By Eamonn Ryan
During the COVID-19 pandemic, many building owners believed they were “doing the right thing.” Outdoor air dampers were opened. Filters were upgraded. Portable air cleaners appeared in hallways and classrooms. Yet infection risk often remained stubbornly high. The reason, as ASHRAE Standard 241 now makes explicit, is simple but uncomfortable: design intent does not equal delivered performance. This is part four of a six part series.
Commissioning as a core requirement, not an afterthought. Freepik.com
ASHRAE Journal Podcast Episode 26 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.
Standard 241 marks a turning point by explicitly addressing a long-standing gap in building practice: how do we know that the clean air we think we are delivering is actually reaching occupants? This is where commissioning, verification and field performance move from optional best practices to central pillars of epidemic readiness.
Lessons from COVID: when systems didn’t do what we thought
As Bahnfleth notes, early in the pandemic many facility managers rushed to “increase ventilation” only to discover that systems were not configured, maintained or even capable of delivering what the controls claimed. Dampers that were commanded open were stuck. Filters were improperly installed. Airflows were assumed rather than measured.
The pandemic exposed a systemic weakness: buildings were trusted without being verified.
Standard 241 responds to that failure by embedding verification directly into the framework of infectious aerosol control. It does not assume that compliance on paper translates into protection in practice.
Laboratory ratings versus real rooms
A central challenge addressed by the standard is the disconnect between laboratory-tested performance and in-situ effectiveness.
Many air-cleaning technologies – portable HEPA units, in-duct filtration, ultraviolet germicidal irradiation – are tested under controlled laboratory conditions. These tests yield valuable metrics such as clean air delivery rate (CADR). But real buildings are not test chambers. Air distribution patterns, room geometry, occupant behaviour and equipment placement all influence outcomes.
As Sherman explains, a device that performs well in a lab may underperform – or behave unpredictably – once installed in a real space. Germicidal UV systems, in particular, cannot be fully characterised without observing how they interact with actual airflow and mixing conditions.
Standard 241 acknowledges this reality rather than ignoring it.
Appendix C: measuring equivalent clean air in the field
One of the most consequential innovations in Standard 241 is Appendix C, which provides a field test method for verifying equivalent clean air (ECA).
Appendix C allows practitioners to measure what matters most: how effectively the installed systems reduce contaminant concentration in the occupied space. Instead of relying solely on nameplate ratings or design assumptions, building teams can directly assess whether the required ECA is being achieved under real operating conditions.
This capability is especially important when multiple technologies are used together – such as outdoor air, recirculated filtration and in-room air cleaners. Appendix C enables verification of the combined effect, not just individual components.
As Sherman notes, Appendix C was nearly deferred to a future version of the standard. Its inclusion reflects both the urgency of the moment and a recognition that verification is essential to credibility.
Air distribution: the hard problem we can no longer ignore
Another theme that emerges strongly from the transcript is air distribution. Even with adequate total clean air, poor mixing or short-circuiting can leave occupants underprotected.
The Standard 241 committee openly acknowledges that air distribution remains an unresolved challenge. While the standard addresses it to some extent, both Bahnfleth and Sherman describe this as an area for future refinement.
This honesty matters. Rather than offering false precision, Standard 241 establishes a foundation for ongoing improvement – reinforcing the idea that measurement and feedback, not assumptions, must guide epidemic control.
Commissioning as a core requirement, not an afterthought
Perhaps the most important cultural shift embedded in Standard 241 is the elevation of commissioning from a checkbox to a central requirement.
Commissioning ensures that:
- Systems are installed as designed
- Controls function as intended
- Performance can be verified before IRMM is activated
In the context of infectious aerosol control, commissioning becomes a form of preparedness. When Infection Risk Management Mode (IRMM) is triggered, building operators should not be scrambling to test systems or install new equipment. Everything should already be in place, tested and ready.
As Sherman puts it, the goal is to “push a button”, not to panic.
Trust, but verify: a new norm for epidemic readiness
Standard 241 introduces a new expectation for the building industry: trust is no longer enough.
Design calculations, manufacturer claims and control sequences must be backed by verification. This shift does not imply mistrust of professionals or technologies – it reflects the reality that infection control demands a higher level of accountability.
In this sense, Appendix C and the emphasis on commissioning represent more than technical tools. They signal a new professional ethic: if we claim protection, we must be able to prove it.
