By Eamonn Ryan
The COVID-19 pandemic fundamentally changed how the world thinks about public health. Yet one of its most persistent blind spots remains the role of buildings in disease transmission – and, critically, in disease prevention. While vaccines, masks and social distancing dominated public discussion, the spaces where people spend nearly 90% of their time received far less co-ordinated attention. This is part one of a six-part series.
Not a new idea, but a timely one. Freepik.com
That gap is the focus of ASHRAE Journal Podcast Episode 26, which features two of the most influential figures in building science and indoor environmental quality: William Bahnfleth and Max Sherman. Hosted by Emily Toto, ASHRAE manager of codes, the episode explores the intent, development and implications of ASHRAE Standard 241: Control of Infectious Aerosols, a landmark standard created in direct response to lessons learned during COVID-19.
Together, the conversation poses a critical question that extends well beyond the current pandemic: How do we prepare our buildings for the next epidemic – before it happens?
Why buildings matter in epidemic preparedness
Historically, public health responses to infectious disease outbreaks have focused on human behavior and medical interventions. Buildings were treated as neutral backdrops rather than active participants in disease transmission. Yet COVID-19 made one fact impossible to ignore: airborne transmission occurs indoors, and building systems directly influence how that risk is amplified or reduced.
Bahnfleth, chair of the ASHRAE Environmental Health Committee and former ASHRAE president, explains that prior indoor air quality (IAQ) standards were never designed to address epidemic-scale infection control. Standards such as ASHRAE 62.1 and 62.2 focus primarily on comfort, odour control, and general health – not on managing infectious aerosols during public health emergencies.
Standard 241 fills that void. Its explicit purpose is to establish minimum requirements for controlling infectious aerosols in both new and existing buildings, including renovations. In doing so, it marks a turning point in how the building industry contributes to public health preparedness, epidemic resilience, and disease transmission mitigation.
Learning from COVID-19: from panic to planning
During the early stages of the COVID-19 pandemic, building owners and operators scrambled to respond. Questions arose everywhere: Were outdoor air dampers actually open? Were filters properly installed? Could existing HVAC systems handle higher ventilation rates?
As Sherman, retired senior scientist at Lawrence Berkeley National Laboratory and vice chair of the Standard 241 committee, notes, the industry lacked a defensible answer to a deceptively simple question: “How much clean air is enough?”
The absence of clear, science-based targets led to inconsistent guidance, confusion and in some cases, ineffective or wasteful interventions. Standard 241 was developed specifically to avoid repeating that experience in the future. Rather than reacting in crisis mode, buildings can now be designed and operated with readiness in mind.
A new role for buildings in public health
One of the most significant shifts embodied in Standard 241 is conceptual. It introduces the idea that buildings should operate differently during extraordinary events – what the standard calls Infection Risk Management Mode (IRMM). This mode is only activated when public health authorities, building owners, or occupants determine that elevated risk exists.
Outside of IRMM, buildings operate normally, following established ventilation and IAQ standards. But when an epidemic emerges, pre-planned systems and strategies are activated – without the need for emergency retrofits or last-minute equipment purchases. This approach reframes buildings as adaptive infrastructure, capable of responding dynamically to public health threats.
Bahnfleth describes this as bringing resilience into air quality standards for the first time. Much like preparedness for extreme weather or wildfire smoke events, epidemic readiness becomes something designed in advance rather than improvised under pressure.
Not a new idea, but a timely one
Interestingly, the podcast highlights that this approach is not entirely new. In the early 20th century, pioneers such as Florence Nightingale and physician-engineer John Shaw Billings emphasised high ventilation rates specifically to control contagion. Over time, priorities shifted toward comfort and energy efficiency, and infection control faded from prominence.
Standard 241 represents a return to those original public health roots – updated with modern science, probabilistic risk assessment and advanced HVAC and air-cleaning technologies.
Setting the stage for the next epidemic
The development of Standard 241 – completed in a record 116 days – signals a broader recognition that pandemic preparedness must include the built environment. It does not claim to eliminate risk, nor does it replace medical or behavioural interventions. Instead, it provides a clear, actionable framework for reducing long-range airborne disease transmission indoors.
As future articles in this series will explore, the standard introduces new metrics, operational strategies, and pathways for innovation. But at its core lies a simple, powerful message:
If we want to be ready for the next epidemic, our buildings must be ready too.
