The ventilation rate, for CII buildings, is normally expressed by the volumetric flow rate of outside air being introduced to the building. The typical units used are cubic feet per minute (CFM) or liters per second (L/s). The ventilation rate can also be expressed on a per person or per unit floor area basis, such as CFM/p or CFM/ft², or as air changes per hour.
For residential buildings, which mostly rely on infiltration for meeting their ventilation needs, a common ventilation rate measure is the air change rate (or air changes per hour): the hourly ventilation rate divided by the volume of the space (I or ACH; units of 1/h). During the winter, ACH may range from 0.50 to 0.41 in a tightly insulated house to 1.11 to 1.47 in a loosely insulated house.
ASHRAE now recommends ventilation rates dependent upon floor area, as a revision to the 62-2001 standard, in which the minimum ACH was 0.35, but no less than 15 CFM/person (7.1 L/s/person). As of 2003, the standard has been changed to 3 CFM/100 sq. ft. (15 l/s/100 sq. m.) plus 7.5 CFM/person (3.5 L/s/person).
Ventilating a space with fresh air aims to avoid “bad air”. The study of what constitutes bad air dates back to the 1600s, when the scientist Mayow studied asphyxia of animals in confined bottles.] The poisonous component of air was later identified as carbon dioxide (CO2), by Lavoisier in the very late 1700s, starting a debate as to the nature of “bad air” which humans perceive to be stuffy or unpleasant. Early hypotheses included excess concentrations of CO2 and oxygen depletion. However, by the late 1800s, scientists thought biological contamination, not oxygen or CO2, as the primary component of unacceptable indoor air. However, it was noted as early as 1872 that CO2 concentration closely correlates to perceived air quality.
The first estimate of minimum ventilation rates was developed by Tredgold in 1836. This was followed by subsequent studies on the topic by Billings in 1886 and Flugge in 1905. The recommendations of Billings and Flugge were incorporated into numerous building codes from 1900-1920s, and published as an industry standard by ASHVE (the predecessor to ASHRAE) in 1914.
Study continued into the varied effects of thermal comfort, oxygen, carbon dioxide, and biological contaminants. Research was conducted with humans subjects controlled test chambers. Two studies, published between 1909-1911, showed that carbon dioxide was not the offending component. Subjects remained satisfied in chambers with high levels of CO2, so long as the chamber remained cool. (Subsequently, it has been determined that CO2 is, in fact, harmful at concentrations over 50,000ppm)
ASHVE began a robust research effort in 1919. By 1935, ASHVE funded research conducted by Lemberg, Brandt, and Morse – again using human subjects in test chambers – suggested the primary component of “bad air” was odor, perceived by the human olfactory nerves. Human response to odor was found to be logarithmic to contaminant concentrations, and related to temperature. At lower, more comfortable temperatures, lower ventilation rates were satisfactory. A 1936 human test chamber study by Yaglou, Riley, and Coggins culminated much of this effort, considering odor, room volume, occupant age, cooling equipment effects, and re-circulated air implications, which provided guidance for ventilation rates. The Yaglou research has been validated, and adopted into industry standards, beginning with the ASA code on 1946. From this research base, ASHRAE(having by the replaced ASHVE) developed space by space recommendations, and published them as ASHRAE Standard 62-1975: Ventilation for acceptable indoor air quality.
As more architecture incorporated mechanical ventilation, the cost of outdoor air ventilation came under some scrutiny. In cold, warm, humid, or dusty climates, it is cost preferable to minimize ventilation with outdoor air to conserve energy, cost, or filtration. This critique (e.g. Tiller) led ASHRAE to reduce outdoor ventilation rates in 1981, particularly in non-smoking areas. However subsequent research by Fanger, W. Cain, and Janssen validated the Yaglou model.
