By Eamonn Ryan
When the COVID-19 pandemic forced building owners, engineers, and policymakers to confront airborne disease transmission, one question surfaced again and again: How much clean air is enough to reduce infection risk? This is part two of a six-part series.
A foundation for epidemic-ready buildings. Wirestock | Freepik.com
It is a deceptively simple question – and one the building industry struggled to answer in real time.
That challenge sits at the heart of ASHRAE Journal Podcast Episode 26, hosted by Emily Toto, with guests William Bahnfleth and Max Sherman, the chair and vice chair of ASHRAE Standard 241: Control of Infectious Aerosols. As the conversation reveals, the lack of a common, defensible metric during COVID-19 was not merely inconvenient – it was a fundamental barrier to effective action.
Standard 241 responds with a breakthrough concept that reframes how buildings manage airborne infection risk: Equivalent Clean Air (ECA).
Why traditional ventilation metrics fell short
Before COVID-19, most building professionals relied on outdoor air ventilation rates as the primary indicator of indoor air quality. Standards such as ASHRAE 62.1 and 62.2 specify minimum ventilation rates based on occupancy and space type, largely to control odours and general contaminants.
But during an epidemic, ventilation alone does not tell the full story.
As Sherman explains in the podcast, focusing exclusively on outdoor air created confusion and inefficiency. In some climates, dramatically increasing ventilation carried steep energy penalties. In others, systems simply could not deliver the airflow needed. Meanwhile, technologies such as filtration, ultraviolet germicidal irradiation (UVGI), and portable air cleaners were widely discussed – but without a common way to compare their effectiveness.
The result was a fragmented response, driven more by rules of thumb than by a unified risk framework.
Introducing equivalent clean air
ECA changes that equation. Rather than asking how much outdoor air a space receives, ECA asks a more fundamental question: How much air is effectively cleaned of infectious aerosols – by any means?
In Standard 241, clean outdoor air, filtered recirculated air, disinfected air (such as UV-treated air), and other validated air-cleaning approaches are all translated into a single metric: equivalent clean airflow. This allows different strategies to be compared, combined and optimised.
Bahnfleth describes ECA as a powerful unifying concept – one that brings together dilution, filtration and disinfection into a single performance-based framework. Instead of prescribing one solution, the standard specifies how much equivalent clean air per person is required when a building enters Infection Risk Management Mode (IRMM).
Flexibility without guesswork
One of the most important advantages of the ECA approach is flexibility. A building does not need to rely solely on increasing outdoor air above minimum ventilation requirements. Instead, designers and operators can choose the most practical mix of strategies for their climate, building type and system capabilities.
For example:
- A school may increase filtration efficiency and add portable air cleaners
- An office building may combine moderate ventilation increases with in-duct UV
- A residential building may rely heavily on filtration and localised air cleaning
All of these approaches can be translated into equivalent clean air and evaluated against the same performance target.
This flexibility also addresses one of the most common concerns raised during the pandemic: energy use. As Dr. Sherman notes, Standard 241 does not require energy-intensive solutions. In many cases, compliance can be achieved with little or no increase in outdoor air, minimising energy impacts while still improving infection control.
Where do the numbers come from?
Determining how much equivalent clean air is ‘enough’ was one of the most technically challenging aspects of developing Standard 241. During COVID-19, the ASHRAE Epidemic Task Force was often asked for a number – but could not responsibly provide one.
For Standard 241, that changed.
As described in the podcast, the project committee developed a sophisticated risk assessment model that combines absolute risk, relative risk and probabilistic approaches. The goal was not to eliminate risk – an impossible task – but to equalise risk across different types of spaces, so that an hour spent in a classroom carries roughly the same infection risk as an hour spent in an office or retail space.
Importantly, this is a population-level risk framework, not a guarantee of individual protection. It accounts for uncertainty in community infection rates, variability in infectious aerosol emission, and differences in occupancy patterns. The complexity of this analysis is intentionally hidden from users, who instead rely on clear lookup tables provided in the standard.
A foundation for epidemic-ready buildings
Equivalent Clean Air represents more than a technical innovation – it signals a shift in how buildings are expected to perform during public health emergencies. Rather than improvising under pressure, building professionals now have a common language and a quantifiable target for reducing airborne transmission risk.
As future articles in this series will explore, ECA underpins many other features of Standard 241, from technology evaluation to commissioning and operational planning. Together, these elements move the industry from reactive measures to intentional epidemic preparedness.
In the post-COVID era, the question is no longer whether buildings influence disease transmission. The question is whether they are designed to manage it effectively. ECA provides the metric that finally makes that possible.
