Working as one: How the building envelope, ventilation and filtration support healthy indoor air

Indoor air pollution is often two to five times higher than outside, impacting everything from daily focus to long-term respiratory health. DR ANNA O’DONOVAN explores why building performance and occupant well-being are inseparable—and how a whole-system approach creates healthier indoor spaces.

Healthy indoor air results from decisions taken across the design, construction, operation and maintenance of a building. Three elements are especially important here: a well-designed envelope, a ventilation strategy that brings in and distributes fresh air, and filtration and air cleaning that remove contaminants.

These elements are interdependent. A tight envelope without adequate ventilation can trap pollutants; ventilation without effective filtration can introduce outdoor contaminants; and filtration alone cannot fully compensate for uncontrolled leakage. As we increasingly expect buildings to support occupant health and wellbeing, it’s time we look at them from the perspective of a healthier indoor environment.

Why indoor air quality matters

We spend roughly 90% of our time indoors, making indoor air quality a significant daily health factor. Indoor pollutant levels are consistently two to five times higher than outdoors, and during activities such as painting or chemical cleaning, concentrations can spike to a thousand times background outdoor levels. Hundreds of compounds have been identified in indoor air and dust, showing how complex these exposures have become.

Health effects associated with indoor pollutants include irritation of the eyes, nose and throat; headaches, dizziness and fatigue; and, with longer exposure, respiratory disease, heart disease and even cancer.

Particularly worrisome are PM2.5 and smaller particles, which can penetrate deep into the respiratory system, may enter the bloodstream and, because of their large surface area relative to their size, can carry other toxins with them. Indoors, they can remain suspended for hours.

According to the World Health Organization (WHO), exposure to indoor allergens, such as dust mite allergen, pet dander and mould, together with pollen, increases the risk of developing asthma. In Ireland, radon also deserves attention: the Environmental Protection Agency (EPA) identifies it as the second biggest cause of lung cancer after smoking.

Thankfully, there is a positive side too, because healthy air supports how well people think and work. In a controlled Harvard study led by Joseph G Allen, average cognitive scores in office settings were 61% higher under low-VOC ‘green’ conditions (VOCs, or volatile organic compounds, are gases released by many paints, adhesives, cleaning products and furnishings) than under conventional office conditions, with still higher scores when ventilation was increased.

 

The envelope: where healthy air begins

A high-performance envelope is designed to minimise infiltration, and modern assemblies do this well. But no envelope is perfectly airtight. Pressure differences created by wind, stack effect, mechanical systems or entrances will draw some outdoor air inward, carrying in whatever it holds. So we must accept that air will get in.

This is why the envelope cannot be considered separately from how air is delivered and cleaned. A well-sealed envelope reduces leakage around windows and service penetrations. Air entering through a designed ventilation path can be filtered before it reaches occupants; air seeping through cracks bypasses supply-air filtration. Once indoors, particles carried in by leakage may still be removed by recirculating HVAC filtration or a suitable portable air cleaner, but even so, reducing leakage remains the more predictable, energy-efficient starting point.

The tighter the envelope, the more indoor air quality depends on a deliberate strategy rather than chance. This is especially pertinent in Ireland, where Part L of the Building Regulations is driving greater airtightness, while Part F sets the corresponding ventilation obligations.

Ventilation: diluting and removing what accumulates

Once a building is tightly sealed, ventilation carries much of the responsibility for keeping the air healthy. In a leaky older building, air changed itself, however wastefully and unpredictably. In a modern airtight one, carbon dioxide, moisture, VOCs, and other chemicals accumulate unless ventilation removes them. Basically, good ventilation turns a tight building into a healthy, or healthier, one rather than a stuffy one.

 

Part F and ASHRAE Standard 62.1

For Irish projects, Part F of the Building Regulations is the starting point. ASHRAE Standard 62.1-2025: Ventilation for Indoor Air Quality is also an influential international benchmark for commercial and public buildings. It defines acceptable indoor air quality as air containing no known contaminants at harmful concentrations, and with which most occupants express no dissatisfaction. It requires outdoor air to be assessed and treated where necessary, with intakes away from sources such as flues, loading areas and cooling towers. HVAC systems have rightly been called the lungs of a building; when poorly maintained, they can circulate dust, moisture and microbial contamination as easily as prevent it.

 

Filtration: Managing what remains in the air

Filtration cleans the air that passes through it. In a central system, this may be outdoor supply air, recirculated air or both. Portable air cleaners draw in room air and return it after filtration. They can therefore capture particles generated indoors and those entering through leakage, provided the unit is appropriately selected, positioned and operated.

Filters deal with a wide range of particle sizes, from pollen and dust mite allergen down to PM2.5 and smaller. Coarser particles are captured more readily, but they matter too, particularly for people whose asthma or allergies are triggered by pollen, mould and dust mite allergen.

Filters rated ISO ePM1 50% or higher under EN ISO 16890 (broadly comparable to MERV 13 and above in US literature although not directly interchangeable) can capture a substantial proportion of fine particles, including PM2.5. Higher-efficiency filtration may increase airflow resistance and fan energy, or reduce airflow, so efficiency, pressure drop and system capacity should be considered together.

ASHRAE Standard 241

Importantly, ASHRAE Standard 241, developed to reduce airborne disease transmission, introduced equivalent clean airflow. It recognises that clean air can be delivered through outdoor air, filtered recirculated air, or technologies including portable air cleaners. This gives filtration a recognised role alongside outdoor-air ventilation and makes clear that room air, including air entering through leakage, can be cleaned. Standard 241 concerns infectious aerosols, however, and supplements rather than replaces normal ventilation and indoor-air-quality requirements.

 

A whole-system approach, validated by evidence

Healthy indoor air is not the product of any single component. It results from the interaction of the envelope, mechanical systems, occupants and materials. The strategy may be best seen as complementary approaches: source control through low-emission materials and careful product selection; improved ventilation; and filtration and air cleaning to remove airborne particles.

It is worth noting that central to this whole-system approach is confidence that products and systems perform as claimed. Whether through certification against published standards or bespoke laboratory testing, verified performance, preferably by an independent third party, allows specifiers and facilities teams to stand over the systems in their care, rather than relying on self-declared claims or marketing language.

As the building-services sector continues to deliver the airtight, energy-efficient buildings the climate demands, the parallel task is to ensure those buildings support healthier air. That whole-system approach is the thinking behind the proposed ASHRAE Standard 235P, still at a very early stage of development, which would address indoor environmental quality as a whole, spanning air quality, thermal comfort, lighting and acoustics rather than treating ventilation in isolation.

This is good news for occupants because it brings healthier indoor air and day-to-day building performance into the same domain. Given how much of our lives is spent indoors, air quality deserves the same considered attention we give to energy performance.

 

In conclusion

Ultimately, modern buildings shouldn’t just shelter us or save energy—they must actively support occupant health and well-being. Emerging frameworks like the proposed ASHRAE Standard 235P signal a crucial shift toward viewing indoor environmental quality as a complete ecosystem, bringing air quality, acoustics, lighting and thermal comfort under one unified umbrella.

Achieving this vision requires a coordinated strategy across every phase of a building’s lifecycle, from initial design to daily facility management. When building envelopes, ventilation and filtration operate as one, we don’t just build smarter, more efficient structures, we create healthier, happier, and more productive spaces for everyone inside.

About the Author

Dr Anna O’Donovan is a medical and lifestyle author who translates complex scientific research into engaging, evidence-based content that helps businesses and consumers understand the critical role of indoor environments in human health. With a clinical background in both medicine and dentistry, and more than six years of writing experience focused on healthier indoor environments, she brings a unique perspective to her work. Her focus lies at the intersection of health, air quality, and the built environment.