Proper ventilation requires also that there be a movement or circulation of the air within the space and that the temperature and humidity be maintained within a range that allows adequate evaporation of perspiration from the skin. It was formerly believed that the discomfort, headache, and lethargy commonly associated with poor ventilation were caused entirely by the increase in the amount of carbon dioxide and the decrease in the oxygen content of the air. There is evidence to show, however, that the deleterious effects result largely from interference with the heat-regulating mechanism of the body. Lack of air currents and the increase in relative humidity and temperature (especially noticeable in crowded, poorly ventilated places) prevent normal evaporation of perspiration and loss of heat from the surface of the skin.

Natural ventilation depends on winds outside and convection currents inside a building. Winds raise air pressure slightly on the windward side of a building and lower it slightly on the lee side. The pressure difference promotes circulation into the building on the windward side and out of it on the lee side. Convection currents are caused by the sinking of colder and therefore heavier air, which displaces the warmer air. A building may have a roof ventilator to allow the rising warm air to escape. If there is an opening to the outside at the bottom of the building, fresh, cool air will be drawn in.

A simple roof ventilator is essentially an opening in the roof with a cover to keep out rain and to prevent winds from interfering with its functioning. Natural convection is an appreciable aid to ventilation in a large building only if it contains sources of large amounts of heat. A further useful adjunct is a fan in the roof ventilator. The addition of distribution ducts to the fan and a system for forcing air into the building provides greater efficiency. Outlets are designed to attain maximum mixing of air and to move large amounts of air at low velocity so that temperature layers are eliminated. Factories have special suction hoods and enclosures to draw away localized dust, fumes, and heat. Incoming air may be cleaned of dust by filters or electrostatic precipitators.

Deep mines, underwater tunnels, and other subterranean and submarine environments require elaborate mechanically operated systems for maintaining the air supply in a healthful condition. The lives of those working in, or traveling through, such areas depend upon a constant supply of fresh air; not only must the systems used be highly efficient, but there should be provision for emergencies in case of failure of the apparatus in operation. An outgrowth of studies of problems of ventilation is the development of methods of air conditioning. Such systems, unlike ordinary methods of ventilation, are independent of outdoor atmospheric conditions and can, therefore, maintain the indoor atmosphere at the most healthful temperature and humidity and can free the air of dust and other undesirable materials. They accomplish this, however, at a considerable cost in energy.