CADR of potted plants in reviewed studies

In total, 196 CADR p values were computed from the 12 reviewed chamber studies. A histogram expressing this entire dataset is provided in Fig. 1a, which possesses a wide spread of nearly four orders of magnitude (ranging from 0.0004–0.2 m3 h−1 plant−1 at 10th and 90th percentiles), a median CADR p = 0.023 m3 h−1 plant−1, and a mean (standard deviation) of 0.062 (0.089) m3 h−1 plant−1. Even though these CADR p values represent best-case scenarios (as they were computed assuming negligible chamber sorption and leakage), their magnitudes are exceedingly small. For context, typical gas or particle air cleaners possess average CADR values on the approximate order of ~100 m3/h [65,66,67].

Fig. 1 a Histogram of the CADR p dataset assembled from the reviewed chamber studies outlined in Table 1. CADR p computations are detailed in the SI. b The CADR p data resolved by publication (labeled by first author and reference number) and measured VOC Full size image

Figure 1b resolves all 196 datapoints contributing to the Fig. 1a histogram by type of VOC measured, labeled by the study‘s first author and reference number. This figure thus explores the possibility of constraining CADR p for each VOC. Some of the data preliminarily indicates that certain VOCs may be more efficiently removed by potted plants; for instance, Kim et al. [44,45,46] observed better formaldehyde removal than for xylene, and Wolverton et al. [49] observed a much lower TCE removal than for formaldehyde and benzene. However, these trends are not consistent throughout all studies; for instance, Yang et al. [50] observed similar removal of TCE, benzene, and toluene. Also, not enough studies assessed the same combinations of VOCs sufficient for a definitive trend to be established. Furthermore, some results vary largely from study-to-study even for the same VOC.

More notably, however, the variance of CADR p values belonging to a particular study is much smaller than the variance of the dataset as a whole (intra-study values range 1–2 orders of magnitude, as compared to the total CADR p range of ~4 orders of magnitude). For example, of the 46 CADR p values calculated from Kim et al. [44,45,46], 32 of them (70%) reside above 0.1 m3 h−1 plant−1, making up 84% of the total 38 CADR p greater than 0.1 m3 h−1 plant−1. On the other end of this spectrum, all CADR p values belonging to Irga et al. [43] and Yang et al. [50] were less than 0.001 m3 h−1 plant−1, making up all but one other CADR p below 0.001 m3 h−1 plant−1. The one remaining CADR p existing in this lowest-performing interval belongs to Zhang et al. [52], who also conducted an experiment with chloroform, despite their use of genetically modified plants shown to enhance VOC uptake. We believe these trends suggest that the varying VOC removal performance among different research studies may be an indicator of differences among removal measurement methodologies, which should be further investigated. These perhaps include measurement techniques, plant and rhizosphere health, and other characteristics and relative sizes of the chamber, soil, pot, or the plant itself (e.g. VOC sorption onto competing surfaces).

Effectiveness in typical buildings

Using the entire CADR p dataset (Fig. 1a), Eq. 6 was used to compute four sets of total CADR/V loss rates, binned into four distinct plant density (ρ p ) cases separated at logarithmic intervals (0.1, 1, 10, and 100 plants/m2). In Fig. 2, these loss rates are compared directly to a distribution representing the AER typical of US residences [54, 55] and another representing AERs typical of US offices [53]. Again, these two types of loss rates can be directly compared to demonstrate their relative impacts on VOC removal. The two boxes corresponding to ρ p values of 0.1 and 1 plants/m2 are barely visible, so their corresponding loss rates are almost certain to be negligible, even if plants exhibiting the highest plausible CADR p are used. For a ρ p = 10 plants/m2, some of the loss rates due to VOC removal by the plants from the upper end of the CADR p distribution may comparable to air exchange losses in particularly tight buildings, but the median CADR/V is still negligible compared to the median AER for both residences and offices.

Fig. 2 Boxplots of VOC loss rates due to: (left) CADR/V over four cases of plant density (ρ p ); compared to (right) the VOC loss rates due to air exchange rates (AER, λ) in residences (Res.) or offices (Off.) Full size image

This assessment is in strong agreement with the conclusions of Girman et al. [60] and Levin [63]. Using similar mass balance calculations and the most generous selection of the early published Wolverton et al. [49] data, Levin [63] determined that a ~140 m2 house (1500 ft2) would require 680 houseplants (i.e., ρ p = 4.9 plants/m2) for the removal rate of VOCs by plants indoors to just reach 0.096 h−1. Achieving these rates of plant density throughout a building is obviously not attainable. Even ρ p = 1 plants/m2 would rule out any useful occupant-driven architectural programming being applied to a building, and it would take a theoretical ρ p = 100 plants/m2 for the entire CADR/V loss rate distribution to be comparable to the AER distributions on a whole.

A parametric analysis was used to predict the required ρ p necessary to achieve a desired effectiveness for various combinations of AER and representative CADR p . The analysis computed ρ p required for varied Γ between 0 to 1 and AER between 0.1 and 10 h−1, thus exhausting all Γ possibilities and all reasonably expected indoor AERs in typical buildings. The CADR p was set at one of three discrete cases. The first was a low CADR p case, corresponding to the 10th percentile of the complete CADR p dataset (0.00014 m3 h−1 plant−1); the second used the median of the CADR p dataset (0.023 m3 h−1 plant−1); while the third used the 90th percentile (0.19 m3 h−1 plant−1). The ρ p predictions are presented as contour plots in Fig. 3, which are binned at factor-of-ten intervals from ρ p < 1 to ρ p > 10,000 plants/m2.

Fig. 3 Contour plots displaying the results of a parametric analysis, where binned plant density (ρ p ) was computed over continuous and exhaustive ranges of effectiveness and AER, and three cases of plant performance as an air cleaner: a a weak case being the 10th percentile of the CADR p dataset (0.00014 m3 h−1 plant−1), b the median CADR p case (0.023 m3 h−1 plant−1), and c a strong case being the 90th percentile of the CADR p dataset (0.19 m3 h−1 plant−1) Full size image

At the strongest-case CADR p assumptions (Fig. 3c), an effectiveness of ~20% may be realized in an extremely low-AER building (e.g. λ < 0.2 h−1) if one potted plant is used per square meter of the indoor floor area. This effectiveness quickly falls off if an even slightly higher air exchange rate is experienced. But, as was stated, this ρ p = 1 plants/m2 is too dense to be practical within a building, and it barely registers as effective under the most generous CADR p and AER assumptions. Under the more likely plant-removal characteristics (Fig. 3a, b), any legitimate effectiveness, even in buildings with the lowest air exchange, would require ρ p values that are not only impractical or infeasible indoors, but are ludicrously large. Note again that the analyses in this section were carried out with a best-case CADR p dataset, which computed CADR p assuming neither chamber leakage nor surface sorption contributed to observed losses, so even these impossibly large ρ p values essentially represent a lower bound.

Other considerations

The conditions within sealed chambers do not scale up to the conditions of real indoor environments, which have high AER, large volumes, and persistent VOC emissions. Our conclusion that plants have negligible impact on indoor VOC loads is consistent with the results of field studies that did not observe real VOC reductions when plants were placed in buildings. Despite potted plants not appreciably affecting indoor VOC concentrations, conducting chamber experiments on plants can remain a consequential effort. There is much to still be learned pertaining to the mechanisms of botanical uptake of VOCs. And, other applications of botanical filtration do exist (although passively cleaning indoor air is not one of them). Potential usefulness for further research perhaps lies in plant-assisted botanical bio-trickling purifiers (colloquially, “biowalls” or plant walls), which mechanically pull air through a porous substrate supporting plants and their root ecosystems [68,69,70]. These may create a more effective means of VOC removal because of their size, exposed rhizosphere, and controlled and continuous airflow. Some recent studies suggest that biowalls may yield CADRs on orders of 10–100 m3/h for certain VOCs [71, 72], with the potential to make worthy contributions to indoor VOC removal. However, more biowall field assessments and modeling endeavors are required to better hone our understanding of their true air cleaning and cost effectiveness.

Regardless of application, more rigor is required in future chamber experiments to remove methodological ambiguities. First-order loss must be used to interpret results, and chamber leakage and surface sorption (to the chamber walls as well as to the pot and soil) must be accounted for. A standardized metric to be used in mass balance calculations, such as the CADR, should also be a critical aspect of future experimental reporting. Research also suggests that the plant itself is less crucial to VOC removal than the microbial community which resides within the rhizosphere/soil system of the plant [73, 74].

The issue of bringing plant life into the indoor environment is also a complex one, not settled by a potted plant’s (in)ability to reduce airborne VOCs. Indoor plants, by helping to create a more biophilic indoor environment, may have a positive impact on occupant well-being [75], which may also translate into productivity improvements for businesses. However, plant introduction may also come with certain costs or trade-offs. One potential associated downside of plants indoors may be increased humidity. Also, plants have been shown to produce certain VOCs under particular conditions [76, 77]. So even if a potted plant works to slightly reduce, for instance, the persistence of formaldehyde indoors, its net impact on total VOC concentrations and overall indoor air quality is less clear. Spores and other bioparticle emissions may also be produced by plants, which have been observed from biowall systems [65, 74, 75]. Continued rigorous laboratory and field studies are required to develop a more complete and nuanced understanding of the interplay between plants and indoor environmental outcomes.