Since the early days of the pandemic, HVAC professionals have been saying, “It’s all about the air.” While there has been an increase in awareness from public health officials of the importance of good IAQ in mitigating viral spread, in my estimation, the embrace of IAQ as a defense mechanism has been too slow in coming.

Recent events, though, give me hope that change is coming in the near future. Two events, in particular, are worthy of discussion.

On May 12, 2023, the Centers for Disease Control and Prevention (CDC) posted guidance that calls for a higher standard for IAQ. Highlights of the guidance include:

• Recommendation for at least five air changes per hour of clean air¹ in occupied spaces
• Updated minimum filter recommendation to Minimum Efficiency Reporting Value (MERV) 13
• Information bulletin regarding maintenance and energy cost considerations for various ventilation strategies

While the guidance does not have the rule of law – only building code has the rule of law – the CDC has validated the fact that maintaining IAQ is an important part of public health.

Equally – or perhaps more – important was the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) publishing a draft standard for public review. The new standard, “Control of Infectious Aerosols – ASHRAE Standard 241P” has been released for public review, with the hope that it will be formalised by the end of June 2023.

The standard, once implemented, will for the first time establish minimum guidelines for control of infectious aerosols, and also provide a roadmap for clients to meet those guidelines.

Highlights of the draft standard include:

1. Minimum equivalent outdoor air for infection control rates – prescriptive rates. This guideline (cfm/person) establishes minimum airflow rates for infection control (that is, when owner or public health officials decides that we are at risk of infection). The airflow rates are quite high as compared to normal airflow rates, as would be expected. Also, it is important to emphasise that the equivalent outdoor rates include “normal” ventilation – vis-à-vis outdoor air provided by an air-handling unit – as well as equivalent outdoor air, provided by an air-cleaning device.

2. Arrangement for air inlets and outlets. This highlights the need for the HVAC designer to take into account the potential for short circuiting of supply to return air diffusers. This is an important factor in HVAC design and operation, since improperly placed supply and return diffusers will negate the dilution effectiveness of HVAC airflow distribution.

3. Mechanical ventilation guidance. The standard provides a calculation methodology for a variety of space types (single zone, multi-zone, 100% outdoor air systems and unventilated spaces). For recirculation of air, which is a major concern for infectious aerosol control, the standard specifies a 50% minimum efficiency for infectious aerosol removal or inactivation. For many systems operating at minimum airflow, this will require either the use of an air-cleaning device or a more elaborate filtration system.

4. Commissioning for infection control. The standard provides guidance for commissioning of Existing Buildings as well as New Construction. The proposed methodology follows the traditional Cx approach but with a focus on infection control. Key attributes of the guidance document include testing of various building management modifications for infection control mode, visual check of air filtration systems and confirmation of air-cleaning technology functionality.

5. Roadmap for assessment, planning and implementation of IAQ for existing facilities. The roadmap gives guidance on how to evaluate existing buildings. The Building Readiness Plan (BRP) will provide the end-user an overall assessment of the building’s IAQ. Important factors, such as building controls, filtration levels and air-cleaning technologies are documented, so that the user has an assessment of the building’s state of readiness for an infectious aerosol event.

• • The standard also prescribes methods to measure outdoor air flow levels. Specifically, the standard requires a certified testing and balance report within the past 1.5 years, which I recommended similarly in a previous article, and/or calculations based on existing BMS or CO2 readings.

6. Procedure for measuring effectiveness and safety for filtration and air-cleaning technologies. The standard, for the first time, defines a universal laboratory testing standard for air-cleaning devices – both within the room and duct-mounted systems. The test, conducted in a controlled environment, ensures that air-cleaning devices are tested per a common protocol. The test will be approved by an independent party, which will also have to be approved by the local authority having jurisdiction (that is, code official).

7. Operations and Maintenance guidance. Provides guidance to develop a Building Readiness Manual, which will be available on site. The manual should include:

• • Essential facility supplies: Filtration system inventory, belts, tools, etc. The guide specifies to ensure supplies for a period of three months – most likely in recognition of the supply shortage of filters we experienced in the early months of the pandemic.
  • Modes of operation: The manual should include an operating procedure for Normal Mode, Infection Risk Management Mode (IRMM) and Shutdown Mode (more definition for this mode will be forthcoming). Basically, this will allow operators to have guidance on what modifications need to be made when IRMM is dictated.
  • Facility operating schedule: Define guidance for various airflow control strategies, namely occupancy sensors and demand control ventilation for IRMM conditions.
  • Operator training: Give guidance regarding IAQ training for systems operation (that is, simple test methods, filter ratings and their meaning, and ventilation requirements for various space types).
  • Minimum maintenance guidance for common equipment: The standard gives minimum inspection guidelines for equipment, such as airflow stations, filters and ventilation openings.

8. Systems Manual Guideline: The standard provides guidance regarding what information is relevant for a systems manual, which will be very useful when infectious aerosols are a concern. Information such as equipment inventory, airflow quantities by space, filtration equipment, air cleaning equipment and expected outcomes and troubleshooting guidance will assist facility operators when the information is needed for a crisis event.

Jeremy McDonald

The listing of items, above, constitutes a brief overview of the standard. Please note that the 39-page standard has a lot of complexities which are very much generalised for the purpose of writing this article.

In summary, the standard is an excellent first draft of what a healthy IAQ future will look like. I commend the ASHRAE team members for their hard work and attention to “lessons learnt” from the pandemic. It is clear that there was a lot of thought put into the standard, and we will all benefit from improved IAQ, both in our day-to-day lives, and when we next need to confront a challenging infectious aerosol event.

A closing note: About two years into the pandemic, I was having a conversation with a high-ranking public official at the state level. We both shared our concern that the hard lessons we learnt from the pandemic, namely the importance of IAQ, would be soon forgotten once the pandemic receded. With the CDC statement and ASHRAE’s new infectious control standard, I am confident that the painful lessons from the pandemic will be incorporated into new preventative maintenance practices, which will benefit generations for years to come.

¹ This can be achieved through any combination of central ventilation system, natural ventilation or air-cleaning devices.

Jeremy McDonald is a principal of Guth DeConzo Consulting Engineers, in New York. Before the pandemic, he was an adjunct professor at Rensselaer Polytechnic Institute. Recently, he was the technical consultant to the New York State Energy Research and Development Authority in development of an IAQ guideline for Higher Education in NY: “Covid-19 Response Guide, State University of New York”. He may be reached at jmcdonald@guthdeconzo.com.