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HVAC/R Jul 11, 2023

Sustaining indoor air quality

At a glance

  • Understand the impact of indoor air quality (IAQ) on both humans and information technology (IT) equipment.
  • Understand the repercussions of inadequate ventilation and unsatisfactory IAQ on occupants and business.
  • Review commonly missed requirements of ASHRAE 62.1 and best practices to maintain acceptable IAQ for data centers.


The connection between IAQ and design has historically been an important—and sometimes overlooked—design consideration.

When designing ventilation systems, it is imperative the designer understand the applicable code requirements, standards such as ASHRAE Standard 62.1: Ventilation and Acceptable Indoor Air Quality, and programs like the US Green Building Council’s LEED rating system.

Ventilation is the process of supplying or removing air from a space to control contaminant levels, humidity, or temperature. In a typical process, outdoor air and return air are mixed, which effectively dilutes indoor particulates and contaminants. The resulting mixed airstream is filtered and conditioned before being supplied into the space.

Designers should always investigate outdoor air quality in their region and survey the immediate surroundings to determine the local air quality and a system’s ability to maintain acceptable IAQ. The US Environmental Protection Agency (EPA) collects air quality data and has an interactive map that shows locations of air-quality monitoring stations across the United States. These air-quality monitoring stations provide data to assist engineers with designing ventilation systems.

Deficient ventilation systems, such as those operating with inadequately sized outdoor air quantities or improper ventilation control, can impact occupant health and productivity. Sick building syndrome and building-related illness are some of the negative impacts on occupant health, with symptoms ranging from headaches, nausea, and chest pain to asthma, Legionnaires’ disease, and sinusitis. On the other side of the spectrum is excessive ventilation, which increases energy use and increases indoor concentrations of outdoor contaminants if ambient air quality is unsatisfactory.

Studies of sick building syndrome prevalence and the amount of outdoor air provided show when the ventilation rate is increased, the prevalence of sick building syndrome is reduced. Studies also indicate a direct correlation between performance and amount of outdoor air introduced in office buildings. It’s important to balance the outdoor airflow rate with the additional power and energy required to condition the air. This is where codes, standards, and other rating systems are useful tools.

ASHRAE Standard 62.1 is frequently referenced in codes for ensuring acceptable IAQ. ASHRAE Standard 62.1 defines acceptable IAQ as air in which there are no known contaminants at harmful concentrations as determined by applicable authorities and where 80 percent or more of people do not express dissatisfaction when exposed to the air.


IAQ for human occupants

ASHRAE Standard 62.1 specifies the minimum ventilation rates and related measures to ensure acceptable IAQ and minimize adverse health effects. The standard applies to spaces intended for human occupancy in buildings except for dwelling units in residential occupancies with nontransient occupants. The origin of Standard 62.1 dates to 1973. It has been revised multiple times since; the latest version is 2022.

In addition to outlining the design requirements, the standard also provides requirements for installation, commissioning, and operation and maintenance of equipment. Ensuring compliance and acceptable IAQ requires coordination and collaboration among stakeholders and continued efforts on the operations and management side.

Some frequently overlooked parts of Standard 62.1 as it applies to commercial buildings are as follows.


Design related

  • Quality of ambient air and its ability to maintain acceptable IAQ should be investigated during design. The US EPA established the National Ambient Air Quality Standards as authorized by the Clean Air Act and quality standards for six primary pollutants: carbon monoxide, lead, nitrogen dioxide, ozone, particle pollution or particulate matter, and sulphur dioxide.
    • Refer to Table 1 for the US EPA established air pollutant standards.
    • For buildings in areas where PM10 (particulate matter with a 10-micrometer diameter or smaller) threshold is exceeded, filters or air cleaning devices with minimum efficiency reporting value 8 should be provided to treat outdoor air before introduction into buildings.
    • For buildings in areas where PM2.5 (particulate matter with a 2.5-micrometer diameter or smaller) threshold is exceeded, filters or air cleaning devices with minimum rating of MERV 11 should be provided to treat outdoor air before introduction into buildings. Refer to Table 2 for filter ratings and common applications.
    • For buildings in areas where the most recent 3-year average annual fourth-highest daily maximum 8-hour average ozone concentration exceeds 0.100 parts per million, ozone cleaning devices with a volumetric removal efficiency of minimum 40 percent should be provided. The devices need to be operated when ambient ozone levels are expected to exceed 0.100 ppm.

  • Following are the exceptions to the ozone cleaning requirement:
    • Design outdoor airflow is 1.5 air changes per hour or less.
    • The system is equipped with controls that can sense ambient ozone level and reduce outdoor air to 1.5 air changes per hour or less while still complying with the other requirements.
    • Direct-fired makeup air units are used to heat outdoor air introduced into the building.

  • An observational survey of the building site and immediate surroundings is required during expected hours of occupancy. The intent is to identify local contaminants that could impact IAQ if introduced into the building.

  • Exhaust ducts that convey Class 4 air should be negatively pressurized relative to ducts, plenums, or occupiable spaces they pass through to eliminate the possibility of contaminant leakage. Positively pressurized exhaust ducts that convey Class 2 or Class 3 air should not extend through plenums or occupiable spaces other than the space the exhaust air is drawn from. However, positively pressurized ducts that convey Class 2 air and are sealed in accordance with SMACNA Seal Class A are an exception to the requirement. SMACNA Seal Class A requires all transverse duct joints, longitudinal seams, and duct penetrations be sealed to minimize air leakage. Refer to Table 3 for air classification based on subjective contaminant concentration.

  • Filters with minimum rating of MERV 8 are required upstream of cooling coils that handle latent loads and other components with wet surfaces such as evaporative humidifiers.

  • For buildings that use mechanical cooling equipment, dehumidification is needed to ensure indoor humidity levels do not exceed 15.5°C (60°F) dewpoint at any time (occupied and unoccupied hours) when the ambient dewpoint is in excess of 15.5°C (60°F). Among other exceptions, the requirement does not apply to overnight unoccupied periods that do not exceed 12 hours, provided the indoor relative humidity does not exceed 65 percent during that time.

  • Drain pans beneath wet components such as cooling coils and direct evaporative humidifiers should begin at the leading face or edge of the device and extend downstream a distance of half the vertical dimension of the device or as necessary to limit water carry-over beyond the drain pan to 0.0044 ounce/square foot of face area per hour under peak sensible and peak dewpoint conditions.

  • Access doors or panels are required in infrastructure such as equipment, ductwork, and plenums to allow for inspection, cleaning, and maintenance of the following components:
    • Air cleaners
    • Drain pans and seals
    • Fans
    • Humidifiers
    • Mixed air plenums
    • Outside air plenums
    • Upstream and downstream of each heating, cooling, and heat-recovery coil comprised of more than four rows and direct evaporative coolers, air washers, heat wheels, and other heat exchangers
    • Upstream of heating, cooling, and heat-recovery coil comprised of four rows or fewer



Construction and start-up related

  • Filters should be installed at equipment before start-up to prevent fouling.
  • Contaminants generated by construction should be confined to the construction area, and migration to occupied areas should be minimized.
  • Drain pans should be field tested under conditions most restrictive to condensate flow to ensure they drain properly and water stagnation is eliminated.


Operations and maintenance related

  • The standard has detailed requirements for maintenance activities for system components that impact IAQ, such as cooling towers, cooling and heating coils, louvers, bird screens, and mist eliminators. Continued compliance is imperative to maintaining IAQ over the life of a facility. While the designer is not responsible for the maintenance of a mechanical system, they are responsible for ensuring the building automation system has features to allow for regular maintenance.



IAQ for data centers

Mechanical systems for data centers are unique in that their purpose is to maintain an operating temperature range and acceptable IAQ for IT equipment. In the United States, the Toxic Substances Control Act influences materials of construction for IT equipment and restricts the use of certain materials, similar to the European Union’s Directive 2002/95/EC: Restriction of Hazardous Substances.

The following best practices ensure satisfactory IAQ in the data center critical environment, based on ASHRAE resources such as ASHRAE TC9.9 white papers:

  • Recirculated air in the data center should be filtered using a minimum of MERV 8 filters.
  • Air introduced into data centers by systems such as makeup air units, direct airside economization, direct evaporative cooling, etc. should be filtered using MERV 11 or MERV 13 filters.
  • Gas phase filtration should be incorporated where gaseous contamination is a concern. The corrosion rates, as measured by copper and silver foil coupons, should be within the following thresholds:
    • Copper reactivity rate of less than 300 Angstrom/month
    • Silver reactivity rate of less than 200 Angstrom/month

Occupants rarely think about the air they breathe—and they shouldn’t have to. Maintaining good indoor air quality is imperative for occupant health and comfort as well as the longevity of IT equipment. If you have questions about indoor air quality, refer to ASHRAE for the latest research and resources.

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This content was previously published by Consulting Specifying Engineer