In cities many can take such an air bath in one room every day when they can spare ten minutes, the best time being before dressing and after undressing. But also during the day, especially in summer, we should spare a few moments to take such a bath. In winter the room must, of course, be warmed for the purpose. In combination with the bath, breathing exercises can also be performed to great advantage in the way described in the chapter on the advantage of open air. In order to avoid catching cold in such an air bath it is advisable to rub the entire skin surface of the body. This causes much blood to be brought to the skin; thus more blood is conveyed to the sudorific glands which are thereby enabled to absorb a larger quantity of harmful matters from it, while at the same time more air can be taken in by the pores. While rubbing—which is best done by a brush—we can also move about, and while standing also beat the periphery of the body with a thick rough towel in the same way as in Finland they do with brushwood after hot baths. It is not painful and is very effective in producing a hyperæmia of the surface of the skin. When the skin is thus better nourished with blood it also offers more resistance to germs that may cause diseases of the skin, and such better nutrition and its improved hygienic condition is a safeguard against catching cold. Such an occurrence is more likely to occur in persons who neglect a thorough cleansing of the body daily by baths and who go warmly clad; in such cases the muscles which contract the pores are less active, and because of this more body heat is lost and catching cold more easily takes place. When we feel warm more blood circulates through the skin, and when afterward cold air reaches us the pores, in a normal person, contract quickly and the blood is retained in the interior of the body. Thus we are prevented from giving off too much warmth from the body and catching cold. Persons who have been trained since their early childhood to cold water and cold air show a great facility for reaction against cold. When cold air reaches the skin, which on account of warm surroundings has become warm, filled with blood, red, and moist, the skin will contract and become pale, the easy conduction of warmth will be checked, and the heat retained in the body. Thus such persons will not so easily catch cold as others whose skin is not kept in good hygienic condition.