The same characteristics which make a fabric warm give to it the ability to care for the perspiration. A loosely woven mesh material will absorb moisture more readily than one closely woven. The air in the meshes also aids in the evaporation of the perspiration. Sometimes there is danger of too rapid evaporation, therefore too rapid cooling, when the outside air has easy access to the undergarment. Those materials, like leather and rubber, which allow no air to pass through prevent chilling by convection, but at the same time may become very uncomfortable because they do not permit the evaporation of perspiration and the skin becomes chilled by the moist garment in contact with it.
Thus far we have spoken almost entirely of keeping the body warm in cold weather. It is also essential to keep it cool in very hot weather. Garments which allow the rapid evaporation of perspiration and the removal of heat by convection are cool in summer. In the tropics special kinds of cloth which allow rapid passage of air are used by the white man. Special colors are also necessary, but the subject of color will be discussed more fully in a later paragraph.
The difference in the ability of fibers to absorb moisture without seeming wet is called their hygroscopic power. Wool may absorb thirty per cent of its own weight without changing noticeably in appearance; silk also absorbs much moisture without seeming wet. Cotton and linen have much less ability to do this. While wool absorbs a great deal of moisture it takes it up slowly; for this reason a layer of cotton next the skin, with a layer of wool just outside, seems to be very satisfactory. The cotton absorbs the moisture and passes it onto the wool, from which it is evaporated slowly, thus preventing rapid cooling of the skin. Undergarments are woven which carry out this idea; a thin knit fabric of cotton is combined with a thin fabric of wool in such a way that there is an air space between the two. This garment has also the advantage over all wool that it does not irritate the skin.
Probably few people in active life need woolen underwear; it is warmer than is necessary; it is irritating, and when one exercises the perspiration is not absorbed quickly enough and the skin becomes wet, a bad condition. Those who are leading a sedentary life do need woolens because their skin is not kept warm by muscular activity. Wool is also a good protection against sudden exposure and extreme cold, but it need not be worn in direct contact with the skin. Silk has many points in its favor; it is attractive, a poor conductor of heat, is easily laundered when pure, and absorbs water easily. The prohibitive feature of silk for most people is, of course, its price. With the present adulterations, aside from the union suits, hose, and undershirts, few silk materials fulfill all the requirements of a thoroughly hygienic garment next the skin.
To perform its functions most satisfactorily a garment must be clean, for the excretions of the skin clog the pores of the cloth touching it, just as they clog the pores of the skin itself if it is not properly washed. Ventilation and evaporation of perspiration cannot go on so well if the garment is thus clogged. Choice of material which is easily laundered is therefore important, and is another point in favor of cotton rather than wool.
Outer clothing has not so many essential requirements; its chief function from a hygienic standpoint is that it be warm in winter and cool in summer, and that it allow some ventilation. The outer clothing may be easily changed, and should, therefore, furnish the chief difference in warmth between dress in a steam-heated house and street dress. Furs are warm chiefly because of the air held in and of the impenetrable character of the skin. They are warmer when the fur is worn inside, since it then holds a layer of still air. Furs swathed closely about the neck are unhygienic, as the skin becomes sensitive and colds are easly contracted.
Moist air conducts heat more readily than dry air, therefore on a damp, cold day there is greater need for active exercise or warmer clothing.
Color is an important factor in clothing only in connection with outside garments. The old tradition that red flannels are warmer than white has no foundation, unless it be that the red dye makes them a trifle more irritating. In outside clothing certain colors absorb more heat from the sun's rays; black, blue, green, and red absorb more, yellow and white less. Black also absorbs odors more readily. White reflects the sun's rays most, black least.
In the tropics outside garments are made from white materials and lined with dark green or some other dark color. Certain rays of the sun are reflected by the white, but others pass through. These are absorbed by the dark material, and the skin is thus protected. It has been supposed that the color of the black man's skin is a protection from certain poisonous sun's rays from which the white man must protect himself in an artificial manner. This theory is now questioned.
To summarize, clothing must be such that it will aid the body in maintaining constant temperature whatever the temperature of the outside air may be.
Clothing material must be of such texture that it not only aids in maintaining the right temperature, but also takes care of perspiration in such a way that the skin is cooled rapidly in hot weather and not too rapidly in cold weather.
The outside clothing must perform a function different from the under clothing, and so should differ in nature.
A thick garment is not necessarily the warmest one, neither need a garment be heavy. Light and warm are not opposing terms.
Access of air to the skin should be regulated, not prevented.
Free movement of the body should not be interfered with, neither should clothing be so warm that exercise is uncomfortable.
Cleanliness is all important both for the best functioning of the skin and for the comfort of one's self and one's friends.
Every person must decide what his or her needs are and dress accordingly. General rules cannot be laid down for all. Much is determined by living conditions, amount of exercise, age, health, etc. If not enough clothes are worn then more food is required, and the organs must do more work to keep the body temperature normal.
There has been a reaction from the woolens of the past, and doubtless some have gone to the extreme of not dressing warmly enough. Common sense, together with knowledge of bodily needs, will do much to improve conditions.
Harrington, Charles. Manual of Hygiene. Hough and Sedgwick. The Human Mechanism.