Characterisation of fungal and bacterial dynamics in an active green wall used for indoor air pollutant removal

https://doi.org/10.1016/j.buildenv.2020.106987Get rights and content

Highlights

  • Green wall bacterial community varied amongst plant species.

  • The aerosolised fungal load emitted by green walls did not exceed WHO guidelines.

  • No aerosolised Legionella species were identified.

Abstract

Indoor air quality (IAQ) is of growing public health concern which has prompted the use of plants to phytoremediate air pollution in interior spaces. Active green walls are emerging as a means of reducing indoor contaminants and have demonstrated efficacy comparable to conventional air filtering technologies. However, the use of active airflow through organic substrates has the potential to emit bioaerosols into the surrounding environment, where the potential risk to human health is largely unknown. In this study, we demonstrate that two indoor green walls (with and without active airflow) contribute significantly to the ambient fungal load, however concentrations remained well below WHO safety guidelines. Bacterial dynamics within the rhizosphere/substrate of the operational botanical biofilters displayed variability across plant species. Phyla-wide distribution generally aligned with previous literature; however, differences from those previously reported were observed at the genus level, possibly due to geographic location, substrate composition, or plant species selection. Targeted assessment of Legionella aerosol contamination, an under-addressed potential pathogen for these active systems, yielded no positive identification during the sampling period. We conclude that active green walls host a unique bacterial profile and do not emit harmful levels of fungal propagules or pose significant risk of aerosolised Legionella species, provided systems are well monitored and maintained.

Introduction

In modern societies, humans spend up to 80% of their time indoors [1], where air quality is often more polluted than outdoors [2,3]. Due to the accumulation of air pollutants, and the extended duration of exposure associated with an indoor lifestyle [4], domestic and commercial indoor air pollution is responsible for up to 5% of the global disease burden [5], equating to costs of approximately US$90 billion annually [6].

Since the 1980s, the use of plants in interior spaces to phytoremediate air pollution has grown considerably in popularity [7,8]. The efficiency of botanical systems in improving indoor air quality has been significantly enhanced by the development of active botanical biofiltration, or active green wall systems [9]. Active green walls use ornamental plants grown along a vertical plane with the addition of mechanical air induction to actively draw polluted air through the plant growth substrate and foliage [10]. During this process, air pollutants are delivered directly to the rhizosphere where they may be metabolised/sequestered by microbes, the predominant mechanism for contaminant degradation [[11], [12], [13]]. Additionally, particulate matter (PM) may be filtered by the substrate and root structures [14].

While botanical biofiltration is still an emerging technology, there is substantial evidence for its practical potential, along with growing commercial interest [[15], [16], [17], [18], [19]]. In their current state, botanical biofilters have comparable removal efficiencies to those of conventional indoor technologies such as MERV (minimum efficiency reporting value) 4, 6, 10, 11 and 13 filters for the removal of PM (PM10 and PM2.5) [20]. In addition, botanical biofilters are capable of reducing indoor concentrations of volatile organic compounds (VOCs) and other pollutants such as CO and CO2 [[21], [22], [23], [24], [25]], which cannot be removed by most conventional systems, other than by dilution [26].

Despite the benefits of active green wall technologies, there is a potential for systems that use active airflow through biologically active substrates to emit bioaerosols into the surrounding environment [27]. It has indeed been proposed that active green walls may provide a favourable environment for the proliferation of pathogenic fungal or bacterial species, with the use of mechanically assisted air flow increasing the risk of the aerosolisation of water containing microbial bioaerosols. Currently, research which has assessed bioaerosol emissions from active green walls are limited to assessments of total fungal and bacterial loading. While there are no documented cases where harmful levels of fungal [[28], [29], [30], [31]] or bacterial aerosols [20,28,31,32] have been detected in active green wall emissions, there is a paucity of research that has comprehensively characterised bioaerosol emissions, and we propose that assessments of this kind are essential to fully understand the implications of biowall systems for indoor air quality (IAQ).

Limited research has specifically investigated the aerosolised release of pathogenic bacteria from green walls [33], such as the ubiquitous bacterial genus Legionella. Legionella are free-living motile bacteria that can infect other microorganisms or form chemo-resistant biofilms [[34], [35], [36]], and several species are the causative agents of legionellosis [[36], [37], [38]]. L. pneumophila serogroup 1 is responsible for up to 90% of infections worldwide, with the exception of in Australia, New Zealand and Thailand, where L. longbeachae is the dominant pathogen, and is responsible for up to 40% of infection [[39], [40], [41], [42]]. The dispersal mechanisms of these two species vary significantly [33,43]. Where L. pneumophila requires aerosolisation through water droplets for infection to occur [33,37,38,44], L. longbeachae requires physical contact from contaminated soils with the eyes or mouth [42,43]. Due to the nature of the components used in active green walls, there is some concern that Legionella spp. may proliferate within irrigation systems and botanical substrates and become aerosolised in the event of over-watering or physical disturbance.

In this study, we aimed to determine whether an established active green wall in a modern urban office building contributed significantly to the release of fungal and bacterial aerosols, with specific focus on bioaerosols that have implications for IAQ and human health. We assessed the culturable indoor aeromycota, characterised the bacterial community composition using 16S rRNA amplicon sequencing approaches, and performed targeted enumeration of the pathogen Legionella spp. to examine potential risks to public health.

Section snippets

Site description

Aerosol sampling was conducted on four floors (levels 12–14 and 17) of a newly built commercial office building, made of steel and glass near Sydney's Central Business District. The building featured standard heating, ventilation and air conditioning (HVAC) systems with no additional filtration technology. One active and one passive green wall span the interior of two stories (levels 13 and 14), each covering 60 m2, in a semi-open plan café and meeting/reception space, with a floor space of

Fungal bioaerosol assessment

Active and passive green wall sites featured significantly higher fungal densities than the reference sites across the three-month sampling period (p = 0.001 and p = 0.009 respectively; Fig. 2). Temporal differences in fungal density were not significant, nor were interactions amongst factors or with foot traffic (p > 0.05). Despite elevated concentrations of aerosolised fungal propagules, total concentrations remained well below the World Health Organisation guideline for indoor air [61] of

In situ bioaerosol analysis

While potting soils have been implicated as a source of human pathogens [[62], [63], [64], [65]], studies documenting the dispersal of aerosolised fungal pathogens from indoor contaminated soils is limited [66]. Several studies have found that neither potted plants nor complex biowall structures contribute significantly to allergenic or pathogenic airborne fungal density [[66], [67], [68], [69], [70]], unless considerable physical disturbance or agitation occurs [71]. Nonetheless, as active

Summary and conclusion

Fungi are ubiquitous soil inhabitants and have strong associations with plants. The installation of botanical material indoors, either as simple potted plants or complex active green walls, is likely to contribute to the ambient fungal load [29,70,103,104]. Fungal aerosols in the ambient indoor environment proximal to active and passive green walls remained well below WHO guidelines and the systems did not release detectable harmful fungal bioaerosols. The concentrations detected were

Authors' contributions

RF, PJI and FT designed the study; RF collected samples; RF and PJI analysed fungal samples; RF and NW analysed bacterial samples; RG and RF analysed the data; RF interpreted data; RF, TP, RG, PJI, FT and JS drafted the manuscript.

We would like to acknowledge and express our gratitude to Production Design Engineer Laura Dominici from the Department of Environment, Land and Infrastructure Engineering (DIATI), Politecnico di Torino (Italy), for the contribution of “Fig. 1: Schematic

Declaration of competing interest

We state that there was no conflict of interest.

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