NASA Clean Air Study

The NASA Clean Air Study, formally titled "Interior Landscape Plants for Indoor Air Pollution Abatement," was a scientific investigation conducted by the National Aeronautics and Space Administration (NASA) and published on September 15, 1989.^[1] Led by environmental engineer B.C. Wolverton at NASA's Stennis Space Center in collaboration with the Associated Landscape Contractors of America (ALCA), the study evaluated the potential of common houseplants and their associated soil microorganisms to remove indoor air pollutants, particularly volatile organic compounds (VOCs) such as benzene, trichloroethylene (TCE), and formaldehyde.^[1]^[2] It addressed concerns over "sick building syndrome" in energy-efficient structures and air quality in sealed space habitats, promoting phytoremediation as a low-energy alternative to mechanical filtration.^[2] The research emerged from NASA's work on sustainable life support for space missions, where plants could contribute to bio-regenerative systems.^[2] Conducted amid rising awareness of indoor pollution from building materials and furnishings, the study tested hardy foliage plants and found that pollutant removal primarily occurs in the root-soil zone via symbiotic microbes.^[1] While results suggested plants could purify air in controlled settings, subsequent research as of 2024 has indicated limited effectiveness in typical ventilated homes, requiring far higher plant densities than initially extrapolated for meaningful impact.^[1]^[3]^[4] These findings have influenced commercial applications and guidelines, though real-world efficacy varies with ventilation, density, and maintenance.^[2]

Background and Origins

Historical Context

In the 1970s and 1980s, concerns over indoor air pollution intensified due to the global oil embargo of 1973, which prompted stricter energy conservation measures and the construction of more airtight buildings with reduced ventilation. These designs trapped volatile organic compounds (VOCs) emitted from synthetic materials, furniture, and cleaning products, leading to the accumulation of harmful pollutants like formaldehyde and benzene. This environmental shift contributed to the emergence of "sick building syndrome," characterized by symptoms such as headaches, eye irritation, and respiratory issues among occupants; a 1984 World Health Organization report estimated that up to 30 percent of new and remodeled buildings worldwide were affected.^[1]^[5] NASA's involvement stemmed from its long-standing research into life support systems for sealed space environments, where maintaining breathable air is essential for astronaut health. As early as the 1950s, the agency explored bioregenerative technologies, but the challenges became acute during the Skylab missions in the 1970s, which detected over 300 VOCs from off-gassing materials in the orbital laboratory. With plans for extended missions on space stations and potential lunar bases, NASA recognized the need for efficient air purification methods beyond mechanical filters, viewing biological solutions as a sustainable complement to recycling systems in isolated habitats.^[5]^[1] NASA collaborated with the Associated Landscape Contractors of America (ALCA) in a two-year joint effort, culminating in 1989, to investigate the potential of indoor plants for air remediation, building on prior work at the John C. Stennis Space Center. This effort, supported by NASA's Office of Commercial Programs—Technology Utilization Division, aimed to address both terrestrial and extraterrestrial air quality challenges through phytoremediation. The resulting report, "Interior Landscape Plants for Indoor Air Pollution Abatement," was published that year, marking a key milestone in NASA's broader research on closed ecological life support systems.^[1]^[5]

Objectives and Scope

The NASA Clean Air Study, conducted in the late 1980s, had as its primary objective the identification of common indoor plants capable of removing volatile organic compounds (VOCs) from the air, with the aim of supporting air purification systems in sealed space habitats and potentially applicable to Earth-based enclosed environments.^[6] This initiative sought to leverage phytoremediation—the process by which plants absorb and break down pollutants—to enhance life support technologies for long-duration space missions, where maintaining breathable air in isolated systems is critical.^[6] The scope of the study was deliberately narrow, concentrating on three prevalent VOCs: benzene, formaldehyde, and trichloroethylene, selected due to their common presence in indoor settings from everyday sources such as paints, adhesives, synthetic fibers, and cleaning products.^[6] These chemicals were prioritized because they are indicated as potential carcinogens or teratogens in indoor atmospheres, posing health risks in low-ventilation spaces like spacecraft or tightly sealed buildings.^[6] The research emphasized testing the phytoremediation potential of foliage plants typically used in interior landscapes, simulating conditions in sealed chambers to evaluate their efficacy in such controlled, low-airflow environments.^[6] Notably, the study excluded outdoor or agricultural plants, focusing exclusively on those feasible for integration into NASA's life support systems and indoor human habitats.^[6] This targeted approach ensured the findings would directly inform practical applications for air quality management in confined, human-occupied spaces without broader ecological considerations.^[6]

Methodology

Experimental Design

The experimental design of the NASA Clean Air Study employed sealed Plexiglas chambers to replicate low-airflow indoor environments, allowing for the controlled evaluation of plants' ability to remove airborne pollutants. These transparent chambers, bolted and sealed with wing-nuts, featured volumes ranging from 0.44 m³ (0.76 m × 0.76 m × 0.76 m) for smaller units to 0.88 m³ (0.76 m × 0.76 m × 1.53 m) for larger ones, with additional configurations of 0.69 m³ and 0.87 m³ used in low-concentration tests; small ports facilitated pollutant injection and air sampling, while internal fans promoted circulation without external exchange. Plants, potted in their original soil-filled containers and fertilized with a standard nutrient solution, were placed inside to assess both foliar uptake and root-soil microbial contributions to pollutant remediation.^[1] Volatile organic compounds (VOCs) were introduced into the chambers to simulate indoor pollution levels, with initial concentrations set at 15–20 ppm for high-exposure tests and below 1 ppm for more realistic low-level scenarios. Liquid VOCs, such as benzene and trichloroethylene, were injected via microsyringe onto a metal tray or absorbent material and allowed to evaporate for approximately 30 minutes, while gaseous formaldehyde was added through a controlled scrubber system for 50–120 seconds depending on chamber size. This setup ensured uniform distribution before the 24-hour monitoring period commenced, mimicking off-gassing from building materials or furnishings.^[1] Pollutant concentrations were quantified through serial air sampling at 0, 6, and 24 hours using a Sensidyne-Gastec pump paired with colorimetric detector tubes for initial high levels (1–100 ppm range), supplemented by gas chromatography (Hewlett-Packard Model 5890) with Tenax adsorbent tubes for precise low-concentration analysis. Removal efficiency was determined by calculating the difference in VOC levels over time, expressed as micrograms removed per hour to standardize rates across varying chamber volumes and plant sizes; cooling coils maintained stable conditions by circulating chilled water at 7°C.^[1] Key control variables included a consistent temperature of 30°C ± 1°C and relative humidity managed within typical indoor ranges, though not explicitly quantified in protocols. Illumination was provided continuously at 125 footcandles ± 5 via encircling Damar Gro-lights to support photosynthesis without diurnal cycles, while plants were standardized by total leaf surface area (measured in cm²) rather than height to account for physiological capacity. These parameters ensured reproducible results focused on phytoremediation potential in enclosed spaces.^[1]

Pollutants and Plants Tested

The NASA Clean Air Study focused on three primary volatile organic compounds (VOCs) commonly found in indoor environments, selected for their prevalence in spacecraft materials and potential health risks to astronauts in closed habitats. These included benzene, a solvent emitted from sources such as tobacco smoke, paints, gasoline, inks, plastics, and rubber products; formaldehyde, released from particleboard, carpets, urea-formaldehyde foam insulation, consumer paper products, cigarette smoke, and heating fuels; and trichloroethylene, derived from dry cleaning processes, adhesives, printing inks, paints, lacquers, and varnishes. These pollutants were chosen due to their toxicity, carcinogenicity, association with respiratory irritation, and contribution to "sick building syndrome" in energy-efficient, sealed structures like space stations.^[1] The study evaluated over 20 species of common houseplants, prioritizing those adaptable to low-light indoor conditions, readily available from local nurseries, and supported by prior anecdotal evidence of air-purifying properties, without any genetic modifications. Key species tested included the peace lily (Spathiphyllum "Mauna Loa"), bamboo palm (Chamaedorea seifrizii), English ivy (Hedera helix), snake plant (Sansevieria laurentii), and various philodendrons such as heart leaf philodendron (Philodendron oxycardium), elephant ear philodendron (Philodendron domesticum), and lacy tree philodendron (Philodendron selloum). Additional plants encompassed Chinese evergreen (Aglaonema modestum), ficus (Ficus benjamina), gerbera daisy (Gerbera jamesonii), dracaena varieties like Janet Craig (Dracaena deremensis "Janet Craig"), marginata (Dracaena marginata), mass cane (Dracaena massangeana), and Warneckei (Dracaena deremensis "Warneckei"), pot mum (Chrysanthemum morifolium), green spider plant (Chlorophytum elatum), golden pothos (Scindapsus aureus), and aloe vera (Aloe barbadensis miller). Banana (Musa oriana) was also screened.^[1] In the testing protocol, each plant species was exposed individually to one pollutant at a time in separate sealed chamber trials to assess uptake mechanisms involving leaves, roots, soil, and associated microorganisms.^[1]

Results and Findings

Effectiveness of Individual Plants

The NASA Clean Air Study quantified the removal rates of key volatile organic compounds (VOCs)—benzene, formaldehyde, and trichloroethylene—by various indoor plant species in sealed experimental chambers over 24-hour exposure periods. These rates, expressed in micrograms per hour (μg/h), demonstrated that certain plants excel at phytoremediation, primarily through absorption by leaves and, more significantly, microbial degradation in the root-soil zone. For instance, the peace lily (Spathiphyllum 'Mauna Loa') achieved removal rates of 1,724 μg/h for benzene, 673 μg/h for formaldehyde, and 1,127 μg/h for trichloroethylene, while the bamboo palm (Chamaedorea seifrizii) removed 1,419 μg/h of benzene, 3,196 μg/h of formaldehyde, and 688 μg/h of trichloroethylene.^[1] A comparative analysis across tested plants highlights top performers for each VOC, with gerbera daisy (Gerbera jamesonii) leading in benzene and trichloroethylene removal at 4,485 μg/h and 1,622 μg/h, respectively, and bamboo palm topping formaldehyde removal. Heartleaf philodendron (Philodendron scandens 'Oxford') showed notable efficiency for formaldehyde at 353 μg/h, though its benzene removal was not separately quantified in the primary tests. Other strong contributors included the pot mum (Chrysanthemum morifolium) for benzene (3,205 μg/h) and Janet Craig dracaena (Dracaena deremensis 'Janet Craig') for formaldehyde (2,036 μg/h). The table below summarizes removal rates for the three VOCs where data were available, based on standardized plant sizes and chamber conditions.

Plant Species	Benzene (μg/h)	Formaldehyde (μg/h)	Trichloroethylene (μg/h)
Gerbera daisy (Gerbera jamesonii)	4,485	-	1,622
Pot mum (Chrysanthemum morifolium)	3,205	-	-
Bamboo palm (Chamaedorea seifrizii)	1,419	3,196	688
Peace lily (Spathiphyllum 'Mauna Loa')	1,724	673	1,127
Marginata (Dracaena marginata)	1,263	853	1,137
Warneckei (Dracaena deremensis 'Warneckei')	1,629	-	573
Janet Craig (Dracaena deremensis 'Janet Craig')	1,082	2,036	763
English ivy (Hedera helix)	579	402	298
Mother-in-law's tongue (Sansevieria laurentii)	1,196	1,304	405
Chinese evergreen (Aglaonema modestum)	604	182	-
Heartleaf philodendron (Philodendron scandens 'Oxford')	-	353	-
Mass cane (Dracaena fragrans 'Massangeana')	-	-	420

Rates are derived from total VOCs removed over 24 hours divided by exposure time, using plants with typical leaf surface areas ranging from 1,000 to 15,000 cm².^[1] Several factors influenced these removal rates, including leaf surface area, which correlated positively with absorption capacity—for example, plants with larger foliage like Janet Craig (15,275 cm²) exhibited higher overall performance. Additionally, root-soil interactions played a dominant role, as microbial communities in the potting soil degraded up to 87% of introduced VOCs in some cases, far exceeding foliar uptake alone; covering soil with foliage reduced efficiency, while exposing roots improved it. Plant mass was not directly quantified but implicitly affected rates through biomass availability for metabolic processes.^[1] The study concluded that select plants could remove up to 89.8% of low-concentration VOCs, such as benzene by English ivy, in sealed chambers within 24 hours, underscoring their potential for targeted air purification under controlled conditions.^[1]

Overall Implications from the Study

The NASA Clean Air Study demonstrated the potential for integrating plants and their associated soil microorganisms into Closed Ecological Life Support Systems (CELSS) to support long-duration space missions by recycling air pollutants into biomass and maintaining habitable environments in sealed habitats.^[1] This approach leverages natural phytoremediation processes, where plants absorb volatile organic compounds (VOCs) through leaves and roots, with microbial activity in the soil enhancing degradation, thereby reducing reliance on mechanical systems alone for trace contaminant control in spacecraft.^[6] Extrapolations from the study's sealed chamber experiments suggested the potential for effective VOC reduction in controlled indoor spaces using plant-based systems, providing a scalable model for air purification in enclosed areas like space stations.^[1] On Earth, these findings encouraged the incorporation of houseplants in offices and homes as a supplementary measure to mechanical filtration, offering an economical, low-energy method to mitigate indoor air pollutants associated with "sick building syndrome."^[6] The 1989 report by B.C. Wolverton et al. has influenced subsequent green building practices by promoting biophilic design elements that enhance indoor air quality through vegetation.^[7] Its emphasis on plant-soil systems as a sustainable purification strategy continues to inform standards in energy-efficient architecture, fostering healthier built environments.^[8]

Limitations

Scientific Constraints

The NASA Clean Air Study employed short-term exposure tests lasting 24 hours to assess pollutant removal by plants, which provided snapshots of initial uptake rates but failed to evaluate sustained performance under ongoing pollutant introduction or the time required for plants to metabolize and recover from absorbed compounds.^[1] This approach overlooked dynamic indoor environments where volatile organic compounds (VOCs) are continuously emitted from sources like furnishings and cleaning products, potentially leading to an overestimation of long-term efficacy.^[9] For instance, while the study reported VOC removal rates up to 89.8% for benzene in controlled setups, these figures do not account for pollutant re-emission or plant saturation over extended periods.^[1]^[10] Experiments were confined to sealed Plexiglas chambers measuring approximately 0.44 cubic meters, maintaining constant temperature (30°C ± 1°C), lighting (125 footcandles ± 5), and minimal airflow via internal fans, thereby excluding key real-world factors such as variable ventilation rates, fluctuating CO2 concentrations, and complex interactions with ambient humidity or dust.^[1] These idealized conditions isolated plant responses but ignored how natural airflow might dilute or redirect pollutants away from plant surfaces and roots, as well as the influence of elevated CO2 on photosynthetic rates and VOC processing.^[9] Furthermore, the setups used basic potting soil without analyzing diverse microbial communities beyond simple bacterial counts, limiting insights into how environmental variables could alter soil microbiome activity in phytoremediation.^[10]^[11] The study tested a limited number of plant specimens per species—often just one or a few pots with leaf areas ranging from 713 cm² to 15,275 cm²—without replicates or rigorous statistical analysis to quantify variability in removal efficiency across individuals or conditions.^[1] This small-scale approach, common in early phytoremediation research, introduced potential bias from plant health differences or chamber inconsistencies, as percent removal calculations relied on single-point measurements rather than variance assessments like standard deviations or confidence intervals.^[10] Critics note that such designs hinder reliable extrapolation, as natural plant variability (e.g., due to age or stress) was not factored in, reducing the robustness of findings on species-specific performance.^[11] Although the study identified the root-soil zone as the primary site for VOC degradation, driven by associated microorganisms, its methodology placed disproportionate emphasis on overall plant uptake, particularly through foliar pathways, while underestimating the full extent of root zone and microbial contributions due to limited dissection of these processes.^[1] Tests comparing plants with and without soil showed markedly lower removal without microbes, yet the analysis did not deeply probe microbial diversity or symbiotic dynamics, leading to an incomplete model of biological interactions.^[10] Subsequent reviews highlight that foliar absorption accounts for only 20-26% of certain VOCs like BTEX in similar setups, with rhizosphere microbes handling the majority, a nuance the original work acknowledged but did not quantify comprehensively.^[11]

Practical Applicability

Translating the findings of the NASA Clean Air Study to real-world indoor environments on Earth presents significant challenges due to the impractical scale required for meaningful volatile organic compound (VOC) reduction. The study suggested that substantial plant densities—ranging from 10 to 1,000 plants per square meter—would be necessary to achieve noticeable air purification effects comparable to those observed in controlled chambers. For instance, applying these densities to a typical 1,500 square foot home would require approximately 680 plants, far beyond what most households can accommodate.^[12] Realistic building conditions further diminish the applicability of the study's results, particularly the role of ventilation. The experiments were conducted in sealed chambers without airflow, unlike typical homes and offices where natural or mechanical ventilation systems dilute and remove VOCs at rates that exceed the purification capacity of plants. This ventilation effect reduces both the accumulation of pollutants and the relative efficacy of phytoremediation in everyday settings.^[3] Maintenance demands also pose barriers to practical implementation, as indoor plants require consistent care including adequate lighting, watering, and space allocation to thrive. Neglect can lead to issues such as mold growth from overwatering, dust accumulation on leaves, or infestations by pests, which may inadvertently degrade air quality rather than improve it. Additionally, certain plants can introduce allergens through pollen or mold spores, complicating their use in sensitive environments.^[3] Popular media coverage has often overstated the study's implications for terrestrial air purification, fostering unrealistic expectations that a few houseplants can substantially clean indoor air. This misinterpretation stems from the study's original focus on sealed space habitats, where high plant densities were more feasible, leading to widespread but unsubstantiated claims about everyday benefits.^[13]

Subsequent Research and Developments

Replications and Extensions

Following the original NASA Clean Air Study, B.C. Wolverton conducted extensions in 1993 that expanded the scope to include additional volatile organic compounds (VOCs) such as xylene and ammonia, alongside formaldehyde. In this work, conducted with J.D. Wolverton, experiments demonstrated that certain plants, including peace lilies and chrysanthemums, effectively removed these pollutants from indoor air at rates comparable to those observed for benzene and trichloroethylene in the initial study, with removal efficiencies reaching up to 50-75% over 24 hours in sealed chambers.^[14] Wolverton's 1996 publication further built on these findings by evaluating a broader array of 50 houseplants, confirming similar phytoremediation capabilities for the tested VOCs and recommending species like chrysanthemums for their high removal rates of xylene, while emphasizing the role of plant-soil systems in sustained air purification.^[15] A 2014 systematic review by Dela Cruz et al. analyzed independent studies attempting to replicate the NASA findings, revealing inconsistent results across varying experimental conditions such as chamber size, light levels, and pollutant concentrations. The review found that while some replications confirmed partial VOC removal, particularly for formaldehyde (with efficiencies of 20-50% in controlled settings), overall success was limited, and no study fully reproduced the original high removal rates due to differences in methodology and environmental variables. In 2019, Cummings and Waring published a meta-analysis in the Journal of Exposure Science & Environmental Epidemiology, reviewing 12 chamber studies and testing plant efficacy in larger, ventilated chambers simulating real indoor environments. Their experiments with common houseplants like spider plants showed significantly reduced VOC removal rates—often below 10% over typical exposure times—when airflow was introduced, contrasting sharply with the sealed-chamber results of the original study.^[16] Collectively, these replications indicate that the NASA Clean Air Study's results are robust under ideal, static conditions but diminish substantially in dynamic settings with ventilation and realistic pollutant loads.

Role of Soil Microbes and Modern Insights

Subsequent research has elucidated the critical role of soil microbes in the phytoremediation process initiated by the NASA Clean Air Study, shifting emphasis from foliar absorption to rhizospheric degradation. A pivotal 2004 study by Orwell et al. demonstrated that microorganisms in the potting soil of indoor plants, rather than the plant leaves alone, are the primary agents for removing volatile organic compounds (VOCs) like benzene from the air, with removal rates significantly higher in soil-associated systems compared to bare soil controls. Root exudates from plants stimulate these microbial communities, providing carbon sources that enhance bacterial activity; species such as Pseudomonas have been identified as dominant VOC degraders in the rhizosphere, capable of metabolizing benzene and other pollutants through enzymatic pathways.^[17] This mechanistic insight, building on earlier NASA work, underscores that the soil-plant microcosm functions as a biofilter, where microbes convert VOCs into harmless byproducts like carbon dioxide and water. Modern evaluations, however, temper enthusiasm for standalone houseplants in practical air purification. The 2024 American Lung Association report concludes that houseplants offer only minimal VOC reduction—typically less than 1% in well-ventilated rooms—far below levels needed for significant health benefits, and recommends integrating them with mechanical systems like HEPA filters for effective indoor air management.^[3] This assessment aligns with meta-analyses showing that achieving meaningful pollutant removal would require impractical densities of plants, such as 10 to 1,000 per square meter of floor space.^[18] As of 2025, ongoing research advances hybrid approaches combining plants with biofilters for urban applications, particularly in green buildings and offices. European Union-funded initiatives under Horizon Europe are testing these systems, such as active botanical biofilters using species like Epipremnum aureum (pothos) to target office VOCs, achieving up to 80% removal efficiency in controlled airflow setups when integrated with microbial-enhanced substrates.^[19] These developments emphasize scalable, low-energy solutions for enclosed spaces.^[20] As of November 2025, EU pilots continue to report sustained efficiencies of 70-80% in real office settings with these integrated systems. Contemporary challenges, including climate change-driven increases in indoor VOC exposure from wildfire smoke infiltration, highlight the limitations of natural plants alone, as smoke can elevate benzene and other compounds by factors of 10–100 indoors during events.^[21] Emerging microbial engineering addresses this by augmenting plant microbiomes with bioengineered bacteria, enabling 30-fold greater VOC degradation than unmodified systems, as demonstrated in 2024–2025 prototypes like Neoplants' enhanced pothos.^[22] This focus on engineered rhizospheres promises more robust indoor air solutions amid rising environmental pressures.