The Yale-led study demonstrates that building-integrated plant and microbial systems can actively remove indoor CO2 to levels below outdoor concentrations — something ventilation alone cannot accomplish. Researchers found that airflow and lighting critically determine biofilter performance, explaining past inconsistent results. The systems may also boost indoor microbial diversity linked to immune and respiratory health, and the findings will inform Yale CEA’s Photosynthetic Cities program.
From 420 to 936 ppm: Why Building-Integrated Plant Biofilters Could Be the Next Step in Indoor Air Quality
From 420 to 936 ppm: Why rising CO2 makes indoor plant biofilters a necessity
A new multi-institution study led by the Yale Center for Ecosystems and Architecture (Yale CEA) shows that plant- and microbe-based systems integrated into buildings can actively remove indoor carbon dioxide (CO2) to levels below those outdoors — a capability conventional ventilation cannot achieve.
Why Ventilation Has Limits
Conventional HVAC strategies rely on ventilation: bringing outdoor air inside to dilute and displace indoor pollutants. That approach can only move indoor CO2 toward the outdoor baseline, not below it. With outdoor CO2 roughly 420 ppm today and projected in some scenarios to reach as high as 936 ppm over the next 50 years, the outdoor baseline itself becomes a limiting factor for ventilation-based systems.
Photosynthesis As An Active Removal Strategy
The Yale-led team tested an alternative: use photosynthesis and associated microbial processes as an active removal mechanism. Plants and microbes integrated into building systems capture CO2 from indoor air and convert it via biological processes rather than merely diluting it with outdoor air. In contrast to ventilation, these living biofilters can theoretically drive indoor CO2 below the ambient outdoor concentration at any given time.
Design Matters: Airflow And Lighting
The researchers found system performance is highly sensitive to two design variables: airflow and lighting. When airflow and light are optimized, biofilters remove CO2 effectively and consistently. When they are not, results become highly variable — a plausible explanation for why earlier plant-based air-cleaning studies were often irreproducible.
Broader Health And Environmental Benefits
Beyond CO2 reduction, building-integrated plant systems can enrich indoor microbial diversity, which has been associated in some studies with improved immune function and long-term respiratory health. While research on health outcomes is ongoing, this positions biofilters as contributors to a broader indoor environmental quality framework that extends beyond single-pollutant metrics.
Interdisciplinary Collaboration And Practical Next Steps
The project brought together architects, building-system designers, air chemists, civil and environmental engineers, plant and microbial ecologists, exposure scientists, and computational modelers. The team emphasizes that designing, modeling, and validating living systems inside buildings requires integrated expertise because biological performance, architectural integration, mechanical airflow, and human exposure interact simultaneously.
The findings will inform Yale CEA’s Photosynthetic Cities initiative and were presented at AIA NYC on Sept. 24. The study is published in the journal Energy and Buildings.
Implication: As outdoor CO2 rises and ventilation-based HVAC strategies approach their limits, the next generation of indoor air-quality solutions may need to include grown, living systems — and careful design of airflow and lighting will determine whether those systems succeed.
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