Millions of Americans work in buildings with mechanical heating, ventilation, and air-conditioning (HVAC) systems; these systems are designed to provide air at comfortable temperature and humidity levels, free of harmful concentrations of air pollutants. While heating and air-conditioning are relatively straightforward operations, the more complex processes involved in ventilation are the most important in determining the quality of our indoor air.

While many of us tend to think of ventilation as either air movement within a building or the introduction of outdoor air, ventilation is actually a combination of processes which results in the supply and removal of air from inside a building. These processes typically include bringing in outdoor air, conditioning and mixing the outdoor air with some portion of indoor air, distributing this mixed air throughout the building, and exhausting some portion of the indoor air outside. The quality of indoor air may deteriorate when one or more of these processes are inadequate. For example, carbon dioxide (a gas that is produced when people breathe) may accumulate in building spaces if sufficient amounts of outdoor air are not brought into and distributed throughout the building. Carbon dioxide is a surrogate for indoor pollutants that may cause occupants to grow drowsy, get headaches, or function at lower activity levels. There are many potential sources of indoor air pollution, which may singly, or in combination, produce other adverse health effects. However, the proper design, operation and maintenance of the ventilation system is essential in providing indoor air that is free of harmful concentrations of pollutants.

Sources of Indoor Air Pollution

Indoor air pollution is caused by an accumulation of contaminants that come primarily from inside the building, although some originate outdoors. These pollutants may be generated by a specific, limited source or several sources over a wide area, and may be generated periodically or continuously. Common sources of indoor air pollution include tobacco smoke, biological organisms, building materials and furnishings, cleaning agents, copy machines, and pesticides.

Health Problems and Ventilation

Harmful pollutants from a variety of sources can contribute to building-related illnesses, which have clearly identifiable causes, such as Legionnaire's disease. HVAC systems that are improperly operated or maintained can contribute to sick building syndrome (SBS); SBS has physical symptoms without clearly identifiable causes. Some of these symptoms include dry mucous membranes and eye, nose, and throat irritation. These disorders lead to increased employee sick days and reduced work efficiency.

A committee of the World Health Organization estimates that as many as 30 percent of new or remodeled buildings may have unusually high rates of sick building complaints. While this is often temporary, some buildings have long-term problems which linger, even after corrective action. The National Institute for Occupational Safety and Health reports that poor ventilation is an important contributing factor in many sick building cases.

Controlling Indoor Air Pollution

Control of pollutants at the source is the most effective strategy for maintaining clean indoor air. Control or mitigation of all sources, however, is not always possible or practical. Ventilation, either natural or mechanical, is the second most effective approach to providing acceptable indoor air.

In the past, most buildings had windows that opened; airing out a stuffy room was common practice. In addition, indoor-outdoor air pressure differences provided ventilation by movement of air through leaks in the building shell. Today however, most newer office buildings are constructed without operable windows, and mechanical ventilation systems are used to exchange indoor air with a supply of relatively cleaner outdoor air.

The rate at which outdoor air is supplied to a building is specified by the building code. Supply rates are based primarily on the need to control odors and carbon dioxide levels; carbon dioxide is a component of outdoor air, but its excessive accumulation indoors can indicate inadequate ventilation. Supply rates, hereafter referred to as ventilation rates, are commonly expressed in units of cubic feet per minute per person (cfm/person).

Economic Considerations of Air Quality

It is generally agreed that poor indoor air can adversely affect employee health and productivity. These costs to industry have been estimated to be in the "tens of billions of dollars per year" (Report to Congress on Indoor Air Quality, 1989). Improvements in the indoor air environment may substantially increase employee moral and productivity. Therefore, it is important to include indoor air quality controls in operation, maintenance, and energy conservation strategies.

Resolving Air Quality Problems

To improve indoor air quality; areas to address include:

HVAC system operation and maintenance: Operate the ventilation system in a manner consistent with its design. Perform maintenance and inspections on a regular basis, as prescribed by the manufacturer.

Record keeping: Maintain records of all HVAC system problems, as well as routine maintenance and inspection activities. Document the nature of complaints concerning the indoor air environment, as well as steps taken to remedy each complaint. These records may be useful in solving future problems.

Pollution control: Identify pollution sources. Implement source removal or special ventilation techniques (including restrictions on smoking).

Occupant activities: Eliminate practices which may restrict air movement (e.g., furniture placement relative to air vents).

Building maintenance activities: Increase ventilation rates during periods of increased pollution, e.g., during painting, renovation, and pesticides use; schedule use of pollutant sources to minimize the impact on indoor air quality.

Ventilation standards and codes: Keep abreast of revisions to ventilation standards and building codes affected by those standards.

Energy conservation: Reexamine energy conservation practices with regard to indoor air quality considerations, employee health, and productivity costs.