The influence of rainfall on health necessarily depends to a great extent on the configuration of the land, but assuming that the latter admits of adequate drainage, more depends upon its distribution than upon its amount; for the sanitary influences of rain are, in the main, mechanical, and depend on the “laying” of the dust and the washing away of infective and otherwise deleterious material. A heavy shower of sufficient duration will carry away, viâ the river to the sea, the deleterious products of human occupation during a preceding drought; but to do this the rain must be heavy while it lasts, for a prolonged drizzle in a warm climate simply turns the soil into a particularly efficient cultivation ground for the germs of infective diseases, and the attendant gloom of the sky stops entirely the beneficent germ-killing power of the sun’s direct rays. A prolonged drizzle, never exceeding the absorbtive and drainage capacities of a given site, marks the maximum of unhealthiness in all climates, and is possibly even more obnoxious when associated with heat than with cold: so that the most pleasant tropical climates are those that combine frequent short but heavy showers with intervals of bright sunlight, a continuously overcast sky being everywhere unfavourable to health.
To judge, then, the influence of rainfall on health, we require three data—the total rainfall, the number of rainy days, and the aspect of the sky in any given season of the year; for the beneficent influence of light on the animal organisation is at least as marked as it is on plants, though while the latter fact is a matter of common observation, the former does not meet with the recognition which it deserves. We bleach our celery by protecting it from the light, but are apt to forget that, while the consequent reduction in the amount of its characteristic essential oil makes its eatable, the plant could hardly survive but for the application of lime and other artificial antiseptics, which we are obliged to apply to make up for the lack of the natural protection. Whether the process of “earthing up” be soothing to a celery plant or otherwise is a question of which we have no means of judging, but there can be no doubt that in such matters man is far more practical in the treatment of plants than of himself, and that in tropical climates he often suffers by shrinking too much from the immediate effects of the sun’s rays. In this as in all other affairs, moderation is, of course, desirable, but the commoner mistake is undoubtedly to shirk too much all exposure to the sun, whereas those whose avocations take them most into the open are generally the healthiest in the Tropics as elsewhere. Contrast the ardent sportsman who spends the broiling days of May and June in the pursuit of large game, with the lady who spends her days in a darkened bungalow, and there can be no question as to which suffers the most from “the effects of climate”; nor is the difference, as is often suggested, purely one of sex, for it will be noticed that female medical practitioners and missionaries and other ladies whose occupations involve their being much in the open are commonly at least as healthy as men similarly situated.
In all hot countries the period of the rains is the sickly season, but this is due not so much to any direct evil effects of damp on the human system as to the fact that the agents and carriers of disease, i.e., low plant organisms and mosquitoes and other suctorial insects, find in heat moisture and puddles the conditions that best favour their growth and multiplication; in other words, the unhealthiness of this season can be largely obviated by suitable measures of sanitation, so designed as to impede this growth and multiplication of noxious agencies in the immediate vicinity of human habitations.
Effect of Winds on Health.
—Save only in so far as it necessarily raises and transports dust, and that the latter may consist not only of mineral particles but may contain also deleterious organic matter and the germs of certain diseases, the action of wind, being equivalent to so much the freer ventilation, may always be considered desirable in hot climates. Given a free current of air, the highest air temperatures are borne with comparative comfort, whereas in stagnant air the sense of oppression is unbearable. The existence of a steady breeze from a known direction also makes it possible to artificially cool houses by placing in the doorway facing the direction of the wind wetted mats, which cool the air passing through them by the agency of evaporation. It further makes it possible to live in comfort without the use of punkahs and other artificial means of keeping up a free current of air; indeed, as a matter of fact, the habitability of places situated in the Tropics depends largely on the amount and continuity of the breeze.
As has already been remarked, the amount of dust present in the atmosphere depends mainly, in the first place, on dryness of the air, and in the second on the force of wind; but it is also a fact that under certain conditions, dependent probably on electrical manifestations, a very still atmosphere may yet carry in suspension a large amount of dust, and its presence may become inimical to health by causing irritation to the respiratory organs as well as to the eyes and lining membrane of the nostrils. This is specially liable to be the case when the suspended particles are sharp and angular, as in the case of the micaceous dust, with which, during the dry, hot weather, the air is often loaded in certain sub-Himalayan stations, producing in many persons soreness of the eyes and troublesome, dry cough.
Systematic observations on the amount of solid matter suspended in the air are as yet entirely wanting, but there can be little doubt that they would, if available, be valuable to the student of public health; for the high mortality among tradesfolk whose occupations involve the respiration of a constantly dusty atmosphere is thoroughly well known, and it is most improbable that what is true of dusty trades is not also true of dusty places.
Although generally admitted, especially as an article of popular belief, the influence of the varying electrical states of the atmosphere is as yet so ill-understood that nothing definite can be stated on the subject.