A very common and at the same time injurious form of air-vitiation is that from tobacco smoke. Smoking, especially in a closed space such as a smoking-room or smoking-car, vitiates the air very seriously, for smoker and non-smoker alike. [Sidenote: Dust] As to dust, the morbidity and mortality rates in certain occupations, particularly those known as the dusty trades, are appreciably and even materially greater than in dustless trades. An accumulation of house-dust should be avoided. The dust should be removed--not by the old-fashioned feather duster which scatters the dust into the air--but by a damp or oiled cloth. Dust-catching furniture and hangings of plush, lace, etc., are not hygienic. A carpet-sweeper is more hygienic than a broom, and a vacuum cleaner is better than a carpet-sweeper. The removable rug is an improvement hygienically over the fixed carpet. [Sidenote: Bacteria] The bacteria in air ride on the dust-particles. In a clean hospital ward, when air was agitated by dry sweeping, the number of colonies of bacteria collected on a given exposure rose twenty-fold, showing the effect of ordinary broom-sweeping. [Sidenote: Sunlight] The air we breathe should be sunlit when possible. Many of our germ enemies do not long survive in sunlight. Section II--Clothing Air may be shut out not only by tight houses but also by tight clothes. It follows that the question of clothing is closely related to the question of ventilation. In fact it is a reasonable inference from modern investigations that air-hygiene concerns the skin quite as much as the lungs. Therefore the hygiene of clothing assumes a new and hitherto unsuspected importance. A truly healthy skin is not the waxy white which is so common, but one which glows with color, just as do healthy cheeks exposed to the open air. [Sidenote: Porous Clothes] The hygiene of clothing includes ventilation and freedom from pressure, moderate warmth, and cleanliness. Loose, porous underclothes are already coming into vogue. But effective ventilation, namely such as will allow free access of air to the skin, requires that our outer clothes--including women's gowns and men's shirts, vests, vest-linings, and coat-linings--should also be loose and porous. Here is one of the most important but almost wholly neglected clothing reforms. Most linings and many fabrics used in outer clothes are so tightly woven as to be impervious to air. Yet porous fabrics are always available, including porous alpacas for lining. To test a fabric it is only necessary to place it over the mouth and observe whether it is possible or easy to blow the breath through it. [Sidenote: Air-baths]