Climate and Health in Hot Countries and the Outlines of Tropical Climatology / A Popular Treatise on Personal Hygiene in the Hotter Parts of the World, and on the Climates That Will Be Met Within Them.

by George Michael James Giles

Temperature.

—In health the temperature of the human blood varies but little, whatever may be the climatic conditions to which we are submitted, the normal point being generally taken as 98·4° F. (37° C), though it may range half a degree or so above or below this level without prejudice to health or comfort. The mechanism by which this uniformity of internal temperature is maintained, in spite of the widest differences in the temperature of our environment, depends upon an automatic regulation of the nutritive processes going on within the system.

The various muscular and nervous actions, going on constantly throughout life, derive the force necessary for their production from the oxidation of the various articles contained in our food, and as the oxidation of all its digestible constituents is really nearly as complete as if they had been subjected to combustion, the body gains nearly as much heat from the consumption of its food as if the latter were actually burned. In climates where the temperature of the air is less than that of the blood, a good deal of heat is conducted away from the body by its contact with the air; but where, as in hot climates, the difference is but small, or the temperature of the air may even exceed that of the blood, it is obvious that some further mechanism is required if the temperature of the body is to be kept at the normal level of health. This requirement is met by evaporation from the surface of the body, the amount of which is regulated automatically by a special set of nerves which are known as the vaso-motor system, whose function it is to regulate the calibre of the blood-vessels throughout the body, according to the varying nutritive necessities of the several organs to which they are distributed.

We are all familiar with the fact that either extreme heat or violent exertion will alike bring about free perspiration, and in both cases the object is the same, viz., to cool down the body which tends to become overheated; in the one case by the warmth of the surroundings, and in the other by the activity of the chemical changes going on within the body to provide the force required for the various muscular and nervous actions involved in the work performed. On the other hand, under the influence of cold the skin becomes bloodless and dry, very little blood being allowed to circulate at the actual surface, the bulk of it being kept to the deeper parts of the body, well beneath the protective coating of fat which lies immediately below the skin. Buried in this fat, and opening by delicate tubes on the surface, are enormous numbers of small glandular bodies—the sweat-glands—each of which, when sufficiently freely supplied with blood, pours out a fluid consisting mainly of water, but containing also a little common salt and minute quantities of other mineral constituents, as well as a trifling amount of organic or animal matter, which has served its purpose in the organism and is thrown off in this way as being of no further value.

In the conversion of water into vapour a large amount of heat is absorbed, and it is thus equally possible for the body to be kept at a temperature lower than that of the surrounding air as it is, under more ordinary conditions, to maintain it at a higher; but the endurance of intense heat throws an even greater strain on the organism than that of severe cold, as it cannot be combated in the same way by covering the body with non-conducting clothing, and under such circumstances exertion, involving as it does a further production of heat within the body, becomes well-nigh insupportable. The evil effects of intense heat become all the more marked in proportion as they are prolonged, and the effects of air temperatures approaching or exceeding that of the normal blood continuously, for many days or weeks, without any relief at night, are most debilitating, and render any considerable amount of muscular exertion not only painful but dangerous, even to natives of the country, who, indeed, thoroughly recognise the fact and abstain during such periods from any laborious tasks not absolutely necessary. The exhaustion and incapacity produced by extreme heat are naturally specially marked in persons in whom the sudorific system is ill developed, and there is no doubt that those who suffer from this defect in any marked degree should avoid subjecting themselves to such conditions and be content to remain in more temperate climes. It is obvious that under such conditions a failure in the action of the sweat glands must necessarily result in a rise of the body temperature, and there can be little doubt that this is what takes place in certain cases of simple “heat apoplexy.” This failure of the sudorific system appears to be specially favoured by the overcrowding of too many persons within a limited space. It is only, however, when the temperature stands for long periods above 90° or 95° F. (33° C.) that these distressing effects of heat are at all commonly experienced; most Europeans bearing heat up to this limit even for prolonged periods, if not with comfort, at least without serious detriment to health, and much higher temperatures are well borne during the day, provided that the daily range of temperature is sufficient to secure a definite relief during the night. It will thus be seen that a wide diurnal range of temperature will go far to neutralise the bad effects of a high mean temperature, and the importance of securing information on this point in estimating the possible effects of a given climate on health is therefore obvious.

The proportion of moisture present in the air has at least as important a bearing on health as its temperature. It is obvious that when the air is actually saturated with watery vapour evaporation from the surface of the body must necessarily be stopped, and with it the natural provision for preventing an undue rise of the temperature of the body. Actual saturation combined with high temperature is, however, fortunately rare for anything but short periods, as the absolute amount of water requisite to saturation increases rapidly as temperature rises; and hence warm air, but partially saturated, and therefore still active as an absorbent of evaporation from moist surfaces, may contain a far larger absolute amount of water than saturated air at a lower temperature. In practice, saturated air is only to be found in situations where it is brought into contact with colder surfaces, such as that of the earth, cooled by radiation during the night, as is seen in the production of dew; the dew point being, in fact, the temperature at which the amount of water present in the air suffices to saturate it. Apart from its diminution by the formation of dew, the absolute amount of moisture present in the air, depending as it does on but slowly changing conditions, can naturally also change but slowly, but the relative moisture, or the percentage of the amount required to produce saturation, which is actually present always varies greatly during the twenty-four hours in all places where the diurnal range of temperature is at all considerable; so that relative moisture, when given for any day or other period, always refers to an average. As a rule it is only the stratum of air of a few yards in thickness that is cooled by contact with the soil during the night and is hence concerned in the formation of dew, a fact which is prettily illustrated by the low-lying bands of vapour which hang over the landscape after a clear night on any fine morning in the Tropics, and which clear off as if by magic as the returning sun once more warms up the soil and air. Wherever radiation is impeded by the shelter of trees, by artificial shelter, or by sufficiently dense masses of cloud, the temperature of the soil and air falls but little, and hence, under such circumstances, dew does not fall.

For practical purposes there are no better hygrometers than those that depend on the hygroscopic properties of certain organic substances, such as hair and catgut, and it has been shown by Sresnewsky that the alteration in length of a hair caused by its absorption of moisture is directly proportional to the natural logarithm of the degree of relative humidity, so that such instruments can be graduated for use as scientific instruments. Rapidity of evaporation is, however, proportional not to the relative humidity, but to the difference of the tension of watery vapour present in the air with that of its tension when saturated—in other words, the difference of tension of watery vapour at the temperatures of the dry and wet bulb thermometers, a form of expression which admits of degrees of humidity at different temperatures to be directly compared. In practice, however, this datum is rarely to be found in climatic tables, which is of the less importance, as in its effects on our organisation a difference of 2 or 3 mm. of mercury, from the pressure of saturation at a low temperature, will give a pleasant sensation of dryness, while at a high temperature the same deficit of pressure would be felt intolerably close and sultry. Extreme conditions of either humidity or dryness are, of course, alike unhealthy, though much of the respiratory irritation ascribed to too dry air is, I believe, more truly referable to the dust which usually accompanies such atmospheric conditions; but in any case, there can be no doubt that alike in hot and cold climates it is far healthier for the air to be too dry than too moist. With the effects of damp cold we are all of us only too well acquainted in England, and those who have experienced the effects of damp heat will never need being reminded of its debilitating effects. Fortunately, however, relative humidities exceeding 80 per cent. are but rarely to be found accompanied with really high air temperatures, and are seldom met with except in localities blessed with a copious rainfall, which by cooling the air goes far to render matters tolerable.

The most trying of all climates, however, are those where high temperatures and relative humidity are combined with an absence of rain, and under such circumstances a relative humidity of far less than 80 per cent. gives rise to intolerable closeness and oppression, especially when combined with stillness of air. Typical examples are the autumnal climates of the Red Sea and Persian Gulf, the unbearable character of which is notorious. At Abusher, in the Gulf, for example, in the month of August rain never falls, there is little or no breeze, and the mean maximum temperature is 96·5° F. (35·7° C.), while the relative humidity averages 65 per cent., and though neither figure separately is remarkably high as compared with what may be met with elsewhere, the entire combination of conditions is generally admitted to constitute one of the most unendurable climates in the world. On the other hand, in the Algerian Sahara in the summer months the relative humidity may fall as low as 16 per cent., but provided that an unstinted supply of water, to supply the loss by evaporation, be obtainable, most people find crisp, dry heat of this sort rather stimulating than otherwise, and even where the temperature is so high as to become most trying to endurance, the mortality returns of such situations show that dry heat is really favourable to health. The reason of this is obviously found in the fact that a few hours’ exposure to the sun’s rays in such climates suffices to kill the germs of nearly all specific contagious diseases, and that the breeding of mosquitoes, which are now known to be the carriers of several of the most important and deadly of tropical diseases, is further summarily stopped. A further contributory reason is also found in the fact, that the population is driven to sleep in the open air instead of within more or less ill-ventilated houses, and hence obtains the inestimable benefit of the freest possible ventilation during a large portion of the twenty-four hours, besides reducing the chances of the direct infection of the healthy by the sick to a minimum. As a degree of relative humidity of the air so low as to be in itself irritating to the respiratory mucous membranes is almost unknown, we may practically consider that dryness of climate is everywhere synonymous with healthiness.

Effects of Amount and Distribution of Rainfall.

—As already remarked, it is quite possible for a climate to be damp and yet have little or no rainfall, but such instances are rare, and on the other hand, a heavy rainfall necessarily brings about a coincident increase of relative humidity. Rain is, moreover, necessarily combined with a cloudy sky, whereby the heating of the soil during the day and its cooling by radiation during the night are alike impeded. The immediate effect of a shower of rain is to cool the air, and this for a double reason: first, coming as they do from the higher strata of the atmosphere, the temperature of the raindrops is necessarily much lower than that of the earth’s surface; and secondly, from the multiplied surfaces of the descending drops and from the wetted earth there necessarily occurs a rapid evaporation, whereby a further large amount of heat is absorbed, but unless showers recur at sufficient intervals to continuously diminish air temperature, the temporary remission is apt to be dearly paid for by a period of heat combined with high relative humidity, with its attendant discomforts of reduced evaporation from the surface of the body, prickly heat, and the other discomforts inseparable from tropical damp.