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HYGIENE is the science, PRACTICAL HYGIENE the art, of preserving health. The name has been adopted from the French, from which language it has also been introduced into most other tongues ; it is derived from the Greek liyieta or Lria, health. Writings on health are among the oldest in the world, for the subject has engaged the attention of the profoundest thinkers and the most renowned leaders of men. We have only to point to the elaborate directions in the Mosaic laws for the preservation of health through scrupulous attention to cleanliness, the isolation of the sick, and extreme care in the use of wholesome articles of food and drink. Throughout the whole of their history the Jews enjoyed a remarkable immunity from epidemic disease, the most of the instances in which such disease occurred being represented as those in which they departed from the law and doubtless relaxed the wholesome vigilance enjoined by it. In medi:eval and modern history they have often, even down to our own time, been spared the ravages of epidemics, when their Christian neighbours were perishing around them. Ignorant superstition often gave rise to the idea that they had poisoned the wells, and they fell victims to the fanaticism of the times. It is highly probable that the periodical. cleansing of their dwellings, involved in the thorough search for the leaven which preceded the yearly passover Pesachim, i. 11), had a notable influence in preventing that continuous deposition of organic matter, which is no doubt one most powerful factor in the production of zymotic disease. On the other hand, the filthy habits of the Christian populations offered a premium to plagues of every kind ; for there is no parallel in ancient history to the terrible invasions of disease which from time to time ravaged Europe down to quite recent times.
It is the province of hygiene to seek out and determine the causes of disease, and to formulate rules for their prevention and removal. It may thus be called also preventive medicine, although this term does not quite express all that must be included. The progress of hygiene, such as it was, rested for many ages upon an empirical basis, and indeed to a large extent this is still the case. The subject has, however, in later times at least, been studied to considerable advantage, although much remains to be done. Two centuries ago the mortality of London was 80 per 1000 - at the present day it is under 23. A century ago ships could barely keep the sea for scurvy, whilst jails and hospitals were in many cases the hotbeds of fatal disease ; now those conditions are rectified, or at least the means of rectifying them are known. Thirty years ago the English troops at home died at the rate of 20 per 1000 - now their death-rate is less than one-half of this. A knowledge of the causes and modes of propagation of disease being necessary in order to provide rules for its prevention, it is obvious that hygiene must be largely dependent upon the advances made in pathology and aetiology ; hence the impossibility of any very marked progress in former times, by reason of the imperfection of the collateral sciences, and the want of the appliances more recently made available for inquiries of such a difficult and recondite character. Within this century, however, and especially within the last forty or fifty years, it has been possible to follow out the subject on a more strictly scientific basis, and so to lay a foundation, at least, on which to build a structure, which may one day entitle hygiene to a place among the more exact sciences.
The special subjects which hygiene embraces are the following: - I. Those which concern the surroundings of man; such as meteorological conditions, roughly included under the head of climate ; the site or soil on which his dwelling is placed ; the character, materials, and arrangement of his dwelling ; the air he breathes ; the cleansing of his dwelling, and the arrangements for the removal therefrom of excreta and other effete matters.
II. Those which concern the personal care of health; such as the food he eats and the water and other beverages he drinks; clothing; work and exercise; personal cleanliness; special habits, such as the use of tobacco, narcotics, die. ; control of sexual and other passions.
Certain points not directly included in the above ; such as the management of infancy ; the prevention of disease; the hygiene of the sick-chamber; and the disposal of the dead.
It is obvious that it is impossible to draw any hard and fast line in these divisions, and that they must constantly run into and overlap each other, Such a division, however, gives a general idea of the scope of the science, and a brief consideration of the different sections will enable us to furnish a slight sketch of the nature of the subject.
regards health, so long as reasonable care is observed. In a healthy body an adaptation to circumstances rapidly takes place, and an equilibrium is soon established. Thus, it used to be supposed that great heat increased the temperature of the body, but later observations have shown this to be erroneous, and that the balance is soon re-established by the process of transpiration ; - should this, however, be arrested, then a rise of temperature may take place, and disease of a febrile character be established.
The soil or site of the dwelling is, however, of greater moment, and much that has been attributed to climate has been more truly due to locality. Soils are generally divided into moist and dry, permeable and impermeable, and again subdivided according to formation, composition, slope, &c. Healthy soils are those which are dry and permeable, or which have such a slope as renders drainage easy ; on the other hand, soils which are flat, moist, and impermeable are generally unhealthy. Soils containing much organic matter are to be avoided, such as alluvial soils generally, as well as all marshy districts. The air in soils is generally more or less impure, hence the unadvisability of occupying dwellings below the ground level or situated immediately on its surface. The water in the soil is a question of great importance, apart from the mere moisture. At varying distances from the surface, but everywhere, there exists a great subterranean lake or sea, known as the ground-water or water-table, which is constantly in motion, both vertically and horizontally. Its horizontal movement is towards the nearest water-course or towards the sea ; its vertical movement is determined by rainfall chiefly. Much importance has been attached to it, and the following points may be consideredas accepted by most hygienists : - (1) a permanently high ground-water, that is, within 5 feet of the surface, is bad, while a permanently low ground-water, that is, more than 15 feet from the surface, is good ; and (2) violent fluctuations are bad, even with an average low ground-water; a comparatively high ground-water with moderate and slow fluctuations may be healthy. According to the school of Pettenkofer, it is the ground-water which determines the spread of certain forms of disease, such as cholera and enteric fever. A previously high level, succeeded by a fall, with a certain height of temperature in the soil-air, is the condition believed by them to be the one most favourable for disease production. Healthy soils are the granites, metamorphic rocks, clay-slate, limestone, sandstone, chalk, gravel, and sand ; unhealthy are - clay, sand and gravel with clay subsoil, alluvial soil, and marsh-lands, with the exception of peatiands. Among the unhealthy soils ought also to be included all " made " soils, particularly those that are formed so often in towns from rubbish of all sorts. Such soils ought not to be occupied as building sites for at least two years.
The sanitation of dwellings involves numerous points. The site has been considered in the previous section, but the importance of excluding soil emanations must be insisted upon. The placing of a dwelling in any spot of ground tends to exert an extractive force upon the soil, because the air of the dwelling is almost always warmer than the external air, and there is therefore a constant danger of sucking up the more or less impure soil-air into the dwelling. Not only is this a recognized source of disease, but fatal cases of direct poisoning have sometimes resulted, as when coal-gas has -escaped into the soil below or near a dwelling. An impervious foundation is therefore necessary, although this precaution is too often neglected, even in high-class dwellings. Houses ought to be so arranged that they may receive plenty of light, not merely for work or convenience, but as a matter of health. Sunlight, for full health, is as necessary as air, and this is now so strongly recognized in America that in many of the hospitals in that country rooms are provided where patients may take a " sun-bath."
The materials of which houses are built are various. Wooden dwellings have advantages, but there is always the danger of fire. Brick or stone is most commonly used, but very good dwellings may be made of concrete or even of mud. Probably the best material is good, sound, well-burnt brick. Dryness must be secured by means of damp-proof courses along the foundations, hollow walls, and cementing or slate-hanging externally. Non-absorbent surfaces internally are important, although some writers, such as Pettenkofer, &c., have been inclined to attribute the unhealthiness of dwellings to the impermeability of the walls obstructing air change. But where air can pass, organic matter can lodge and become a source of danger. It is better, therefore, to have non-absorbent surfaces as much as possible, and to provide for ventilation in other ways. Paint that can be washed is therefore better than paper ; if the latter is used it had better be glazed. Care should be taken to scrape off all old papers beneath, as they and the paste used with them tend to decompose and become injurious to health. Ceilings ought to be impervious as well as walls, and floors ought to be made of well-fitting seasoned wood, caulked, and oiled or varnished so as to make them water-tight.
Proper cubic space is a matter of great importance, for upon it depends the renewal of air. The air of an air-space can seldom be changed oftener than three times an hour, hence the space ought to be large enough to allow of such rate of change providing enough of air for respiratory purposes. Tho furniture of rooms, especially sleeping rooms, ought not to be too massive; whilst curtains and hangings too often form traps for dust and organic matter.
The warming of houses is important, and is generally badly and wastefully done. The open fire-place has great advantages, but it is in many cases insufficient. Where any general system is employed it is better to warm the air in the room itself, as by pipes conveying hot water or steam, than to warm it before delivery. Overheated rooms are a source of ill-health. For sitting-rooms 60° to 65° is quite enough ; for a study or work-room 60° is sufficient, even in some cases less than this. A sleeping-room need never be above 60°, often with advantage below it. Fresh air ought not to be sacrificed to temperature, except under extreme circumstances. Dwellings should not be occupied for some time after building, till they are thoroughly dry. Rheumatism, chest diseases, &c., are very apt to arise from neglect of this precaution.
Scrupulous attention to cleanliness is necessary in dwellings, and there is wisdom in their periodical vacation for a certain time, so as to let them lie fallow, as it were, and interrupt the continuity of deposit of organic matter. Dwellings ought to be scattered over as wide an area as possible, for statistics show that sickness and death-rate are often inversely proportional to the amount of area per head occupied by a community. The area per head in London is estimated at double that of Paris and many other cities, whilst at the same time its death-rate is smaller than that of any other large city in Europe.
Air is the prime necessity of life. Food or water may be abstained from for a considerable time, and we may thus have an opportunity of replacing either should we doubt its purity or wholesomeness, but tho atmosphere around us we must breathe or die. Hence the paramount necessity for having it pure. But, although this is apparently so obvious, attention to its importance has been very generally omitted. Air consists of a mechanical mixture of oxygen and nitrogen, in the proportion of nearly 21 per cent. of the former to 79 of the latter, with small quantities in addition of carbonic acid, moisture, organic matter, &c. By respiration and combustion air becomes vitiated, the oxygen diminishing and the carbonic acid and organic and suspended matter increasing. Within certain limits the amount of carbonic acid is in itself immaterial to health, but it is important as a measure of the amount of organic matter, which is really the dangerous impurity. Air vitiated by respiration is also much more dangerous than when the carbonic acid is partly the result of combustion. It is now pretty generally admitted that air cannot be con_sidered as really good and fit for respiration in which the respiratory impurity reckoned as carbonic acid much exceeds two parts in 10,000 by volume. On the other hand, if air can be kept down to this point, the condition may be looked upon as satisfactory. The amount of impurity given off by living beings varies of course with size, weight, age, sex, and work, but it may be allowed that under ordinary circumstances it amounts to about six cubic feet of carbonic acid per head in ten hours during repose. This requires an hourly supply per head of 3000 cubic feet of fresh air for its dilution, and this amount should be largely increased during work or in sickness (see HOSPITAL). The diseases which have been shown to arise from the effects of vitiated air are widely prevalent, including such as consumption and other forms of scrofulous disorders, bronchitis and pneumonia, sore throat, &c. Crowded and ill-ventilated places also tend to increase the virulence and the rapidity of spread of the various communicable diseases.
Cleansing, including the removal of slops, excreta, &c., forms one of the most important and also most difficult of questions. The main principle is that all should be immediately and effectually removed from the house and its neighbourhood, and that there should be no possibility of reflux of foul air from drain or cesspool. The system of water-carriage is certainly the cleanest and most convenient, especially among large communities, but other systems find advocates. In villages and isolated houses the earth system and other dry methods have many advantages. The question of the disposal of sewage is a very large one, into which it is impossible to enter here. Hitherto all or almost all the material has been wasted by being poured into rivers or the sea, the streams being thus polluted and the shores rendered offensive. The object to be aimed at is to utilize a product of undoubted fertilizing influence, without endangering the health of the community. The diseases to be apprehended from imperfect methods of sewage removal are enteric fever, cholera, diphtheria, sore throat, and an aggravation of most other diseases, especially those of an eruptive character. Ashpits ought to be especially attended to, their neglect being attended with much danger.
Water-supply, although included under the head of food and beverages, merits special consideration, so important is its relation to health, both directly as a drink and indirectly with reference to its many other uses. It is required for drinking, cooking, the cleansing of person, clothes, and dwelling, and the flushing of closets, sewers, and drains. The hygienic requirements are that water should be good in quality and sufficient in quantity. Good water should be clear, colourless, quite free from suspended matter, of a good lustre, and should have a pleasant sparkling taste, the latter qualities being due to the carbonic acid and atmospheric air dissolved in it. In its chemical composition it ought to be as free as possible from organic matter. The evidence in favour of communication of disease by means of drinking water is now very extensive, and we may cite diarrheea, dysentery, ague, enteric fever, and cholera as among the diseases which may be conveyed through this channel. Numerous parasites also find their way into the human body by this means. Hard water is objectionable for cooking and washing, nor can it be recommended for drinking, although some insist upon a certain amount of hardness being essential. In the case of water being impure, boiling, distilling, or filtering may be resorted to. The two former are the most efficacious, but the last has advantages of convenience if properly carried out. Charcoal filters, if properly cleansed, or renewed sufficiently often, are useful, but it is better to have a material that purifies without risking any deterioration of the water itself. Such filters as the spongy iron and the carferal effect this. All filters, however, require the medium to be cleansed or renewed periodically. In a house the chief dangers are from dirty cisterns and from pipes being connected with drains and closets. All supply should be on the constant system, and no pipe supplying a closet should be resorted to for drinking purposes. All overflow pipes should deliver in the open air. The quantity of water required per head may be stated at a minimum of 12 to 16 gallons per diem where there is no general system of drainage, and about 25 gallons with drainage. In towns more than this is necessary, and from 30 to 50 gallons are desirable. In sickness generally double the amount is necessary that is required in health. The source of the water ought to be pure, - springs, deep wells, and upland surface water being the best. Shallow wells and rivers to which sewage gains access are most to be avoided.
For food and beverages the reader is referred to the article DIETETICS.
Work and Exercise. - The kinds of work performed by man are of course very various, but they may be reduced more or less to a uniform standard, which is usually reckoned as so many tons (or pounds) raised through one foot, or, tersely, as foot-tons or ton-feet. A fair day's work is generally taken at 300 foot-tons, a laborious day's work at 450, and the maximum to be expected, except under very special conditions, at 600. For this work a certain time should be allowed, as the strain increases (almost in a geometrical ratio) with the velocity. Usually speaking 50 foot-tons an hour is a fair amount, and this ratio is equal to a walk of three miles for an average man. The amount for mental work has not been accurately calculated, but it may be safely assumed that a man of sedentary occupation ought to take exercise of a physical kind varying from 50 to 100 foot-tons per diem. In all cases his food ought to be proportioned to his work, for it is now recognized that man is a machine, whose work depends upon the energy derived from the food he eats.
Clothing and Personal Cleanliness. - Clothing should fulfil the functions of preserving warmth in cold weather, providing covering without being too oppressive in hot weather, keeping out wet in wet weather, and yet allowing sufficient transpiration for health. At the same time it ought to admit of frequent change and cleansing. Dr Parkes has pointed out that it is probably due in some measure to cleaner habits with reference to clothing that the diminution of typhus fever should have been so marked in recent times. Personal cleanliness is also a matter of great importance, a daily general bath being advisable for every one. For animals as well as human beings it has been shown that cleanliness is conducive to improved appetite and general health. Filth is one of the prime factors in the production and propagation of most of the devastating plagues known to mankind.
Prevention of Disease. - This is a large question, on which we can only briefly touch. Much depends upon our knowledge of aetiology or the remote causes of disease. The best rule for preventing disease is to follow out carefully the principles of general hygiene, laid down with reference to pure air, pure water, proper food, cleanliness, &c. Some diseases may be more specially provided against, such as paroxysmal fevers by the use of quinine, and smallpox by vaccination, but for the great majority of diseases 'the freest ventilation, and to all diseases this plan may be applied with more or less effect. In those diseases which are known to be communicable, such as scarlet fever, isolation of the patients is an effectual means of arresting the spread ; but the poisons of others, such as measles and hooping-cough, are so subtle that isolation can only be looked upon as a measure of doubtful success. Much stress has been laid upon disinfection as a means of preventing disease, and if properly carried out it has some efficacy. But it is a mistake to place too implicit reliance upon it as ordinarily practised. In dealing with clothing, bedding, &c., the best method is the application of heat, at or above the boiling-point of water, which may be done by means of dry heat, superheated steam, or boiling water. In fumigating places, burning sulphur or the vapour of chlorine or nitrous acid is used, but to be effectual the air must be rendered for the time irrespirable. The solid and liquid disinfectants (so-called) are chloride of lime, the permanganates, carbolic acid, and a great many similar substances, many of which have been made the subjects of patents. A large number of them are merely deodorants. It may be stated generally that disinfectants are useful as adjuncts to other hygienic measures, but that they cannot replace them, except to a small extent, and in a very imperfect way.
Bibliography. - E. A. Parkes, Manual of Practical hygiene, edited by F. de Chaumont ; G. Wilson, Handbook of Hygiene; A. Wynter Blyth, Dictionary of Hygiene (after Tardieu); F. de Chaumont, Lectures on State Medicine; A. H. Buck, hygiene and Public Health, New York ; Michel Levy, Traile d'Hygiene Publique et Priree; Von Pappenheim, Handbuch der SanitWs-Polizei; Roth and Lex, Handbuch der Milit(irisehe Gcsundheitspfiege; and numerous works and monographs on special departments. (F. DE C. )