Developing A Science-Based Approach To Cleaning Indoor Air Pollution With Houseplants

Residential and commercial buildings consume approximately 30% of total global energy [1]. In recent years, incentives to improve energy efficiency have facilitated the development of thermally insulated buildings, which require less energy for heating, ventilating, and air-conditioning (HVAC) systems. However, potentially toxic gases and particular matter (PM) can be released by a variety of indoor sources and activities of occupants, including furnishings, paints, varnishes, waxes, carpets, solvents, cleaning supplies, office equipment such as copiers and printers, gas cooktops, and cigarettes [2]. Air pollutants common to different indoor environments include carbon monoxide and dioxide (CO and CO₂), volatile organic compounds (VOCs; e.g., formaldehyde and benzene), nitrogen oxides (NO and NO₂), and polycyclic aromatic hydrocarbons (PAHs) [3]. Since people in industrialized countries spend more than 80% of their lives indoors, the build-up of air pollutant concentrations to dangerous levels, especially in modern energy-saving but air-tight constructions, represents one of the priority concerns for human health today [4]. In fact, continuous exposure to air pollutants, the concentration of which indoors can even be higher than outdoors, may cause respiratory and cardiovascular diseases eventually contributing to the so-called ‘sick building syndrome’ (SBS; see Glossary) and ‘building–related illnesses’ (BRI). One of the major concerns regards formaldehyde, a widespread hazardous air pollutant that is released over the long-term from aging furniture and pressed-wood products, and that is likely to have carcinogenic effects in humans [5]. Plants can absorb and catabolize almost any airborne pollutant, although this phytoremediation capacity has been poorly applied indoors. To date, plants selected for indoor environments have appealing aesthetic features, and their associated microbiome has been greatly overlooked. Here, we outline how the development of computerized self-sufficient biosystems, which combine the best-performing indoor plant species with new air-cleaning and sensor technologies, will represent a cost-effective solution to radically improve the quality of life of people living in ‘smart’ and more sustainable buildings.

Plants can improve indoor air quality (IAQ) by simultaneously taking up CO₂ and releasing O₂ through light-dependent photosynthesis, and increase air humidity by water vapor transpired from leaves through microscopic leaf pores, namely stomata [6]. The capacity of leaves to exchange gases and, thus, to take up any pollutants from indoor air, is limited by physical constraints related to stomatal and mesophyll resistance (Figure 1). Plants actively modulate stomata apertures in response to variable environmental conditions (i.e., light availability or air humidity) through a complex signaling network of hormones (i.e., abscisic acid, ABA) that are exchanged between roots and shoots [7]. In addition, indoor air pollutants can passively collect on the external surfaces of the complete root–soil system of the plant and, thus, be effectively removed. Processes driving ‘non-stomatal’ adsorption not only depend on the total area, anatomical features, morphological traits, and chemical properties of the plant surface, but are also related to the characteristics of the soil substrate [8]. In particular, the capacity of the leaf surface to remove air pollutants is influenced by the presence, shape, and density of trichomes [9]. More specifically, the amount of cuticular wax and the lipid composition of the membrane of epidermis cells coating the plant surfaces have an important role in the adsorption of pollutants, especially lipophilic VOCs, such as benzene, following deposition [10].

Pioneer studies conducted by NASA during the 1980s successfully demonstrated that plants are able to remove airborne pollutants [11], although these findings were based on a simplistic experimental approach [12]. Recently, more accurate experimentation simulating the long-term exposure of foliage to typical indoor concentrations of air pollutants [13] highlighted that stomatal (dependent) absorption is 30–100 times higher than the amount passively adsorbed through non-stomatal deposition [14]. These results further suggest that, after entering the plant leaf, some pollutants are metabolically degraded and/or translocated to shoots and roots [15]. Plants have enzymes able to catalyze the degradation of pollutants, such as the oxidation of formaldehyde [16] or the hydroxylation and cleavage of the aromatic rings of benzene and toluene [17]. Airborne pollutants absorbed through leaf stomatal uptake can undergo transformation following enzymatic oxidation and conversion into different bioproducts via conjugation with plant endogenous compounds (i.e., sugars, amino acid, organic acids, and peptides). After being catabolized, the assimilated pollutants might be either re-expelled (re-emission into the air or ejection via root exudates) or further metabolized to be finally used as both carbon and energy sources [17]. Plant enzymatic mechanisms of scavenging and detoxification maintain a decreasing concentration gradient of pollutants between the air and the interior of the leaves, allowing a steady and continuous uptake when stomata are open. However, only a few of the enzymes responsible for the metabolic transformation of airborne pollutants are currently recognized [18] and the fate of many other toxic gaseous compounds inside plants remains unknown.

Therefore, although there is clear evidence that plants can improve IAQ, real applications have been hampered by our limited understanding of the processes affecting stomatal uptake and the factors influencing the size of the non-stomatal sink for air pollutant absorption (see Outstanding Questions).

So far, plants used indoors have been selected on the basis of the preferences of consumers, who typically favor aesthetic features, good survival, and low maintenance needs. Most ornamental plants currently grown indoors are broadleaf evergreen species from the understory of warm tropical and subtropical climates; to thrive under dense forest canopies, such plants have optimized their leaf photosynthetic performances under low-light intensities [19]. However, adaptation to shade implies the presence of large leaf surface areas and reduction of stomatal apertures [20], which both favor the deposition of pollutants on plant surfaces rather than uptake by the leaves. The suboptimal quality and intensity of light that usually occurs indoors limit stomatal opening, which, in turn, restrains the flux of pollutants that could enter the leaves and be removed by plant enzymes.

Besides the ability to survive in the light-limited conditions provided by traditional illuminating systems and to acclimate to frost-free environments, unambiguous recommendations to drive the selection of suitable indoor plants are lacking. Therefore, scientific criteria should be outlined urgently to screen the optimal-performing plant species in indoor environments on the basis of the morphological (i.e., leaf shape, size, and hairiness), anatomical (i.e., composition of epidermis and mesophyll layers, and stomatal density and size) and physiological (i.e., CO₂assimilation rate and activity of detoxifying enzymes) properties that reflect the capacity of the plant to phytoremediate air.

A comprehensive understanding of the plant biochemical processes involved in the degradation of indoor air pollutants could be achieved through the combined application of advanced ‘omics technologies (genomics, proteomics, and metabolomics). A readout of the metabolic state of the plant provided by ‘omics profiling allows us to characterize and quantify large pools of molecules and, thus, help identify physiological functions underlying the catabolism of air pollutants by plants. The discovery of the major metabolic pathways, genes, and enzymes involved in air phytoremediation will enable the determination of biomarkers for screening (and, thus, ‘phenotyping’) the most appropriate plant species for improving IAQ. In the near future, the number of species used to phytoremediate indoor air could be widened by editing the genome of plants through precise and targeted DNA modifications aimed at overexpressing or inserting genes encoding detoxification enzymes.

Plants do not ever stand alone. Populations of microorganisms (bacteria and fungi) are ubiquitously associated with plants both belowground (i.e., in the soil rhizosphere) and aboveground [i.e., on the leaf surfaces (phyllosphere], where they can reach an impressive concentration of 10⁶–10⁷ microbial cells cm⁻² [21]. The plant microbiome can actively participate in the removal of airborne pollutants through non-stomatal adsorption [22]. However, the contribution of different microbial species inhabiting the leaf surfaces and the plant root system to removing air pollutants from various indoor environments is not yet established. Plants select different microbial species on both leaves and roots depending on genetic features and environmental conditions [23]. The introduction of microbiomes associated with various plant species into indoor environments, already inhabited by different population of microbes [24], may improve the removal of indoor air pollutants, but gives rise to concerns about their effects on human health, including allergies and lung inflammation.

Plant species suitable for cleaning indoor air should simultaneously demonstrate high physiological performance when exposed to limiting environmental conditions (i.e., low-light intensities or suboptimal growth temperatures) while being able to effectively lower the levels of harmful air pollutants. The potential of plants to improve IAQ can only be approximated because quantitative assessments mostly rely on the percent decays of heavily-concentrated air pollutants measured in small chambers enclosing plants under static conditions [25]. More sophisticated predictive models that are able to reproduce the ability of the plant to phytoremediate indoor air should be developed on the process-based mechanisms of the dynamic stomatal- and non-stomatal removal of pollutants. Therefore, an accurate experimental approach is required to quantify, over time, the flux of indoor pollutants sequestrated by the foliage (i.e., the amount of pollutants per unit time and leaf area) and the whole plant under highly controlled environmental conditions.

In addition, when introduced indoors, plants can become a source of pollution. In fact, some plant species, having scented leaves or flowers, release biogenic VOCs (mostly isoprenoids) that can react with ozone and other oxidants, such as hydroxyl (-OH) and nitrate (NO₃) radicals [26], in indoor air. Ozone-initiated oxidation [27]leads to the production of highly reactive secondary gas-phase pollutants (i.e., hyperoxides, aldehydes, and carbonyls) that can be more harmful than their precursors [28] and contribute to the formation of indoor particles [29, 30], with potential adverse effects on human health. As a consequence, the overall capacity of plant–microbe systems to phytoremediate air should be tested in realistic scenarios simulating the chemistry, transport and deposition rates of pollutants under the low mixing (stagnant) air conditions that occur indoors.

Existing air-cleaning technologies that integrate heating, ventilation, air conditioning with electrofilters, ultraviolet (UV) lights, photocatalytic materials, fuel cells, and catalyzers are promising tools to improve IAQ. However, the application of these commercial air-cleaning systems is often limited by the high costs associated with frequent maintenance and their significant energy consumption. In future buildings, improving IAQ could become more cost-effective and sustainable thanks to the use of ‘biosystems’ that combine air-cleaning technologies with the most optimally performing indoor plants (Figure 1). The introduction of sensor-controlled LED lighting, fertilization and ventilation systems can reduce energy requirements to provide optimal conditions for plant growth and enhance indoor air phytoremediation. In particular, the use of a targeted spectrum of light provided by LED illumination will simultaneously improve the efficiency of photosynthetic CO₂ assimilation and stimulate stomata opening, thus increasing the sink strength of foliage for airborne pollutants [31]. The development of such biosystems would enable the engineering of indoor environments through the real-time monitoring of air quality retrieved from low-cost wireless network sensors (WNS) communicating with logging stations, such as personal computers and smartphones. In addition, user-friendly software will serve as a decision support system (DSS) to plan and efficiently regulate the air-cleaning biosystem by suggesting the most suitable plant species, the optimal number of plants, positioning and the environmental requirements that would maximize air phytoremediation accordingly to both pollutant levels and the physical characteristics of the interior spaces (i.e., volume, temperature, and relative humidity). However, new ‘green’ indoor environments could be designed only by pursuing an inter- and multidisciplinary approach that integrates innovative technologies with a deep understanding of the air phytoremediation capacity of plants and their associated microbiome, thoroughly assessed and modeled to improve human well-being.

The ability of plants to phytoremediate indoor air pollutants has been overlooked for too long. The selection of plants suitable for indoor phytoremediation should follow unambiguous scientific criteria that reflect their capacity to sequestrate airborne pollutants, instead of only taking into consideration their aesthetic features. The capacity of indoor plants to remove air pollutants needs to be quantitatively assessed in realistic scenarios and modeled on the process-based mechanisms of both (stomatal) uptake and (non-stomatal) deposition. However, some crucial points must be addressed when planning indoor phytoremediation: (i) the production of secondary gas-phase pollutants triggered by emission of plant biogenic VOCs may negatively impact the IAQ; and (ii) the microbiome associated with indoor plants needs to be managed to realize the benefits of removing pollutants without any risk to human health.

The possibility to integrate smart sensor networks and computerized technologies for air cleaning with highly performing indoor plant species provides the opportunity to improve indoor life while also reducing energy consumption. The development of ecosustainable, cost-effective (and possibly self-sufficient) plant-based biosystemsable to enhance IAQ will positively affect both human society and the housing industry in an unprecedented fashion that could lead to a postmodern version of ecoarchitecture. If this was to happen, society as a whole would experience a paradigm shift in the way in which dwellings and indoor spaces are perceived and designed since plants, that have so far been used only as a decorative tool, would become a key player in everyone’s life.