-risks to man in oxygen-rich environmments such as those obtained with hyperbaric oxygen therapy. Experimental evidence shows that the risks of ignition and the subsequent burning-rate are strikingly increased when compared with normal conditions. The risk of ignition will be further increased as more equipment fitted with electronic instruments is brought into use and by grease or petrol stained clothing. The fires are more serious because of the high burningEstablished methods of fire extinction are inrate. effective under these conditions. Some precautions have already been urged but others might include: administration of hyperbaric oxygen by oronasal tube whenever possible; replacement of a patient’s clothing by a tight-fitting, fire-proofed garment; and the incorporation of an automatic flame-detector and suitable water supply into the system. Some of this work was communicated to the Second International Research in Burns, Edinburgh 1965. All of it rests of workers from many fields in several Service and Government departments. We are grateful to the Director General, Medical Services R.A.F., for permission to publish this paper. Requests for reprints should be addressed to D. M. D.
Congress on on the help
IGNITION BY ELECTROSTATIC SPARKS IN HYPERBARIC OXYGEN P. R. PURSER
RECOMMENDATIONS
A code of practice should be established for the construction, instrumentation and use of hyperbaric oxygen chambers. From the fire prevention and control viewpoint this code should include those recommendations made by Roth, Brown and Smith, Clamann and Denison. To these we should like to add three further points: 1. Hyperbaric oxygen is administered much more safely by delivering pure oxygen through a well-fitting oronasal mask to a patient lying in compressed air, than by immersing the patient in compressed oxygen. (The indications for the second mode of administration are probably limited to cases in which it is intended to oxygenate ischaemic regions of open wounds lying deeper than 2 mm. from the surface. Those shallower than 2 mm. would be adequately oxygenated by compressed air and those with satisfactory blood-supplies would receive sufficient oxygen from oronasal delivery to the respiratory tract.) 2. Whenever possible patients should be stripped of all clothing, particularly if it is contaminated by dirt, grease, or solvents and then reclothed in a uniformly tight-fitting, singlelayered garment made from a lightweight open-weave material that has been fire-proofed. 3. Any chambers that can be filled with pure oxygen at a pressure greater than 0-4 atmosphere should be fitted with an automatic flame-detection system with a performance equal to or better than that of the system described by Pignatelli 11; it should also be equipped with a water-sprinkler system delivering a spray with a density of at least 5 ml. per minute per sq. cm. throughout the envelope of possible movements of the occupants (the system should provide this spray within 2 seconds of initiation, and it should respond to activation of the flame-detection sensors and to manual overrides placed inside and outside the chamber); and an open-ended hose, fed 11.
Pignatelli, A. B. R.A.F. Report no. 351.
Hants).
Institute of Aviation Medicine 1965.
(Farnborough,
PRINCIPAL
PHYSICIST,
M.Sc. Lond. THOMAS’S HOSPITAL,
ST.
LONDON
S.E.1
WITH the increasing use of hyperbaric oxygen chambers in medicine the associated fire hazards become increasingly important. Denisonhas shown the catastrophic nature of fires in an oxygen environment and the difficulty in extinguishing them. Fire precautions, therefore, should emphasise prevention rather than cure. Fortunately, no fires have been reported from the medical use of hyperbaric oxygen in this country, but several have been reported from other uses of hyperbaric chambers.2 In most instances the cause of the fire was sparking or overheating in electrical equipment. But in many cases some electrical equipment, such as electrocardiograph leads for monitoring patients and a microphone for communication, is essential. It therefore becomes necessary to know whether there is a minimum electrical power level below which solid materials cannot be ignited. Denison et awl. have shown that compressed air gives a greater fire hazard than air at atmospheric pressure, but that it is considerably less dangerous than pure oxygen. One might conclude that, whenever hyperbaric oxygen has to be administered, it should be through a oronasal mask in a compressed-air chamber. Unfortunately, this cannot always be done. In radiotherapy with hyperbaric oxygen, a mask would increase the level of carbon dioxide and the consequent risk of convulsion; many of the patients have tumours of the head and neck which makes the use of a mask impracticable. Denison, D. M. Institute of Aviation Medicine, report no. 294, 1964. Roth, E. M. NASA spec. Publ. no. 48. U.S. Government Printing Office, 1964. 3. Denison, D. M., Ernsting, J., Cresswell, A. W. Institute of Aviation Medicine, report no. 343, 1965. 1. 2.
1406 IGNITION BY ELECTROSTATIC SPARKS OF MATERIALS IN OXYGEN
inlet was closed. The smallest condenser was connected across the electrodes and charged to 5 kV. The plate electrode was then moved in until a spark occurred. If no fire resulted, the test was repeated ten times, and then ten times with each of the larger condensers. Three samples of each material were tried at atmospheric pressure, and three more at 3 atmospheres pressure. After each fire, the chamber was thoroughly cleaned and dried before starting
atmospheres before the
again. RESULTS
The figure gives the number of abortive tests. X indicates ignition.
In 1963 Dr. I. Churchill-Davidson, who is conducting a trial of radiotherapy with hyperbaric oxygen at St. Thomas’s Hospital, requested an assessment of the consequent fire hazard. There was little relevant published work at the time. Guest et awl. had shown that inflammable vapours (in their case, anxsthetic gases) mixed with oxygen could be ignited by electrical sparks, with a minimum of 1 microjoule in energy. Voigtsberger5 had shown that clothing materials in an environment of pure oxygen could be ignited by sparks from grinding metal on metal, or by electrical sparks of as little as 0-5 millijoule in energy. Voigtsberger’s work, however, did not set a minimum energy, did not extend to higher pressures than 1 atmosphere, and used fairly unrealistic experimental conditions; charged condensers were connected across widely spaced electrodes with the test sample hung between them; the spark was initiated by means of a high frequency discharge from an auxiliary electrode. Voigtsberger’s work was repeated under slightly more realistic conditions and the investigation was extended to obtain minimum ignition energies for oxygen at 1 and 3
atmospheres
pressure. METHOD AND MATERIALS
A small brass pressure chamber was constructed of about 20 ml. capacity. This had an observation window, inlet and outlet points for gas, a fixed insulated electrode, and a movable earthed electrode. The fixed electrode, made from a car spark-plug, had a domed end of 1 mm. radius. The movable electrode was a circular brass plate 1 cm. in diameter to which samples could be tied; this could be moved by a screw thread towards or away from the fixed electrode. Condensers of 100 pf, 0-001 f or 0 01 f capacity could be connected across these electrodes. The materials tested were antistatic rubber, a light cotton dress material, and a woollen serge; further samples of the last two, lightly smeared with a lubricant oil, were also used. In each test, the sample was tied to the plate electrode, the window was closed, and oxygen was flushed through until a Beckman D2 Analyser in the outlet registered 99% oxygen. The system could then be closed off at atmospheric pressure, or the outlet could be closed and the pressure raised to 3 Guest, P. G., Sikera, V. W., Lewis, B. U.S. Bureau of Mines; report no. 4833, 1952. 5. Voigstberger, P. Moderne Unfallverhütung, 1962, 7, 66. 4.
A preliminary experiment showed that when the chamber was filled with air at atmospheric pressure none of the samples could be ignited, even when the largest condenser was used. The results of the tests in oxygen are shown in the accompanying table. There was no clear difference between ignition at 1 atmosphere and at 3 atmospheres. None of the samples could be ignited when the smallest condenser was used. If the usual practice is followed of reckoning the spark energy as 1/2 capacity x voltage,2 this gives a minimum ignition energy between 1 and 10 millijoules. The difference between this and Voigtsberger’s results must be attributed to the difference in experimental conditions.4 DISCUSSION
The question arises whether this energy is available. In their work on the risk of anxsthetic explosions, Guest et al.4 found that man in indulging in fairly normal activities could build up a potential of several kV and could have a capacitance of about 100 pf. This gives an available energy of about 1 millijoule, which, taken together with their minimum ignition energy of 1 microjoule, indicates a clear risk. The magnitude of the risk is shown by the Ministry of Health report6 recording 22 anxsthetic explosions in seven years attributable to static
electricity. This static build-up, however, can easily be avoided in the one-man radiotherapy chambers by making the patient wear cotton clothing, by using antistatic rubber on the couch, and by earthing the patient through one of the monitoring leads. Can electrical leads in the chamber provide this energy? No exact answer can be given. Almost certainly, the lowvoltage high-impedance circuitry for microphones and patient monitoring could not provide this energy. Certainly, mains voltages with inductive loads could do so. Until the maximum safe power-level is known, it would be wise to restrict electrical leads to a minimum and to a maximum of a few tens of mW of power transfer-a quite feasible restriction for transistorised equipment. CONCLUSIONS
radiotherapy with hyperbaric oxygen the fire risk is negligible if suitable clothing is worn by the patient, if antistatic rubber covers the couch, and the patient is earthed, if inflammable vapours are excluded and if electrical leads are restricted (see above). If these precautions are not taken, the result may be catastrophic. In compressed-air chambers the fire hazard is less, and less stringent conditions might be allowed, but further In
work is needed. I should like to thank Dr. I. Churchill-Davidson, of St. Thomas’s Hospital, Dr. E. M. Guenalt, of the Safety in Mines Research Establishment, and Flight Lieutenant D. Denison, of the R.A.F. Institute of Aviation Medicine, for valuable advice; and Mr. J. Burns of St. Thomas’s Hospital for design and construction of the
apparatus. 6.
Ministry of Health Report Office, 1956.
on
Anæsthetic
Explosions. H.M. Stationery
Compressed Air at Absolute Pressures not Exceeding 5 Atmospheres The risks of igniting clothing rise moderately with pressure; but although the fire seems to be similar in character to that in air at atmosphere, the burning-rate increases moderately with pressure. The nap of clean denim material or the body hair of dead pigs will not propagate fire. Such fires can certainly be controlled by a dense water spray (of the type recommended later). In a single anaesthetised dog ventilated on pure oxygen while lying in compressed air at 3 atmospheres, clothed in unproofed denim material, ignition of its clothing revealed no gross difference due to oxygenation.
from the sprinkler supply, to douse those areas such as the crutch and axillae inevitably shielded from the general spray. SUMMARY
Little is known of the fire-risks to man in oxygen-rich environmments such as those obtained with hyperbaric oxygen therapy. Experimental evidence shows that the risks of ignition and the subsequent burning-rate are strikingly increased when compared with normal conditions. The risk of ignition will be further increased as more equipment fitted with electronic instruments is brought into use and by grease or petrol stained clothing. The fires are more serious because of the high burningEstablished methods of fire extinction are inrate. effective under these conditions. Some precautions have already been urged but others might include: administration of hyperbaric oxygen by oronasal tube whenever possible; replacement of a patient’s clothing by a tight-fitting, fire-proofed garment; and the incorporation of an automatic flame-detector and suitable water supply into the system. Some of this work was communicated to the Second International Research in Burns, Edinburgh 1965. All of it rests of workers from many fields in several Service and Government departments. We are grateful to the Director General, Medical Services R.A.F., for permission to publish this paper. Requests for reprints should be addressed to D. M. D.
Congress on on the help
IGNITION BY ELECTROSTATIC SPARKS IN HYPERBARIC OXYGEN P. R. PURSER
RECOMMENDATIONS
A code of practice should be established for the construction, instrumentation and use of hyperbaric oxygen chambers. From the fire prevention and control viewpoint this code should include those recommendations made by Roth, Brown and Smith, Clamann and Denison. To these we should like to add three further points: 1. Hyperbaric oxygen is administered much more safely by delivering pure oxygen through a well-fitting oronasal mask to a patient lying in compressed air, than by immersing the patient in compressed oxygen. (The indications for the second mode of administration are probably limited to cases in which it is intended to oxygenate ischaemic regions of open wounds lying deeper than 2 mm. from the surface. Those shallower than 2 mm. would be adequately oxygenated by compressed air and those with satisfactory blood-supplies would receive sufficient oxygen from oronasal delivery to the respiratory tract.) 2. Whenever possible patients should be stripped of all clothing, particularly if it is contaminated by dirt, grease, or solvents and then reclothed in a uniformly tight-fitting, singlelayered garment made from a lightweight open-weave material that has been fire-proofed. 3. Any chambers that can be filled with pure oxygen at a pressure greater than 0-4 atmosphere should be fitted with an automatic flame-detection system with a performance equal to or better than that of the system described by Pignatelli 11; it should also be equipped with a water-sprinkler system delivering a spray with a density of at least 5 ml. per minute per sq. cm. throughout the envelope of possible movements of the occupants (the system should provide this spray within 2 seconds of initiation, and it should respond to activation of the flame-detection sensors and to manual overrides placed inside and outside the chamber); and an open-ended hose, fed 11.
Pignatelli, A. B. R.A.F. Report no. 351.
Hants).
Institute of Aviation Medicine 1965.
(Farnborough,
PRINCIPAL
PHYSICIST,
M.Sc. Lond. THOMAS’S HOSPITAL,
ST.
LONDON
S.E.1
WITH the increasing use of hyperbaric oxygen chambers in medicine the associated fire hazards become increasingly important. Denisonhas shown the catastrophic nature of fires in an oxygen environment and the difficulty in extinguishing them. Fire precautions, therefore, should emphasise prevention rather than cure. Fortunately, no fires have been reported from the medical use of hyperbaric oxygen in this country, but several have been reported from other uses of hyperbaric chambers.2 In most instances the cause of the fire was sparking or overheating in electrical equipment. But in many cases some electrical equipment, such as electrocardiograph leads for monitoring patients and a microphone for communication, is essential. It therefore becomes necessary to know whether there is a minimum electrical power level below which solid materials cannot be ignited. Denison et awl. have shown that compressed air gives a greater fire hazard than air at atmospheric pressure, but that it is considerably less dangerous than pure oxygen. One might conclude that, whenever hyperbaric oxygen has to be administered, it should be through a oronasal mask in a compressed-air chamber. Unfortunately, this cannot always be done. In radiotherapy with hyperbaric oxygen, a mask would increase the level of carbon dioxide and the consequent risk of convulsion; many of the patients have tumours of the head and neck which makes the use of a mask impracticable. Denison, D. M. Institute of Aviation Medicine, report no. 294, 1964. Roth, E. M. NASA spec. Publ. no. 48. U.S. Government Printing Office, 1964. 3. Denison, D. M., Ernsting, J., Cresswell, A. W. Institute of Aviation Medicine, report no. 343, 1965. 1. 2.
1406 IGNITION BY ELECTROSTATIC SPARKS OF MATERIALS IN OXYGEN
inlet was closed. The smallest condenser was connected across the electrodes and charged to 5 kV. The plate electrode was then moved in until a spark occurred. If no fire resulted, the test was repeated ten times, and then ten times with each of the larger condensers. Three samples of each material were tried at atmospheric pressure, and three more at 3 atmospheres pressure. After each fire, the chamber was thoroughly cleaned and dried before starting
atmospheres before the
again. RESULTS
The figure gives the number of abortive tests. X indicates ignition.
In 1963 Dr. I. Churchill-Davidson, who is conducting a trial of radiotherapy with hyperbaric oxygen at St. Thomas’s Hospital, requested an assessment of the consequent fire hazard. There was little relevant published work at the time. Guest et awl. had shown that inflammable vapours (in their case, anxsthetic gases) mixed with oxygen could be ignited by electrical sparks, with a minimum of 1 microjoule in energy. Voigtsberger5 had shown that clothing materials in an environment of pure oxygen could be ignited by sparks from grinding metal on metal, or by electrical sparks of as little as 0-5 millijoule in energy. Voigtsberger’s work, however, did not set a minimum energy, did not extend to higher pressures than 1 atmosphere, and used fairly unrealistic experimental conditions; charged condensers were connected across widely spaced electrodes with the test sample hung between them; the spark was initiated by means of a high frequency discharge from an auxiliary electrode. Voigtsberger’s work was repeated under slightly more realistic conditions and the investigation was extended to obtain minimum ignition energies for oxygen at 1 and 3
atmospheres
pressure. METHOD AND MATERIALS
A small brass pressure chamber was constructed of about 20 ml. capacity. This had an observation window, inlet and outlet points for gas, a fixed insulated electrode, and a movable earthed electrode. The fixed electrode, made from a car spark-plug, had a domed end of 1 mm. radius. The movable electrode was a circular brass plate 1 cm. in diameter to which samples could be tied; this could be moved by a screw thread towards or away from the fixed electrode. Condensers of 100 pf, 0-001 f or 0 01 f capacity could be connected across these electrodes. The materials tested were antistatic rubber, a light cotton dress material, and a woollen serge; further samples of the last two, lightly smeared with a lubricant oil, were also used. In each test, the sample was tied to the plate electrode, the window was closed, and oxygen was flushed through until a Beckman D2 Analyser in the outlet registered 99% oxygen. The system could then be closed off at atmospheric pressure, or the outlet could be closed and the pressure raised to 3 Guest, P. G., Sikera, V. W., Lewis, B. U.S. Bureau of Mines; report no. 4833, 1952. 5. Voigstberger, P. Moderne Unfallverhütung, 1962, 7, 66. 4.
A preliminary experiment showed that when the chamber was filled with air at atmospheric pressure none of the samples could be ignited, even when the largest condenser was used. The results of the tests in oxygen are shown in the accompanying table. There was no clear difference between ignition at 1 atmosphere and at 3 atmospheres. None of the samples could be ignited when the smallest condenser was used. If the usual practice is followed of reckoning the spark energy as 1/2 capacity x voltage,2 this gives a minimum ignition energy between 1 and 10 millijoules. The difference between this and Voigtsberger’s results must be attributed to the difference in experimental conditions.4 DISCUSSION
The question arises whether this energy is available. In their work on the risk of anxsthetic explosions, Guest et al.4 found that man in indulging in fairly normal activities could build up a potential of several kV and could have a capacitance of about 100 pf. This gives an available energy of about 1 millijoule, which, taken together with their minimum ignition energy of 1 microjoule, indicates a clear risk. The magnitude of the risk is shown by the Ministry of Health report6 recording 22 anxsthetic explosions in seven years attributable to static
electricity. This static build-up, however, can easily be avoided in the one-man radiotherapy chambers by making the patient wear cotton clothing, by using antistatic rubber on the couch, and by earthing the patient through one of the monitoring leads. Can electrical leads in the chamber provide this energy? No exact answer can be given. Almost certainly, the lowvoltage high-impedance circuitry for microphones and patient monitoring could not provide this energy. Certainly, mains voltages with inductive loads could do so. Until the maximum safe power-level is known, it would be wise to restrict electrical leads to a minimum and to a maximum of a few tens of mW of power transfer-a quite feasible restriction for transistorised equipment. CONCLUSIONS
radiotherapy with hyperbaric oxygen the fire risk is negligible if suitable clothing is worn by the patient, if antistatic rubber covers the couch, and the patient is earthed, if inflammable vapours are excluded and if electrical leads are restricted (see above). If these precautions are not taken, the result may be catastrophic. In compressed-air chambers the fire hazard is less, and less stringent conditions might be allowed, but further In
work is needed. I should like to thank Dr. I. Churchill-Davidson, of St. Thomas’s Hospital, Dr. E. M. Guenalt, of the Safety in Mines Research Establishment, and Flight Lieutenant D. Denison, of the R.A.F. Institute of Aviation Medicine, for valuable advice; and Mr. J. Burns of St. Thomas’s Hospital for design and construction of the
apparatus. 6.
Ministry of Health Report Office, 1956.
on
Anæsthetic
Explosions. H.M. Stationery