From 420 to 936 ppm: Why rising CO2 makes indoor plant biofilters a necessity

A new study from Yale University and collaborating institutions has shown that building-integrated plant and microbial systems can reduce indoor carbon dioxide concentrations below the levels found in outdoor air — a threshold that conventional ventilation systems are fundamentally incapable of crossing.

The findings were led by researchers at the Yale Center for Ecosystems and Architecture (Yale CEA), in collaboration with teams from NYU, Purdue, Yale School of the Environment, Buro Happold, and other academic and industry partners.

Why ventilation hits a wall

Indoor CO2 is already a documented problem. In cities, people spend more than 90% of their time indoors, where carbon dioxide and other pollutants are routinely elevated above outdoor ambient levels. Conventional HVAC systems address this through ventilation bringing in outside air to dilute and displace indoor pollution.

But ventilation has a hard ceiling. Because indoor CO2 concentrations are typically higher than outdoors, ventilation can bring them closer to outdoor levels, it cannot reduce them below that baseline. As outdoor CO2 continues rising, currently at 420 ppm and projected to reach as high as 936 ppm over the next 50 years, that baseline itself becomes the problem. The ventilation strategy begins failing before the indoor air quality goals do.

The photosynthesis alternative

The Yale-led team investigated a fundamentally different approach: using photosynthesis as the active removal mechanism. Plants and microbes integrated directly into building systems capture CO2 from indoor air and convert it through biological processes and not mechanical ones. Unlike ventilation, this approach removes CO2 from the air rather than diluting it with outdoor air, making it theoretically capable of reducing indoor concentrations below whatever outdoor concentration exists at a given time.

The researchers found that the performance of these systems is highly sensitive to two design variables in particular: airflow and lighting. Getting those parameters right dramatically changes how well the biological system performs. Getting them wrong produces the kind of variable, irreproducible results that have dogged earlier work in this field — a finding that the team says explains why plant-based air cleaning studies have historically been difficult to replicate.

Built-in health dividend

Beyond CO2 reduction, the study identifies an additional benefit: building-integrated plant systems can improve indoor microbial diversity, a factor linked to human immune function and long-term respiratory health. This positions biofilter systems as contributing to a broader definition of indoor environmental quality that extends beyond simple pollutant concentration metrics.

The interdisciplinary team behind the research spans architecture, building-system design, air chemistry, civil and environmental engineering, plant and microbial ecology, exposure science, and computational modeling — an unusually wide collaboration that reflects the problem’s complexity. No single discipline has the tools to design, model, and validate a system where biological performance, architectural integration, mechanical airflow, and human exposure all interact simultaneously.

From lab to practice

The findings will inform the Photosynthetic Cities program, a Yale CEA initiative convened at AIA NYC on Sept. 24, bringing together researchers, architects, policymakers, and practitioners to translate living-system science into actionable design strategies for buildings and cities.

As outdoor CO2 rises and ventilation-based HVAC strategies approach their limits, the study makes the case that the next generation of indoor air quality solutions may need to be grown rather than engineered — and that how they are designed into buildings matters as much as whether they are included at all.

The finding were published in Energy and Buildings.