Until the 1960s, ventilation in homes occurred naturally, obviating the need for intentional ventilation systems. Homes were loosely built, allowing enough outside air to make its way through the home to keep it fresh. By some accounts, this loose construction contributed to as many as three to four air exchanges per hour. Although there were ample air exchanges, there was also an unacceptable amount of energy required to run such a home, and uncontrolled ventilation through air leakage can cause serious harm to a building. Currently, with energy-efficient construction, much of the unintentional air exchange has been eliminated. However, while homes were built of more natural, nonpolluting materials in the past, in recent years indoor air has become at least five to ten times more polluted than outdoor air and it is often too polluted for optimal health. Although minimum air-exchange rates are enforced for commercial structures, this is generally not the case for residential construction, except where exhaust fans are mandated.
Like many other components essential to health, ventilation is considered an “extra” in standard construction. The American Society for Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) has set a standard of.35 air exchanges per hour, or 15 cubic feet per minute per resident, for residential ventilation. Although this may be sufficient to dispel pollutants created by human activity, it may not be enough to dispel the chemical pollution generated by standard construction or the thousands of other chemicals introduced into homes through furnishings, clothing, cleaning products, cosmetics, and other scented products. ASHRAE determines its requirements based on the level at which 80 percent of a test population feels comfortable. It should be noted that it is quite possible to feel comfortable in environments that are polluted enough to be detrimental to health. The human body has the ability to become accustomed to harmful chemicals, much as one might adapt over time to the toxic effects of tobacco smoke. Whether or not the ASHRAE standard is sufficient to meet health requirements is irrelevant because in fact most homes are not equipped with ventilation other than spot exhaust fans and do not meet the ASHRAE recommendations.
With tight construction, ventilation strategies are necessary in a healthy home to ensure fresh air and dispel odors from everyday living. Care should be taken to locate the fresh — air supply away from exhaust-air piping and in the best location for receiving an unpolluted airstream.
