New standard on cutting risk of infectious aerosol spread sets high bar for building ventilation but is work in progress

Building HVAC system

oliver de la haye / iStock

[Editor's note: A correction was made in this story on Oct 30, 2023, to eliminate a quote misstating the standard's position on the use of ceiling fans. The standard doesn't specifically mention ceiling fans.]

The first-ever ASHRAE standard on reducing the risk of indoor infectious aerosol transmission sets new targets for building operators in terms of air system design, installation, operation, and maintenance.

But both ASHRAE  and other experts acknowledge that buildings have not been designed to reduce disease transmission—only to heat, dehumidify, and cool both outdoor air and recirculated indoor air and address off-gassing of volatile organic compounds from people and building materials by diluting them—so compliance may require some heavy lifting.

"The requirements for filter and air cleaner testing incorporated in this standard go well beyond what is found in current standards," the authors wrote in the foreword. "They are a major step in the direction of creating uniform and effective technology-agnostic criteria for characterizing filter and air-cleaner performance and safety."

Tony Havics, a certified industrial hygienist and professional engineer in Indiana, said the standard offers some good guidance but doesn't include the assumptions, criteria, and processes used in its development. It also doesn't tailor its recommendations to specific pathogens or scenarios (ASHRAE didn't specify which virus or other variables they used for their estimation).

"How would you make a decision to manage something?" he asked. "If it was based off of COVID, and I've got measles, do I change my approach?"

ASHRAE, founded in 1894, is a global professional society dedicated to advancing heating, air conditioning, refrigeration, and related fields.

Introducing equivalent clean air flow

Earlier in the COVID-19 pandemic, the White House COVID-19 Response Team encouraged ASHRAE to develop Standard 241, the foreword said. The resulting document advises operators of new and existing buildings and those undergoing major renovations to take steps to reduce the average (not individual) risk of infection with SARS-CoV-2, influenza, and other airborne pathogens.

Rather than indoor-outdoor air exchanges per hour, the document describes an "equivalent clean air flow" target per occupant of pathogen-free air, which ASHRAE calls "a more useful and scalable way to represent requirements." The standard includes a calculator to estimate equivalent clean air flow and a table listing minimum equivalents in settings such as correctional and educational facilities, healthcare, industry, sports and entertainment spaces, and residential dwellings.

Air exchanges per hour are not necessarily better than equivalent clean air flow, Steven Welty, an airborne disease expert in Virginia and member of ASHRAE's Epidemic Task Force, told CIDRAP News. Air exchange is "a less-than-well understood phenomenon for controlling airborne disease transmission," he said. "It depends on how the air is being introduced and how it's being evacuated."

Max Sherman, PhD, of the University of Nottingham and vice chair of the standard committee, said in a podcast on 241 that the group spent considerable time developing the equivalent clean air flow measure. "What we did was combine absolute risk and relative risk and theoretical risk and probabilistic risk in a way that met certain criteria," he said. "For example, we wanted the risk in every space to be more or less the same."

To meet equivalent clean air flow targets, the standard requires outdoor-air exchange and/or the use of better-quality filtration and air cleaning technologies such as high-efficiency particulate air (HEPA) filters (including in-room air cleaners), air filters with a minimum efficiency reporting value (MERV) of at least 11, or ultraviolet (UV) lights to clean both outdoor and recirculated indoor air. The latter methods are more cost-effective and energy-efficient than outdoor/indoor air exchanges, Welty said.

"Outdoor air is very expensive to bring in, because you have to cool and dehumidify it in the summer and warm it in the winter," he said. "The critical thing that makes 241 a challenge is monitoring the people in the room to change the clean airflow rates. Empty rooms need to have a sensor to alert airflows changes, just like packed rooms need to have a sensor to do that."

The critical thing that makes 241 a challenge is monitoring the people in the room to change the clean airflow rates. Empty rooms need to have a sensor to alert airflows changes, just like packed rooms need to have a sensor to do that.

Steven Welty

The standard's other key requirements include:

  • An infection risk management mode (IRMM) that applies during periods of heightened disease-transmission risk. ASHRAE does not specify a trigger point; rather, it says authorities with jurisdiction can determine when this mode is needed. "A similar approach could be taken to developing requirements for systems to mitigate wildfire smoke," the standard foreword said.
  • Assessment and planning requirements for building epidemic readiness, including a plan that lists procedures for evaluating new or existing heating, ventilation, and air conditioning (HVAC) systems. Lab or field testing is needed to ensure they are working properly and contributing to the equivalent clean air flow requirement.
  • Separation of sick and vulnerable people from their healthy counterparts in homes.

Havics said there aren't enough trained workers in the country to meet the standard's ongoing requirements. "There needs to be a lot of education, and then there's going to need to be a lot more people for the testing and balancing as well as inspection," he said.

A work in progress

Standard 241 applies to buildings such as single- and multi-family homes, offices, schools, and healthcare facilities.

Group talking at conference
Caiaimage/Martin Barraud / iStock

The standard authors acknowledge that the document will need to be refined as more evidence emerges, including publication of a risk calculator incorporating the methods used to develop equivalent clean air flow goals, amended air distribution guidelines that account for contaminant-removal effectiveness, and guidance on the use of computational fluid dynamics to comply with air distribution requirements.

ASHRAE acknowledged that the document doesn't address requirements for maintaining indoor air quality, may not substantially lower the risk of disease transmission in all situations due to differences in infectious pathogens and personal susceptibility, and addresses only inhalation of infectious aerosols emitted by someone who is 1 meter or more away from a susceptible person.

But Havics said that it should have expanded the area of interest far beyond 1 meter. "They're talking about well-mixed air in the room," he said. "There are some assumptions that go into it, that '4 feet is the same as 20 feet away,' and that any device put into a space to control air delivers equally across the room. That's not the case."

Instead of testing the air at one location in a room, Havics splits a room into nine sections and tests at two different heights in the center of each section. "I can actually tell you exactly how well air is distributed in a room," he said.

Nor does the standard detail optimal air-filter placement or nonviral contaminants of concern. "Since it was written in 6 months, it's not perfect and will be improved," Welty said. "Think of this as the 1.0."

ASHRAE President William Bahnfleth, PhD, said in the podcast that it was difficult to develop a defensible model. "There are so many choices to be made, starting with what risk do I think is acceptable?" he said. "I think this is the best that I've seen, this idea of the risk being the same per hour no matter which covered space type you're in is really powerful."

It's important to mention that air distribution was a tough nut to crack, and we didn't completely fill it; that is, we had something there but we were not happy with it. That's one of the things that we'll work on in version 2.

Max Sherman, PhD

Sherman added, "It's important to mention that air distribution was a tough nut to crack, and we didn't completely fill it; that is, we had something there but we were not happy with it. That's one of the things that we'll work on in version 2: how to improve the air distribution part, how do things interact, what kind of air mixing is good, what kind of mixing is bad."

Needs ANSI rigor

Bahnfleth said the next version will address risk assessment to develop a performance path and the effects of implementation of the standard on energy usage, as well as a user's guide.

Red air vents
Scouse Smurf / Flickr cc

ASHRAE noted in the foreword that previous position statements on airborne infectious diseases resulted in few changes to building standards and codes but said that guidance from its Epidemic Task Force in 2020 on ventilation, filtration and air cleaning, air distribution, HVAC system operation, commissioning for multiple building types, and a framework for planning effective upgrades has been widely used.

Ultimately, Bahnfleth said ASHRAE would like to turn 241 into an American National Standards Institute (ANSI) standard, which forms the basis of building code. Havics said that ASHRAE should have taken the time to gather broader industry input over a longer period and develop it into an ANSI standard rather than release a document that was not ready.

"I like the idea of producing the guidance document, because there's nothing out there," he told CIDRAP News. "But this is one of those times you get it right the first time, and then you don't have to fix it five times."

Still, 241 "makes people be aware of and accept airborne disease transmission," Welty said. "Airborne transmission is a very complicated subject, and it intimidates a lot of people. Because it intimidates a lot of people, they don't want to deal with it and would rather dismiss it. Now 241 shows them how to take actions in order to reduce it when levels may rise again in the fall and winter months."

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