August 21, 1914. THE COLLIERY GUARDIAN. 427 Behaviour of Rescue Apparatus in Hot Air.* By J. S. HALDANE, M.D., F.R.S. Some years ago I showed experimentally that in warm atmospheres, within wide limits, it is neither the temperature of the air nor the percentage of moisture present, nor the relative humidity, but the temperature shown by the wet bulb thermometer which, other things (such as clothing, muscular exertion, and velocity of the air current) being equal, determines the ill-effect of external heat on a man.f For instance, if the wet bulb temperature was at 89 degs. Fahr., it made no difference whether the actual temperature as shown by the ordinary dry bulb thermometer was 89 degs. or 130 degs. At a wet bulb'temperature of 89 degs. Fahr., in still air, and with nearly all clothing removed, the body temperature began to rise slowly, but steadily, even during complete rest. In a fair air current a higher wet bulb temperature, up to 94 degs. Fahr, or more, could be borne without rise of body temperature; but during quite moderate muscular work the body tem- perature began to rise at as low a wet bulb temperature as 79 degs. Fahr, in still air, despite the removal of clothing. Wet bulb temperatures as low as 70 degs. or 65 degs. Fahr, may produce heat stroke during con- tinued muscular exertion (£uch as marching) if unsuit- able clothing is worn. At temperatures so low as not to produce sweating, the wet bulb temperature ceases to be of physiological importance; but it must be remembered that during muscular work sweating occurs at a much lower air temperature than during rest. In the experiments just referred to, none of the observations were made at wet bulb tempeiatures exceeding the body temperature, and it might be sus- pected that when the dew point is above the body temperature, and moisture is actually condensing on the body, the connection between the wet bulb tempera- ture and the physiological effect of warm air would no longer hold. In order to investigate this point two experiments were made, in one of which the wet bulb temperature was about 110 degs. Fahr., but the tem- perature (dry bulb) about 152 degs., while in the other both wet and dry bulbs were at about 110 degs. In connection with these experiments, it may be well to refer to the importance of a proper method of observing the wet bulb temperature, The wret bulb temperature, in the ordinary physical sense, is given by the readings of an ordinary chemical thermometer hung up with the bulb covered with muslin wetted with water, in a good air current, or, if the air is still, waved about until the reading is steady. If the wet bulb thermometer is simply hung up in air which is nearly still, the reading is slightly higher, but probably gives a more correct measure of the physiological action of the still air than if the thermometer were waved about, since the body itself is in the still air. The ther- mometer was therefore not waved about when the wet bulb temperature was observed. The bulb itself must be quite freely exposed to the air. If this is not the case, very gross fallacies may arise. In illustration of this it may be mentioned that with a dry and wet bulb hygrometer ordinarily used for making observation underground to comply with the Mines Act, the wet bulb temperature read about 15 degs. Fahr, too high in still air in the first experiment about to be described. The thermometers were protected by a substantial metal case, there being beneath the bulbs a large metal reser- voir for water. The wet bulb readings given by this instrument, when hung up in still air, were grossly misleading, as the wet bulb was more or less boxed in by the metal. The subjects of the two • experiments were Mr. Winmill and myself, and the clothing (flannel shirt and trousers) was the same in each experiment. In the first experiment the room was heated to the maximum extent by running steam through the pipes, and in order to bring the wet bulb temperature up to about 110 degs. Fahr, some steam was allowed to escape into the air beneath the floor. The actual readings of the ther- mometers at the level of the centre of the body averaged 110 degs. Fahr, wet bulb and 152 degs. Fahr, dry bulb. In the second experiment the room was heated by simply blowing in steam. The dry bulb temperature averaged 110-5 degs. and the wet bulb 110 degs. Fahr. The mouth temperatures were as shown in the following table :— * From the second report to the Doncaster Coal Owners’ Committee (Gob-Fire Research) on Self-contained Rescue Apparatus and Smoke Helmets. The main conclusions were given in last week’s Colliery Guardian. f Journal of Hygiene, 1905, vol. v., p. 494. It may be useful to remind readers that the wet bulb temperature is the temperature shown by an ordinary thermometer of which the bulb is kept wet by a thin layer of muslin gauze or similar material; the dew point is the temperature at which moisture begins to be condensed from the air; and the relative humidity is the percentage relation between the proportion of moisture which the air actually contains and the proportion which it would contain if fully saturated with moisture. At an air temperature of 110 degs. Fahr, the difference between the dry bulb temperature and dew point would be 50 per cent, greater than the difference between the dry bulb and wet bulb temperature. Thus at an air temperature of 110 degs. Fahr., with the wet bulb at 80 degs. (as in a number of experiments described below), the dew point would be about 65 degs., and the relative humidity only about 30 per cent. This atmosphere would have the same effect on a man as air at a temperature of 80 degs. Fahr, with the wet bulb and dew point at 80 degs. and the relative humidity 100 per cent. Dry bulb tempera- Dry bulb tempera- ture, 152 degs. F. ture, 110’5 degs. b . Wet bulb tempera- Wet bulb tempera- Time. ture, 11" degs. F. ture, 110’0 degs. F. Just before entering After 5 minutes ... „ io „ >, 15 „ „ 20 „ >, „ fJ. S.H. T. F.W. Degs. F. Degs. K 9b*7 ... 98*9 . 99’0 ... 99*5 . 100*2 ... 100*3 . 100*7 ... 101’0 . 101*5 ... 102*0 . 10r5 ... 102*9 . 7. S.H. T. F. W? Degs. F. Degs. F. .. 99*1 ... 98’7 .. 1< 0*2 ... 99*8 .. 10'»*7 ... 100’3 .. 102’0 ... 101*2 .. 102*6 ... 102*0 .. 1"3’3 ... 102*6 It will be seen that the rises in temperature were 3-8 degs. and 4-0 degs. Fahr, in the first experiment, and 4-2 degs. and 3-9 degs. Fahr, in the second. The results were thus practically identical, so that there remained no room for doubt that it was the wet bulb temperature which determined the physiological effect. If the wet bulb temperature is equal to the body temperature (99 degs. Fahr.) no heat at all is carried away from the body surface, even with a good air cur- rent ; for though water may be evaporated from the skin, the heat thus got rid of in the latent form is balanced by the heat communicated to the body by conduction and radiation. The heat produced inside the body must, therefore, raise its temperature. The rise actually observed in myself during rest in a stand- ing position was at the rate of 4 degs. Fahr, per hour, with the wet bulb at 98 degs. With the wet bulb above the body temperatuie, the heat communicated to the body by the air more than balances any loss of heat, by evaporation. Hence the body temperature will rise more rapidly, and the more air current there is, the more rapid will this rise be. If the dew point reaches the body temperature, all loss of heat by evaporation will cease. But if the dew point exceeds the body temperature, the body acts as a condenser, and heat is communicated to it, not merely by conduction and radiation, but also by the condensation of moisture; and as the temperature of the dew point rises above the body temperature, the heat communicated to the body by condensation becomes rapidly more and more important. When a man has to go temporarily into very hot air, he can protect his skin by suitable clothing, so that his body temperature only rises slowly. With the wet bulb above the body temperature, the more thick and impervious the clothing he wears the better. He can thus stay for some time in dry air at a temperature far above boiling point. If, however, he is breathing the air directly, he is stopped at a much lower temperature by the burning sensation in his air passages, if the air contains much moisture. Some* observations were made on this point in the hot chamber. With the air saturated at 130 degs. Fahr., it was found that no work could be done in the chamber, on account of the sharp burning sensation in the throat when the breathing became at all deeper than usual. Even ordinary quiet breathing was more or less painful. Moving the hands, or walking about, produced an acute burning sensation in the exposed skin. The body temperature rose 4 degs. in less than 10 minutes, and but for the clothing worn would doubtless have risen still faster. Ordinary atmospheric air, without moisture added, would require to be heated far above boiling point to give a wet bulb temperature of 130 degs. Fahr. With the air saturated at 125 degs. Fahr., similar effects were produced, but not in quite so marked a degree. On reducing the temperature of the saturated air to 120 degs., the heat was much less uncomfortable, although on moving the hands about sharply or breathing deeply the feeling of heat was still very unpleasant. A canary was taken into the room, and showed signs of discomfort at once, and came down from its perch. After three minutes it appeared to be becoming considerably affected, and was taken out, but suddenly became unconscious and died just as it was removed. As the natural temperature of a bird is several degrees higher than that of a man, the very rapid fatal effect upon it of the heat was hardly expected. It is clear, however, that a small warm- blooded animal must be affected by heat far more rapidly than a man, since the ratio of its exposed surface (including respiratory surface) to its mass is far higher. The small animal can, therefore, be used as a rapid test of a hot atmosphere which will, in time, be dan- gerous to a man, just as it can be used as a rapid test of a poisonous atmosphere. In a hot mine atmosphere near a fire it may be difficult to say whether a small animal used for testing is affected by heat or by carbon monoxide; but in either case the practical information is given that the atmosphere will, in time, be dan- gerous to a man. Several rough observations were made in the hot chamber on the rise of body temperature in persons standing in the room with the air saturated at 120 degs. Fahr. It w*as evident that the rate of rise was at least 3 degs. or 4 degs. in 15 minutes. More precise results were obtained in experiments made in the large air-tight respiration chamber in my laboratory at Cherwell, Oxford. The following mouth temperatures were obtained in three persons, all of whom wore flannel shirts and trousers :— Time. Dry bulb temperature, 125 degs. F. Wet bulb ,, 121 J. S. H. J. B.S.H. A.E.M. Degs. F. Degs. F. Degs. F. Before entering . 98’4 ... 99’3 .. 98*4 After 4 minutes . 99’7 ... 100’0 .. — „ 8 101’0 .. 101’4 .. — „ 11 „ • 102’0 ... 102*3 .. — „ 15 „ . — — .' 103*4 These observations give a mean rate of rise of 4-7 degs. in 15 minutes. One of the subjects had difficulty in breathing, owing to the feeling of heat in the air passages. On breathing deeply through the mouth, I experienced a very unpleasant burning sensation in the tongue and throat. On moving about, the feeling of heat in the skin was greatly increased. This produced in us all the curious psycho- logical impression that every place towards which we moved inside the chamber was hotter than where we had been before. During work the rise of body tempera- ture would certainly be much faster, and might easily be twice as fast; but work was hardly possible, on account of the burning sensation in the nose and throat. It seemed fairly clear that without special protection from heat a man could not work safely for much more than five minutes in air with the wet bulb at 120 degs. Fahr. A work experiment was done with the air at 113 degs. Fahr, wet bulb. The air was not too hot to breathe pretty freely, and Mr. Macaskill worked the ergometer for 10 minutes at a rate of 4,900 ft.-lb per minute. At the end of this time, and after he had been in the chamber 14 minutes in all, he was much exhausted, and was becoming dizzy and confused. His mouth temperature had risen to 104*8 degs. Fahr. This obser- vation gives a good idea of the rapidity with which the body temperature rises towards a dangerous point when work is done with the wet bulb temperature above the body temperature. If no heat was lost from, or com- municated to, the body, its temperature would rise nearly 1 deg. Fahr, in 15 minutes during waiting about in a standing position, without doing appreciable W’ork, and about 3 degs. in 15 minutes if a quite moderate amount of work was done. The following table, con- structed partly from direct observation, and partly from calculation, will give a rough idea of the rate of rise in body temperature with various wet bulb temperatures in still air :— Wet bulb Rise of body temperature in 15 minutes. temperature. Rest. Moderate work. Degs. F. Degs. F. Degs. F. 70 0’0 0*0 80 0*0 0*6 90 0*3 1*5 100 1’2 3’0 110 2’4 5*0 120 4*5 9*0 With regard to the effect of muscular work on the rise of body temperature, it should be borne in mind that the motion associated with the work brings more air into contact with the skin. This factor has the same influence as movement of the air, and increases the rate of rise of body temperature -when the wet bulb air temperature is above 99 degs. Fahr., but has the oppo- site effect with the wet bulb below 99 degs. The higher the wet bulb temperature is, the greater is the effect of each degree of increase. A man going into a hot atmosphere, wearing a rescue apparatus, may be forced to retire by discomfort pro- duced by either rise of his body temperature or by the air furnished by the apparatus being too hot to breathe. With a caustic soda purifier the air furnished by the apparatus is at first so dry that even if it is greatly heated its wet bulb temperature is too low to cause any burning sensation in the mouth and throat. If, how- ever, the purifier becomes very hot and partly used up, the soda begins to let moisture pass through, and the inspired air may burn the throat. It is a serious defect in an apparatus if this occurs in an atmosphere in which the wearer could do work but for the hot and moist inspired air, or but for the influence of the apparatus in accelerating rise of body temperature. On the other hand the heat of the inspired air is of no practical interest if the atmosphere is so hot that a man could not in any case remain at work in it. These points have been borne in mind in testing rescue apparatus in hot air. With the wet bulb much above 80 degs. Fahr., a man could not remain for long at work, as his temperature would soon rise. The tests with rescue apparatus in hot air have, therefore, been made, as a rule, with the wet bulb below 80 degs. Fahr., which limited the actual temperature to 110 degs. in summer, or in warm and moist weather to rather less. LETTERS TO THE EDITORS. The Editors are not responsible either for the statements made, or the opinions expressed by correspondents. All communications must be authenticated by the name and address of the sender, whether for publication or not. No notice can be taken of anonymous communications. As replies to questions are only given by way of published answers to correspondents, and not by letter, stamped addressed envelopes are not required to be sent. THE SHORTAGE OF PIT TIMBER. Sir,—We notice in your current issue a letter from a correspondent with reference to the desirability of using steel pit props on account of the scarcity of timber. For nearly a quarter of a century Firth's steel props have been in regular use at some of the principal collieries, but since the outbreak of the war we have experienced a rush of orders, and, judging from enquiries to hand, many other collieries are contem- plating their use. We have made special arrangements to increase our output, and hope to meet the demand. We shall also be very pleased to give any information we possess to those interested. William Firth Limited. . (William Firth, director). Leeds, August 17, 1914.