Your questions answered: Indoor air quality and indoor environmental quality

IAQ and IEQ are a focus for mechanical engineers specifying or retrofitting HVAC systems

By Consulting-Specifying Engineer November 24, 2023

Air insights

  • ASHRAE Standard 241 is designed to manage infection risks and can be adopted voluntarily by individual entities.
  • Strategies to reduce outside air in buildings include taping air handling unit filter edges and minimizing outside air damper positions.

Indoor air quality (IAQ) quantifies the minimum acceptable air quality that promotes the health and comfort of the human occupants. This includes the removal of harmful pollutants coming into the occupied space from outside or those being generated inside the space.

Indoor environmental quality (IEQ), on the other hand, is a broader term that includes many factors such as air quality, lighting, acoustics and thermal conditions inside the building that affect the health and well-being of the occupants. A better IEQ rating increases the quality of life of the residents in the building.

In this Q&A, webcast presenters addressed items left unanswered during the live event.

Watch the webcast “HVAC: IAQ and IEQ” at any time.


  • Garett Karalus, PE, CCP, Director of Engineering/Lead Commissioning Agent, IEA Inc., Brooklyn Park, Minnesota
  • Emmy Riley, CEM, BEAP, WELL Performance Testing Agent, Energy Engineering Team Leader & Account Manager, Cyclone Energy Group, Chicago

What strategies can be used to reduce the amount of outside air (OA) brought into a building?

Emmy Riley: Strategies we mentioned in the presentation were taping air handling unit (AHU) filter edges and reducing OA damper positions to their minimums.

Typically, AHUs are shut off after normal business hours and cycled on/off as needed to maintain a setback space temp. The space area component of the required ventilation rate is not being delivered in this case. Is that acceptable?

Garett Karalus: In most space types, it is acceptable for ventilation equipment to be turned off during unoccupied building hours. There are special space types that may require continuous ventilation, usually exhaust ventilation, due to specific hazards. The minimum “area” ventilation rate should be maintained during normal building hours, even if the dynamic occupancy of the space is indicated unoccupied and the “people” ventilation rate should be maintained for anytime the space is close to the design occupancy.

Regarding ASHRAE 241, please restate the best time to implement this procedure considering building readiness procedures.

Emmy Riley: The Building Readiness Plan is the deliverable that would result after the assessment, planning and implementation phases have been addressed. Each entity (e.g., state, city, municipality, individual building) can decide for itself what its acceptable threshold is for when to enter Infection Risk Management Mode. An example of a threshold would be when a certain percent of residents or occupants test positive for an infectious disease.

Is the ASHRAE 241 standard a necessity for all buildings or can owners choose not to adopt?

Emmy Riley: ASHRAE Standard 241 is very new and it’s my understanding that it hasn’t been adopted as a necessity for any building. Moving forward, it’s possible that cities and municipalities would adopt Standard 241 as code (mandatory to implement) during times that it deems necessary to invoke Infection Risk Management Mode (e.g., when infectious disease rates exceed a certain percent of the population). Buildings can choose to adopt Standard 241 as soon as they would like; they could adopt it today, specifying a threshold above which they would enter Infection Risk Management Mode.

Is ASHRAE 241 going to be implemented mandatory solely for health care/hospital facilities? Or other types of facilities as well (commercial, industrial, etc.)?

Garett Karalus: Standard 241 guidelines impact the indoor air environments of all buildings (new buildings, existing buildings and major renovations), aimed to safeguard against pathogens such as COVID-19, influenza and more. This standard empowers building owners, operators and professionals to take proactive measures in safeguarding indoor environments. It’s an essential tool for creating healthier indoor environments and promoting sustainable practices. There may be jurisdictions or building types where this standard is adopted as mandatory, however on its own, it should be viewed as a guideline for all buildings. Like all ASHRAE standards, Standard 241 will be updated regularly going forward to help provide the best guidance to safeguard against infectious aerosols.

Do enthalpy wheels increase infectious disease risk? What can be done to reduce this risk?

Garett Karalus: Enthalpy wheels have a small amount of bypass and leakage into the supply airstream that could present risk, although typically the benefits (more outside air, dilution) of utilizing the energy wheels, if not mandatory for an HVAC unit design, will significantly outweigh the risk with the return air leakage. Also, filters are typically going to be in locations to clean return air, outside air and/or mixed air to further reduce the risk associated with this bypass. Whether an AHU is at risk will depend on design and operational factors and how they impact the static pressure relationship. An inspection should be conducted by an industry professional before making any changes to the HVAC system.