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Indoor Air Quality - A Maintenance Plan

Even if a regular testing program is not possible, we recommend establishing a baseline against which future results may be compared.

Indoor air quality is all about balance: balance between adequate ventilation and energy conservation, design and operation, and performance and prescriptive approaches to design.

Poor indoor air quality can have serious consequences on the occupants of a building and on the owner.

This issue of Engineering Advisor will look at indoor air quality from the regulatory, design and ownership points of view. But first we must consider what we are trying to control.

Indoor Pollutants

Contaminants of the indoor environment may come from outdoor air or from sources within the building that are recirculated with ventilation air. The terms outdoor air and ventilation air are often used interchangeably, but there is an important distinction between them. Outdoor air, also called intake air or first-pass air, describes air brought into the building from the outdoors; ventilation air is typically a mixture of first-pass and recirculated air.

Contaminants include particulates (such as microorganisms, dust, fumes, and smoke that may be retained in the lungs), gases (molecular vapors that produce toxic or annoying effects), pathogenic or allergenic organisms (fungi, associated mycotoxins and dust mites), and microbial contamination. To control these contaminants, a strategy must be employed that accomplishes one or more of the following approaches: introduce fresh air, limit the introduction of contaminants, treat the contaminants, filter out contaminants, and/or control the humidity.

IAQ Standards

The primary industry standard governing indoor air quality is ASHRAE 62: Ventilation for Acceptable Indoor Air Quality. ASHRAE (the American Society of Heating, Refrigeration, and Air-Conditioning Engineers) first introduced Standard 62 in 1973. The latest version was released in 1989. There is constant new activity in the field of indoor air quality, however. Rather than update the standard every 5 to 10 years, ASHRAE has taken the unusual step of placing the standard under continual review, with addenda being issued when necessary.

Design Factors

There are two principal ways to comply with ASHRAE 62. The first and best-known is a prescriptive approach referred to as the Ventilation Rate Procedure. Here, the amount of outdoor air that must be introduced to maintain indoor air quality is defined in terms of cubic feet per minute per person (cfm/person). It is based on the type of facility (e.g. offices, hotels, theaters, restaurants) and the expected occupancy (e.g. number of people/1,000s.f.).

The Ventilation Rate Procedure incorporates a three-step approach: determine the quality of the outdoor air, treat or filter as necessary, and determine the amount of outdoor air required in each space. This approach, while conceptually simple, may become complicated in its application. Different spaces (offices, meeting rooms, cafeteria, etc.) have different outdoor air requirements. When spaces are served with a common air supply, whether constant volume or variable air volume (VAV), the challenge is to supply the proper percentage of outdoor air to each space. Some spaces will be overventilated, but more often spaces will be underventilated.

Occupancy patterns also affect ventilation. Should conference rooms be ventilated all the time at the rate for full occupancy? Obviously not. How should spaces be ventilated at different times of day? What happens when contaminants build up during periods of low ventilation? Various configurations of controls, dampers, diffusers, and fans are used to provide ventilation or outdoor air when and where it is needed.

In addition to systemic and occupancy factors, there are also operational considerations. An air change rate is specified with the assumption that the building systems will be operated as designed. But that is not always the case. Furthermore, even if a space is ventilated adequately, does that ventilation reach the occupant? Air distribution systems should be designed to minimize "bypass." Ventilation Effectiveness is the ratio between the amount of outdoor air delivered to the space and that which actually reaches the occupants. Economizer cycles (designed to bring in fresh air when outdoor temperatures make mechanical heating or cooling unnecessary) will also affect air quality.

An alternative, performance-based approach is the Indoor Air Quality procedure. Rather than simply target a level of outdoor air that must be introduced, the Indoor Air Quality procedure is based on measuring and monitoring potential contaminants. Outdoor air is provided only in sufficient quantity to mitigate any measured problems.

With the Indoor Air Quality method, it may be possible to actually bring in less fresh air and achieve greater comfort and health. ASHRAE 62 provides a list of pollutants to measure. However, as stated in the standard, not all known pollutants are listed. It is possible that contaminants will be introduced that are not being monitored.

Ownership Recommendations

The diligent building owner will ensure that indoor air quality is being maintained. The first step is to have an engineer review the plans. They should state what method was employed in determining outdoor air requirements (Ventilation Rate or Indoor Air Quality). Then, the engineer should confirm that the building was built according to those plans, and that the building management is operating the building as the mechanical systems designer intended.

In one 900,000 square foot class A office building we have inspected, the building manager has taken a proactive approach. Selected floors are sampled quarterly, such that the entire building is tested once per year. The cost, $16,000 per year, is relatively nominal for the peace of mind obtained. Not all contaminants can be evaluated, but representative testing includes:

  • Temperature and relative humidity for comfort conditions,
  • Carbon monoxide, volatile organic compounds (VOCs), and formaldehyde,
  • Carbon dioxide as an indicator of adequate supply and distribution of outdoor air, and
  • A visual inspection of the ventilation system for conditions that could impact health.

Even if a regular testing program is not possible, we recommend establishing a baseline against which future results may be compared. In the absence of a specific inventory of contaminants to test for, ASHRAE 62 recommends monitoring CO2 levels as a good indicator of overall ventilation. The standard is 1,000 parts per million (ppm) or less, outdoor ranges typically being between 250 and 450 ppm.


Indoor air quality is important in maintaining the health and productivity of building occupants. It is an inexact science, and professionals working in the field are continually learning and updating the governing standards. Nevertheless, there are practical measures that building owners can take on a regular basis to maintain or improve the indoor environment and demonstrate concern and responsibility for those who occupy our spaces.


The Engineering Advisor is intended to enhance your knowledge of technical issues relating to buildings. For additional information on any subject, please feel free to call us. Our commitment is to provide you with timely, accurate information

Criterium Engineers, Copyright © 1999