Improper ventilation can significantly multiply the amount of indoor air contaminants in an office, but just opening the windows isn’t always the answer.

Proper ventilation depends on if the system processes are functioning properly.

In fact, buildings with high ventilation rates may suffer indoor air problems due to an uneven distribution of air, or insufficient exhaust ventilation.

An HVAC system that is properly designed, installed, balanced, operated, and maintained can maintain a healthy indoor air quality. It’s when procedures aren’t followed, that indoor air can become unhealthy for building occupants.

Below are common problems within a building:

Intermittent air flow:

  • HVAC system system designs that operate at reduced or interrupted flow during certain portions of the day or in response to thermal conditioning needs may cause elevated indoor contaminant levels and impair contaminant removal.
    • Minimum ventilation rates should be defined by air cleanliness and distribution, temperature and humidity.

Distribution of air:

  • Failure to maintain proper temperature, humidity, and air movement in a building may emit air that is uncomfortably hot or cold. To combat this, occupants to block supply registers with furnishings or other items and thus disrupting air flow patterns.
  • Placement of partitions or other barriers within a space can also impair air movement.
    • Precautions must be taken to maintain comfortable thermal conditions and placement of furnishings must be done wisely.

Building supply and exhaust locations:

Placement of supply vents is crucial.

  • Air supply vents that are installed too close to building exhaust vents pull contaminated exhaust air into the building.
  • If air supply vents are near outdoor sources of pollution such as loading docks, parking and heavy traffic areas, chimneys, and trash depots, that contaminated air can be brought into the building’s ventilation system.
    • Carefully consider the locations of all air supply vents.

Proportion of Outdoor Air:

  • HVAC systems must bring in adequate amounts of outdoor air to dilute and remove indoor contaminants, but it is costly to heat cold winter air and to cool hot summer air. Thus, some HVAC systems reduce or eliminate the amount of outdoor air brought into the system. contaminated air can then build up inside.
    • A continuous supply of fresh air must be provided.

Periods of Operation:

Any time an HVAC system is off, contaminants can accumulate.

  • An HVAC system that begins to operate after building occupants have arrived, or shuts off before the end of the work day can cause an increase in building-and occupant-generated pollutant levels.
  • If the system is off during periods of non-occupancy, building-generated pollutants can accumulate.
    • The ventilation system should be turned on several hours prior to occupancy, and shut down only after occupants have left.


HVAC systems must be properly maintained to ensure healthy indoor air.

If not:

  • Ventilation systems can become clogged hindering their performance and air flow.
  • Humidification and dehumidification systems can cultivate the growth of molds and bacteria.
  • Improperly treated water in cooling towers can lead to the growth of organisms, such as Legionnella. The bacteria can then contaminate the HVAC supply ducts and cause serious health problems.

The EPA does not recommend cleaning your duct unless:

  • There is visible mold growth on the surfaces of the air ducts or on other components of your HVAC system.
  • The duct work has been infested with rodents or insects.
  • The ducts are clogged with excessive amounts of debris.
  • Particulates are being released in to the home from your supply registers.
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