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

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 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).

Ventilation Standards and Building Codes
After achieving industry consensus in 1989, the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) published its "Standard 62-1989: Ventilation for Acceptable Indoor Air Quality." This is a voluntary standard for "minimum ventilation rates and indoor air quality that will be acceptable to human occupants and are intended to avoid adverse health effects." This standard applies to all types of facilities, including dry cleaners, laundries, hotels, dormitories, retail stores, factories, laboratories, sports and amusement facilities, and others. The specified rates at which outdoor air must be supplied to each room within the facility range from 15 to 60 cfm/person, depending on the activities that normally occur in that room.

Standard 62-1989 is a voluntary standard, which means that it becomes enforceable only after a state or locality adopts the standard in its building code. Furthermore, most current building codes pertaining to ventilation are standards only for the way buildings in a particular jurisdiction must be designed; they are not enforceable standards for the way the buildings are operated. A few states, through recently proposed regulations, pending legislation, labor agreements and other mechanisms, are working to apply existing design codes and standards to building operations.

Ventilation System Problems and Solutions
The processes involved in ventilation provide for the dilution of pollutants. In general, increasing the rate at which outdoor air is supplied to the building decreases indoor air problems. The other processes involved in ventilation however, are equally important. Buildings with high ventilation rates may suffer indoor air problems due to an uneven distribution of air, or insufficient exhaust ventilation. Even in a well-ventilated building there may be strong pollutant sources which impair indoor air quality. The closer such a source is to an exhaust however, the more effective the ventilation; local exhaust ventilation, e.g., a chemical fume hood, is most effective. It is good practice to provide separate exhaust systems in areas where copy machines or solvents are used. Providing localized exhaust for these specific sources can result in a reduction of the amount of overall building exhaust ventilation necessary.

As was mentioned earlier, an HVAC system that is properly designed, installed, operated, and maintained can promote indoor air quality. When proper procedures are not followed, indoor air problems may result. Some common problems, and their solutions, are discussed below.

System Design
Intermittent air flow: Designs that specify HVAC system operation at reduced or interrupted flow during certain portions of the day in response to thermal conditioning needs (as in many variable air volume installations) may cause elevated indoor contaminant levels and impair contaminant removal. Minimum ventilation rates should be defined by air cleanliness and distribution, as well as temperature and humidity.

Distribution of air: Failure to maintain proper temperature, humidity, and air movement in a building can lead occupants to block supply registers if they emit air that is uncomfortably hot or cold; this disrupts air flow patterns. Placement of partitions or other barriers within a space can also impair air movement. In addition, locating air supply and return registers too close together can result in an uneven distribution of fresh air and insufficient removal of airborne contaminants. Precautions must be taken to maintain comfortable thermal conditions, and proper placement of supply and return registers, and furnishings.

Building supply and exhaust locations: Air supply vents that are installed too close to building exhaust vents re-entrain contaminated exhaust air into the building, increasing indoor pollution. Placement of supply vents near outdoor sources of pollution, such as loading docks, parking and heavy traffic areas, chimneys, and trash depots, provides a pathway for contaminants into the building's ventilation system. The location of all air supply vents must be carefully considered.

Ventilation Considerations:

> A supply fan with a diameter of 1 will produce approximately 10% of its face velocity at a distance equal to 30 diameters from the face opening.

> An exhaust fan with a diameter of 1 will lose approximately 90% of its face velocity at a distance of 1 diameter from the exhaust opening.

> The use of ducting systems will improve circulation and minimize airflow losses.
Make-up air should be provided where exhaust systems are operating. The make-up air source point should be located so that only fresh, contaminant-free air is introduced into the working space.

> Never use pure oxygen to ventilate a space. An oxygen-enriched atmosphere is extremely explosive.

> Know whether the airborne vapor contaminants you are trying to remove are heavier or lighter than air (which has a vapor density of 1). This will assist you in properly locating exhaust fans at the most effective height.

> All fan motors and control equipment utilized to move combustible or flammable vapors should be of the explosion-proof type. The metallic parts of air-moving devices, including fans, blowers, jet-type air movers, and ductwork should be electrically bonded to a grounded structure.

OSHA Indoor Air Quality Guidance

OSHA Ventilation Guidance

29 CFR 1910.94 OSHA Ventilation Standard

Training Roster

Training Acknowledgement