Clean, oxygen-rich air for breathing is the most pressing environmental requirement for human life. Continuously moving air is also a primary requirement for thermal comfort, to convect away excess body heat and evaporate perspiration. Although the air in a rural outdoor environment contains such natural pollutants as odors, bacteria, pollens, spores, molds, and dust, we usually perceive it as being pure, and all but the most allergic persons find it optimum for breathing.
Air movement in the outdoors varies over a broad range. On the occasional summer day when there is little or no wind, a sweltering, suffocating, claustrophobic sensation is felt. A light breeze has a pleasant, exhilarating, cooling effect. With increasing wind velocity, convective and evaporative heat losses from the skin can become excessive, particularly if the weather is cold. At very high wind velocities, chilling of the body is extreme; respiration may become difficult; solid objects are picked up and carried by the wind; and structural damage to trees and buildings is likely.
In cities, manufactured pollutants crowd the air: carbon monoxide, carbon dioxide, nitrogen oxides, hydrocarbons, sulfur dioxide, hydrogen sulfide, soot, ash, dust, and odors. Most of these substances are generated by the combustion of fuels in vehicles and heating plants, and others are given off by industrial processes. In the confined spaces of buildings, air loses some of its oxygen and gains carbon dioxide through repeated respiration by human lungs. Bacteria and viruses accumulate. Odors build up from sweating, smoking, toilet functions, cooking, and industrial processes. Gaseous pollutants such as radon and formaldehyde may be given off by building materials. The vapor content of the air is raised by breathing, sweating, bathing, cooking, washing, drying, and unventilated gas-burning appliances. Dust and dirt particles float suspended in the air. The temperature of the indoor air may rise above comfortable levels because of solar heat gain, electric lighting, body heat, or the incidental heat given off by industrial processes, cooking, or washing. Air movement is restricted by the walls, floors, and ceilings of the buildings and may become uncomfortably sluggish. Thus buildings require ventilation systems to ensure an optimum velocity of air movement inside and to dilute and replace contaminated air with new air of acceptable temperature, dryness, and cleanliness.
At the same time, the air in buildings often contains odors that we find pleasant, such as the aroma of baking bread, the scent of flowers, or the evocative pungency of incense in a cathedral, and too-rapid ventilation can spoil the experience. But in buildings that are especially prone to dampness or natural overheating, or in rooms that generate heat and odor, such as restaurant kitchens, gymnasium locker rooms, bars, chemistry laboratories, livestock barns, auditoriums, or foundries, rapid rates of air replacement are necessary. Lower rates are usually sufficient for most residential occupancies, lightly occupied offices, warehouses, and light manufacturing plants. Regardless of the volumetric rate of ventilation, however, indoor air speeds must not be so high as to blow belongings about the room (11.1).
Any building ventilation system, from the simplest to the most complex, has four basic components:
1. An air source of acceptable temperature, moisture content, and cleanliness
2. A force to move the air through the inhabited space of the building
3. A means of controlling the volume, velocity, and direction of the airflow
4. A means of recycling or disposing of contaminated air
The simplest ventilation system in buildings is an inadvertent one: it utilizes the outdoor air as its source, the wind as its motive force, the cracks and seams on the windward side of the building as orifices that introduce the fresh air at a controlled velocity and volume, and the leeward cracks and seams as orifices that leak stale indoor air back to the outdoors (11.2). In tightly constructed buildings, this infiltration ventilation is slow, but in buildings with loose-fitting windows and doors, it can become excessive under windy outdoor conditions, causing drafty rooms and wasting energy in cold weather through the leakage of heated air to the outdoors. In general, infiltration should be minimized by the weatherstripping of doors and windows, the provision of a continuous air barrier around the perimeter of a building, and the airtight sealing of construction seams. Even when such measures are employed, some air leakage is inevitable, but it functions as a useful, minimum-level air replacement system for a simple building.
Most schemes for natural ventilation of buildings use windows to control the volume, velocity, and direction of airflow. For this reason, most types of windows are designed to be adjustable to any degree of openness. In most areas, an insect screen must be incorporated into every opening window, to keep out bugs, birds, and small animals while still admitting air and light. Where security is a potential problem, as in banks, jails, mental hospitals, and buildings in high-crime areas, steel bars or heavy metal screens may also be fitted to the openings.
Various patterns of window operation have particular advantages: A casement window opens to the full area of the window opening, and its swinging sash can function to divert passing breezes into the room (11.3). A double-hung window can open to only half its full area, but the half can be at the top, the bottom, or a part of each (11.4). An awning window or horizontal pivoting window admits air while keeping out rain, a function that other window types can perform only if protected by a broad roof overhang or a separate awning (11.5).
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