A Conversation with Bill Bahnfleth on the State of Indoor Air Quality
The world of indoor air quality (IAQ) is at a fascinating and complex crossroads. Post-pandemic awareness is high, new technologies are emerging, and standards are evolving. Yet, a persistent gap remains between what we know is possible and what is actually being implemented in the vast majority of our buildings.
In this week's episode, I had the pleasure of a wide-ranging conversation with Bill Bahnfleth, an ASHRAE Fellow, past president, and Professor at Penn State University. Bill uniquely positions himself as working “in the gap between practice, which frequently lacks rigour and science, which frequently ignores practical concerns.” This perspective makes for a grounded, "rubber-meets-the-road" discussion about where we truly stand as an industry and where we need to go next.
Key Discussion: The Gap Between Aspiration and Application
One of the most powerful themes of our conversation was the disconnect between the cutting-edge of academic research and the realities of the construction and building management industries. As Bill puts it, paraphrasing Dwight Eisenhower, "if you're leading, every once in a while you should look around to see if anyone is following you."
We discussed the common pitfalls that widen this gap:
The Problem of Maintenance: We acknowledged that perhaps the most consistent characteristic of HVAC systems worldwide is that they are not maintained as they should be, leading to a steady degradation of performance from day one.
The Minimum Becomes the Maximum: In the vast majority of buildings—those that aren't high-profile, showcase projects—standards are often treated as a ceiling to aim for, not a floor to build upon. This creates a built environment that chronically underperforms.
The Challenge of Regional Realities: A solution that works perfectly in a case study from the United States may be completely impractical in Europe or India. We explored how culture, economics, climate, and local regulations mean that "one size will not fit all." Any global push for better IAQ must be adaptable and sensitive to these local contexts.
Hearing Bill articulate the complex interplay of these factors, drawing on his extensive global travels and decades of experience, provides a sobering but essential reality check for anyone working to move the needle on healthy buildings.
Key Discussion: Accountability, Enforcement, and the Power of Social Science
If good standards already exist, why aren't they being met? This led us to a critical discussion on enforcement and accountability. We can’t improve what we don’t measure, but the path forward isn’t straightforward.
We touched on several intriguing ideas that are being tested around the world, from New York City's public "report cards" for building energy performance to a quiet but effective code of practice in Ireland requiring ventilation risk assessments. However, as Bill points out, any inspection or reporting regime must be seen as credible and valuable, not just another bureaucratic exercise.
Perhaps the most compelling illustration of our non-technical challenges came up when we discussed radon. Here we have a known carcinogen—the "slam dunkiest of slam dunks," as I called it—that is relatively easy to test for and mitigate. Yet, as a society, we have fundamentally failed to solve the radon problem. This poignant example reveals how deeply issues of economics, awareness, landlord-tenant dynamics, and even perceived property values can stall progress on even the most clear-cut public health risks. It underscores the growing need to integrate social science into our engineering-focused approach.
Key Discussion: A New Paradigm? The Story and Significance of ASHRAE Standard 241
No conversation about the future of IAQ would be complete without dissecting ASHRAE Standard 241, Control of Infectious Aerosols. As the chair of the committee that developed it in record time during the pandemic, Bill offered a fascinating insider's perspective on its creation and profound implications.
We unpacked two of the standard's revolutionary concepts:
Equivalent Clean Air (ECA): This marks a significant shift away from simply prescribing outdoor air ventilation rates. Instead, it provides a unified, performance-based metric that allows for a combination of strategies—including filtration and air cleaning—to achieve a desired level of risk reduction.
Resilience and Modes of Operation: Standard 241 introduces the idea that buildings need to operate differently under different circumstances. It establishes an "Infection Risk Management Mode" (IRMM), creating a framework for buildings to be more resilient and adaptive to public health events.
This part of our discussion dove into how these ideas challenge our traditional thinking and what it might mean for the future of all ventilation standards. Could this risk-based, outcome-focused approach be the model for controlling other contaminants beyond pathogens? In the full episode, Bill walks through the standard's origins and his vision for its long-term impact on the industry.
This is just a snapshot of a conversation that also covered the changing role of ASHRAE as a global organization, the critical need for better collaboration between the engineering and medical communities, and the surprising power of simple habit changes, like the German culture of Lüften (airing the home)
One Take: A Dose of Reality on Mould and Medical Diagnosis
In this week's One Take episode, we cut through the confusion surrounding one of the most contentious issues in the built environment: mould. We examined a landmark 2023 German guideline from the Association of the Scientific Medical Societies (AWMF) on the medical diagnosis of indoor mould exposure. This consensus document, created by a host of medical experts, offers a powerful, evidence-based reality check for building managers and IAQ professionals alike.
The guideline’s primary message is a call for first principles: focus on the building first. It states that any significant mould infestation should be remediated based on the precautionary principle. The priority is to find and fix the underlying moisture problem, treating mould as a hygiene issue that must be resolved, regardless of whether occupants are currently symptomatic.
Perhaps its most striking directive relates to environmental testing. The guideline bluntly states that for medical diagnosis, "indoor mould measurements are rarely useful." From the perspective of these medical societies, if you can see the mould, the task is not to sample it to identify species or spore counts; it is to remove it. They go further, stating that monitoring for mycotoxins or microbial VOCs (the "moldy smell") has no current role in diagnosing a patient's illness, as we lack the science to draw a clear causal link between a measured concentration and a specific health complaint.
Instead, the document defines diagnosable mould-related health effects quite narrowly, focusing primarily on allergies, irritations, and, in rare cases for at-risk groups, infections. The diagnostic process it recommends relies on established medical practice—patient history, skin-prick tests, and blood work—not air sampling reports.
My one take? This guideline is a welcome dose of pragmatism. It pushes back against the search for simple answers in a lab report and directs our attention where it belongs: identifying and rectifying the moisture problems that allow mould to grow in the first place. For anyone managing buildings or dealing with occupant complaints, it’s a solid piece of advice grounded in clear, evidence-based medical science.
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