As has already been stated, PM is one of the most harmful air pollutants to human health and is present both outdoors and indoors. In numerous studies, it has been shown that vegetation that accumulates PM improves outdoor air quality and reduces health risks (Pough et al. 2012; Soreanu et al. 2013). The results from this study clearly showed that spider plants (Chlorophytum comosum L.), well known for absorbing and detoxifying or degrading many harmful compounds such as NOx, CO, formaldehyde, benzene and others (Wolverton and Wolverton 1993; Giese et al. 1994; Costa et al. 1995; Cornejo et al. 1999; Wolverton 2008), are also able to accumulate PM.
PM of both categories, i.e. sPM and wPM, as well as all three determined size fractions were found on leaves of this species. The quantity of total sPM and wPM accumulated on leaf blades depended on the kind of activity taking place in the surveyed rooms and their locations. The lowest amount of PM accumulated on leaves was, as expected, recorded in plants growing in the house in Zielonka. This is due to fact that Zielonka is a suburb of Warsaw with no industrial activity and only low levels of traffic on the nearby roads. The highest amount of PM accumulated on the leaves of plants growing in the office and the apartment can be explained by (i) the location of these rooms (both are in buildings in Warsaw next to roads with heavy traffic) and (ii) the type of activities (printers and photocopies were heavily used in the office, and in the apartment, residents undertook high levels of physical activity and used a stationary bike on a daily basis).
In this study, the share of sPM in the total amount of PM was, in most cases, greater than that of wPM. These results are in agreement with the results of previous studies carried out in the authors’ laboratory on 52 woody plant species growing outdoors (Dzierżanowski et al. 2011; Popek et al. 2011, 2013, 2014), but not with data reported for small-leaved limes grown in Warsaw (Nawrot et al. 2011) or for some species grown in Norway (Sæbø et al. 2012). sPM was also in greater proportion than wPM in the case of large PM and coarse PM, but not usually in the case of fine PM.
The results of this work also showed that irrespective of the activity taking place in the particular rooms, of the three size fractions, the amount of large PM was greatest while the amount of fine PM was the smallest, with the exception of wPM. These results are in line with data on 51 species of trees and shrubs grown outdoors in urban areas (Dzierżanowski et al. 2011; Popek et al. 2011; Sæbø et al. 2012; Popek et al. 2013, 2014).
Fine PM was found at higher levels in the category of wPM than sPM and as such was more firmly attached to the leaves. This means that fine PM is phytostabilised to a greater extent than coarse and large PM. It is worth underlining that phytostabilisation of fine PM in wax lowers the risk to human health, because fine PM and ultra fine PM (which were not determined separately in this study) are most harmful to people. This is because they easily reach the alveolar region of the respiratory tract and can penetrate into the bloodstream and be transported in the blood to all organs (Nemmar et al. 2002). Moreover, highly toxic heavy metals are carried on fine PM (Voutsa and Samara 2002) along with PAH (Caricchia et al. 1999; Kaupp et al. 2000) and EPRFs (Saravia et al. 2013), making them even more toxic.
In this study, it was also shown that the amount of waxes deposited on leaf blades differed depending on the room in which the plants were growing. The highest amount was in the perfume-bottling room. This is probably because this activity is associated with a higher level of VOCs in the air in comparison with the other rooms, which can in turn generate stress for the plants. Popek et al. (2014) reported higher amounts of wax deposited on the leaves of trees growing at shorter distances from the source of PM emission as compared to those growing at further distances, for example inside the parks/forest.
Although the amount of wax deposited on leaves differed significantly between plants growing in various rooms, similar to the data of Dzierżanowski et al. (2011), Sæbø et al. (2012) and Popek et al. (2013), no correlation was found between the amount of wax and the quantity of accumulated PM.
In every room examined, the amount of PM accumulated on the leaves of the spider plants was always significantly higher than the amount of PM deposited on the AL-plates. This was true irrespective of whether comparisons were being made, in the case of leaf blades, between total PM and AL-plates or just sPM (i.e. the part washed with water) and AL-plates. It clearly demonstrates that gravitation is not the sole factor involved in PM accumulation on the plants’ leaves. This is in line with the statement by Pough et al. (2012) that the dry deposition of air pollutants among other things depends on the nature of the surface and is generally higher with vegetation than with other surfaces because of the metabolic uptake by plants, the stickiness of the leaf surface, the large surface area of plants (when compared to the ground area occupied by plants, LAI) and their aerodynamic properties. There is also data showing that the efficiency of pollutant uptake by plants differs depending on genotype, roughness of leaves, pubescence, the amount of wax, the topography of leaves on stems creating turbulence and the distance from the source of emission (Popek et al. 2013 and articles discussed in it).
In the case of spider plants growing indoors, it is highly probable that wax deposited on leaf blades plays an important role. Perhaps there are interactions between the waxes of spider plant leaves and the hydrophobic nature of PAH (which settles on PM). On the other hand, since the amount of PM did not correlate with the amount of wax, other forces must also be involved in PM accumulation on leaves. It is possible that the metabolic uptake underlined by Pough et al. (2012) plays an important role, but electrostatic forces between heavy metal ions settling on PM and leaf blades cannot be ruled out. To answer this question, more detailed and specifically designed studies are needed.
While at this stage there is no answer to questions about the forces involved in PM accumulation on the leaves of plants, this was not actually the objective of this study. The intention was to establish whether indoor plants also accumulate PM and, to the authors’ best knowledge, this is the first data to be presented in literature on PM phytoremediation from indoor air.
This study has proved that spider plants growing indoors accumulate PM. Although growing plants in pots is, in addition to the other positive functions of a plant, only a passive way of cleaning air, even so it contributes to improving air quality. In order to make air phytoremediation more effective, combinations of plants with associating microorganisms (Schwitzguébel 2000) or botanical filtration with biotrickling filters (Soreanu et al. 2013) can be employed besides conventional, albeit more expensive technologies. Therefore, indoor plant cultivation deserves to be popularised more widely since it reduces the risk to human health.
Summary and conclusions
Spider plants (Chlorophytum comosum L.) grown indoors accumulate particulate matter of both categories and all size fractions, irrespective of their location and the type of activity taking place in the examined room. They therefore phytoremediate PM from indoor air.
The amount of PM accumulated on leaves depends on the kind of activity taking place in the particular room.
Fine PM, the most harmful to human health, is accumulated to a greater extent as wPM than sPM because it is attached more tightly to leaves and is thereby phytostabilised more effectively. This reduces the risk to human health to a greater extent.
Of the three size fractions examined, large PM constitutes the greatest proportion of PM accumulated on plants’ leaves.
Accumulation of particulate matter on leaves involves factors/forces other than gravitation.