A new Yale-led study demonstrates the promising potential of plant-based systems to remove carbon dioxide (CO₂) from indoor settings, an innovation that would decrease human exposure to the ubiquitous gas and help maintain healthier building environments as atmospheric levels of CO2 rise.
As much as 90% of human exposure to air pollution occurs indoors, where concentrations of CO₂ and other gases considered pollutants can be up to 10 times higher than outdoor levels, the researchers noted.
Prior research has indicated that CO₂ concentrations that exceed 700 to 1,000 parts per million over extended periods of time can affect human health and cognition. (Currently, outdoor concentrations of CO2 typically hover at about 428 parts per million in the United States, according to NOAA monitoring data.)
Typical heating, ventilation, and air-conditioning (HVAC) systems draw in air from the outdoors to dilute indoor CO₂ concentrations, meaning that they cannot reduce levels inside buildings below those occurring outside. So, as atmospheric CO₂ concentrations exceed the 700 parts per million level, indoor exposures, which often already exceed this threshold, could climb even higher.
For the new study, researchers tested the Active Modular Phytoremediation System (AMPS), a biomechanical wall system that moves air through interconnected modules which contain hydroponic plants. The system uses photosynthesis, the biochemical process through which plants convert sunlight, water, and CO₂ into chemical energy, to both remove CO₂ indoors and release oxygen.
Through a series of experiments and simulations, the researchers found that the plant-based system is a viable means for reducing CO₂ from indoors and that context-specific factors - such as airflow direction and lighting intensity - greatly affect its performance. Their study is published in the journal Energy & Buildings.
"Our findings provide a quantitative framework for using living systems to improve indoor air quality," said lead author Phoebe Mankiewicz Ledins, a G. Evelyn Hutchinson Environmental Postdoctoral Associate for the Yale Institute for Biospheric Studies (YIBS) and Yale's Center for Ecosystems + Architecture (CEA). "This is important because research suggests that continued exposure to even marginally elevated CO₂ can harm people's long-term health. And, as global atmospheric CO₂ concentrations are rising, we need to develop practical systems for ensuring the air indoors is safe to breathe, especially in urban environments.
"This may eventually mean relying on our air-handling systems to reduce indoor CO2 below outdoor levels, and most of our buildings aren't equipped for that."
The research was conducted by an interdisciplinary team, including scholars from the Yale School of Architecture (YSoA), the Yale School of Public Health (YSPH), the Yale School of the Environment (YSE), and Yale School of Engineering & Applied Science (Yale Engineering). Anna Dyson, CEA's founding director and the Hines Professor of Architecture at YSoA, and Michelle Bell, the Mary E. Pinchot Professor of Environmental Health at YSE, are among the study's coauthors.
In the study, the researchers first used real-world data to demonstrate the fundamental link between outdoor and indoor CO₂ conditions in various settings, including urban, suburban, and rural areas. In urbanized areas, they found that CO₂ levels in both residences and office buildings exceed the 700 parts per million during both the afternoon and nighttime.
They also provide projections showing that outdoor CO₂ concentrations could exceed the threshold by 2070 while indoor levels could climb beyond 1,200 parts per million.
While current atmospheric CO₂ levels are not acutely harmful to humans, biomarker evidence shows that prolonged exposure to marginally elevated CO₂ levels may contribute to systemic inflammation, bone demineralization, kidney calcification, and other health problems, the researchers note. (They acknowledge, however, that more research is required fully understand the effects of CO₂ on human health.)
Next, the researchers tested the AMPS approach. The system is composed of rows of 3D-printed modules. Each module contained three types of plants: golden pothos, wood sorrel, and Saint John's wort. They were outfitted with fans that move air through the plants' foliage and the substrate through which the plants take root and grow.
In a study conducted inside an artificially controlled chamber known as a seal, at the Yale Science Building, the researchers evaluated the system's performance while varying the airflow direction, light intensity, growing medium, and lighting conditions under which the plants were cultivated. They then measured the differences in CO₂ levels entering and exiting each module.
They found that airflow direction is a major factor in the system's effectiveness. Under comparable lighting conditions, airflow directed from roots to leaves removed nearly 39% more CO₂ than when airflow moved from leaves to roots, the study found. This was likely due to the relationship between leaf photosynthesis and the metabolism of root-associated microorganisms, although the exact mechanisms underlying this outcome warrant further investigation, researchers said.
To determine how variations in the number and configuration of the modules and lighting and airflow conditions affect the system's ability to lower CO₂ concentrations and the laboratory's usability, the researchers used real-world data taken from an occupied 336-cubic-meter laboratory space to run a series of simulations in which many iterations of the system could be tested.
Depending on airflow direction and lighting, the simulations showed that between 95 and 3,653 modules could be required to offset indoor CO₂ emissions. But at peak performance, with the airflow and lighting optimally calibrated, at least 363 modules were needed to reduce the CO2 concentrations inside to be consistent with or lower than concentration levels occurring outdoors.
"By tuning lighting and airflow direction, we saw a 43-fold improvement in the system's ability to remediate CO₂," Ledins said. "A lot of the criticism of these systems is that they require too many plants to function effectively. We show that that's true if you don't have proper lighting or airflow. We also used architectural modeling within the space to show that with our design, a system made of 363 modules is very reasonable and wouldn't limit the use of the space."
In multiple iterations of the simulation, the researchers created pockets of air that had lower CO₂ levels than outdoors, Ledins said.
"This is something most HVAC systems can't accomplish," she said.
The AMPS approach has potential benefits beyond CO₂ remediation, including the remediation of volatile organic compounds, diversification of indoor microbiomes, and soothing effects from biophilia - the human drive to connect with nature and other living things, according to the researchers. These co-benefits illustrate the opportunity plant-based indoor systems represent to address many aspects of urban life that impact human health and well-being, the researchers said.
The study's other coauthors are Christina Ciardullo of CEA and YSoA, Tian Ma of YIBS and YSE, Rebecca Rubright and Charlotte Welch of Yale Engineering, Krystal J. Godri Pollitt of YSPH, Chunxu Huang and Nusrat Jung of Purdue University, Elizabeth Hénaff of New York University, and Patrick Pease of Buro Happold, an international engineering and design consultancy.
The study was partially funded by the G. Evelyn Hutchinson Environmental Postdoctoral Fellowship, a YIBS program that enables creative collaborative research in the environmental sciences at Yale by supporting talented postdoctoral scholars.
