Indoor Air Quality

The term indoor environmental quality addresses a broad range of environmental conditions that are experienced by building occupants, including but not limited to thermal comfort, visual comfort, acoustic comfort, and acceptable indoor air quality. Interest in green buildings has generated a heightened interest in residences that can provide exceptional indoor environmental quality. Indoor air quality is an important aspect of this broader concern but should be considered in its own right due to its potential for serious impact on the health and well-being of occupants.


Increasing demands for control of indoor air quality tends to require the design and installation of quality HVAC. Of the three primary means of mitigating indoor pollutants—source control, filtration, and dilution—filtration (through system and equipment selection) and dilution (through ventilation and exhaust) are directly related to HVAC design, installation, and operation.

Conventional venting systems interact with the interior of the house for combustion, dilution, and excess air intake. Byproducts of combustion are exhausted to the outdoors. Conventional venting includes the Type B vent, chimneys, and chimney liners. Both masonry and factory‐built chimneys draw combustion air and excess air from within the house, and they discharge combustion byproducts outside. They are both designed to prevent dilution air from entering the system downstream of the appliance (as in Type B vent draft hoods).

Options for air filtration technologies include air filters and gas-phase filters. Particle air filters are very common, though they are not always effective in filtering bigger particles, which settle quickly due to their size and don’t necessarily reach the filter. Gas-phase filters are used to remove gaseous pollutants. They aren’t as common as particle air filters and tend to not last very long. More information is available at the U.S. Environmental Protection Agency’s (EPA) website.


According to the EPA, the efficiency of particle air filters is measured by “the minimum efficiency reporting value (MERV) for air filters installed in the ductwork of HVAC systems.” The relevant standard for this is set by ASHRAE (American Society of Heating, Refrigerating, and Air‐Conditioning Engineers), in particular Standard 62.1.

ANSI/ASHRAE Standards 62.1 and 62.2 are the recognized standards for ventilation system design and acceptable IAQ. First published in 1973 as Standard 62, Standard 62.1 specifies minimum ventilation rates and other measures for new and existing buildings that are intended to provide indoor air quality that is acceptable to human occupants and that minimizes adverse health effects. More information is available here.

For filtration, ASHRAE’S MERV measurement method ranges from a low of 1 to a high of 20, and it also allows comparison of air filters made by different companies.


The EPA provides an interactive Air Quality Index (AQI) calculator, which allows the user to input any number of indoor air pollutants (including carbon monoxide, sulfur dioxide, and ozone) and determine the corresponding indoor air quality. You can calculate from AQI to concentration or from concentration to AQI. The calculator will provide lists of sensitive groups, health effects statements, and cautionary statements based on the entered specifics.

The tool is best used in conjunction with the EPA’s table defining the ranges of the Air Quality Index:

(AQI) Values Levels of Health Concern Colors
When the AQI is in this range: Air quality conditions are: As symbolized by this color:
0 to 50 Good Green
51 to 100 Moderate Yellow
101 to 150 Unhealthy for sensitive groups Orange
151 to 200 Unhealthy Red
201 to 300 Very unhealthy Purple
301 to 500 Hazardous Maroon

More information from the EPA on the specifics of the table, how AQI works, and other details are all available here.

In AGS Online, “Indoor Air Quality,” acceptable indoor air quality is examined as a critical design and operational concern in nonresidential buildings. Numerous instances of “sick-building syndrome” have been reported in the general and technical press. The cost of mitigation in many of these cases has been astronomical. Design for acceptable indoor air quality is no less a concern in residences, even though residential sick-building cases are generally less reported (and often undiscovered). Pollutants are as likely to be found in residences as in commercial/institutional buildings (though their makeup may change), and design attention to this problem may be less focused in the mass construction residential market.