Energy efficient design and indoor air quality are two major challenges facing mechanical engineers today in the field of Heating, Ventilating and Air Conditioning (HVAC). To minimize the loss of energy, building envelopes have been made more energy efficient. This reduces the cost associated with cooling or heating the building. By tightening the building envelopes, the amount of outside air entering the building is reduced. However, that outside air is needed to remove the air contaminants generated indoors. Flushing these pollutants from the indoors to the outdoors has been the most effective way of reducing indoor air contaminants to acceptable levels.

The HVAC industry has responded to these indoor air quality (IAQ) concerns through its professional organization, The American Society of Heating and Refrigerating and Air Conditioning Engineers (ASHRAE). Several years ago, ASHRAE issued IAQ Standard 62, entitled "Ventilation for Acceptable Indoor Air Quality". This standard emphasizes the need for continuous outdoor air ventilation as well as the importance of maintaining indoor humidity levels. This standard has now been integrated, in some fashion, into each of the three major building codes used throughout the United States.

Most people are aware that outdoor air pollution can damage their health, but many do not know that indoor air pollution can also cause harm. Environmental Protection Agency (EPA) studies of human exposure to air pollutants indicate that indoor levels of pollutants may be 2-5 times, and occasionally more than 100 times, higher than outdoor levels. These levels of indoor air pollutants are of particular concern because it is estimated that most people spend approximately 90% of their time indoors.

Studies have found that the quality of indoor air has been linked to many illnesses (sick building syndrome and building related illnesses), and has been shown to have a direct impact on worker and student productivity and comfort. New research strongly suggests that indoor humidity levels have a far greater impact on the health of building occupants than previously suspected. For example, microbial activity (e.g., mold and fungus), which increases at higher indoor humidity levels, has been shown to emit harmful organic compounds. Childhood asthma is now suspected by some researchers to be linked to microbial activity.

In addition to direct health effects, the odors associated with microbial activity are often cited as a primary reason why indoor air quality is considered unacceptable to occupants. When odors are encountered in a building, building managers often respond by increasing outdoor air quantities in an attempt to eliminate odors. This intensifies the problem because increasing outdoor air quantities often results in higher indoor humidity levels, which, in turn, fosters continued microbial activity.