#115 Air Quality Matters: Rethinking Natural Ventilation and the Flaws in How We Measure Dust
Welcome to the latest insights from the Air Quality Matters podcast. This week, we are diving into two distinct but equally critical conversations for professionals in the built environment. First, we explore the highly anticipated update to CIBSE’s AM10 guide on natural ventilation. Then, in our One Take segment, we unpack a recent World Health Organisation (WHO) bulletin that challenges the very foundation of how we measure particulate matter.
Whether you are an architect, an engineer, or a facility manager, these discussions highlight the evolving science and complex realities of the air we breathe.
Main Episode: The Evolution of Natural Ventilation with Ben Jones
In our main episode, host Simon Jones sits down with Ben Jones, Associate Professor at the University of Nottingham and lead author of the newly updated CIBSE AM10: Natural Ventilation in Non-Domestic Buildings.
With more than two decades having passed since the last major revision in 2005, the landscape of building design, climate modeling, and sustainability has shifted dramatically. This conversation offers a fascinating look at the philosophy behind the new AM10 and the enduring role of natural ventilation in modern architecture.
Key Topics Discussed:
The Overhaul of CIBSE AM10: Ben details the massive effort to modernize the guide for 2026. The discussion covers how the document was structured to serve a layered audience, from sales professionals and architects needing a high-level primer to engineers requiring deep-dive calculations and dynamic thermal modeling for climate change targets.
The Physics and Complexities of Airflow: The conversation touches on the fundamental drivers of natural ventilation: buoyancy, temperature differentials, and the often-unpredictable nature of wind. Ben highlights how modern geometry, deep-plan spaces, and "neutral pressure levels" complicate the simple adage that "warm air rises."
The Adaptive Comfort Model vs. Automation: A major theme is the psychological and physiological value of human agency. They contrast rigidly controlled, air-conditioned spaces with naturally ventilated environments where occupants have the power to open a window, ultimately finding that a little thermal variability combined with personal control often yields higher occupant satisfaction.
Modern Design and Regulatory Roadblocks: The realities of 21st-century building design often clash with natural ventilation principles. Ben and Simon discuss the challenges of integrating open airflow pathways with modern acoustic requirements, strict fire compartmentalization, and the growing concern over filtering heavily polluted outdoor air.
I talk about attending a health conference in one of London’s "most technologically advanced" buildings. Despite the cutting-edge automation and green labels, the room’s CO2 monitors hit a staggering 3,200 parts per million. The only recourse the attendees had was to physically wedge the fire doors open. It perfectly encapsulates the danger of over-relying on automation while stripping occupants of environmental agency, a point Ben strongly validates.
Additionally, Ben drops a great perspective on the design process itself, noting that natural ventilation cannot be an afterthought: "If you think you can chuck a passive vent on a normal classroom and you haven't thought about natural ventilation... good luck."
While the nuanced physics of air recirculation, the specific calculation shifts regarding "effective area" for window sizing, and the granular challenges of hybrid ventilation systems. Hearing the discussion firsthand offers a depth of technical context that cannot be fully captured in writing, providing design teams with a much clearer understanding of how to apply the new AM10 principles effectively.
One Take: The Blind Spots in our PM2.5 Global Standards
In a brief but hard-hitting One Take episode, Simon breaks down a recent perspective piece from the bulletin of the World Health Organization titled Air quality standards and WHO guidance on particulate matter measurement.
This episode acts as a massive wake-up call regarding PM2.5, the global gold-standard metric for particulate air pollution. The core issue highlighted in the paper is that our current PM2.5 metric is fundamentally flawed because it is strictly mass-based. As Simon puts it, it simply weighs the dust.
Why "Weighing the Dust" Fails Us
To illustrate this, Simon uses a brilliant analogy: Imagine a box. If you only measure its weight, a single bowling ball will give you a high reading. But what if the box is filled with tens of thousands of marbles? The mass might be lower, but the quantity and composition are entirely different.
In ambient air, the vast majority of particles by sheer number are ultrafine particles (UFPs)—measuring less than 0.1 microns. Because they weigh next to nothing, they barely register on a mass-based PM2.5 scale. Yet, because of their microscopic size, UFPs penetrate deeply into the lungs and bloodstream, driving systemic inflammation, cardiovascular disease, and increased mortality.
The Impact on the Built Environment
The episode connects this global regulatory blind spot directly to indoor air quality management. The vast majority of affordable, optical light-scattering indoor air quality monitors on the market today are effectively blind to ultrafine particles. A facility manager might look at a dashboard showing a perfectly safe PM2.5 mass reading, while the room is actively flooded with millions of dangerous, undetected ultrafine particles from nearby traffic or 3D printers.
The paper proposes a pragmatic solution: introducing a complementary, number-based metric (measuring particles per cubic centimeter) to sit alongside the existing mass-based standard. It is a vital 10-minute listen for anyone specifying sensors or relying on outdoor air to ventilate indoor spaces.
Air quality standards and WHO's guidance on particulate matter measuring 2.5 μm (PM2.5)
The Air Quality Matters Podcast in Partnership with
Particles Plus - Eurovent- Aico - Lindab
The One Take Podcast in Partnership with
SafeTraces and Inbiot
Do check them out in the links and on the Air Quality Matters Website.
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