Use of Living
Pot-Plants to
Cleanse Indoor Air — Research UPDATE
Margaret Burchett, Fraser Torpy & Jane Tarran
Faculty of Science, University of Technology Sydney (UTS)
BACKGROUND
Urban air quality (IAQ), in Australia as elsewhere, remains a
health issue, and since we city dwellers spend 90% of our time
indoors, that is where we breathe most of our air, which is even
more polluted than outdoors. Urban outdoor air pollution is
mainly caused by petroleum-based vehicle and other emissions.
According to the NSW EPA, air pollution is estimated to kill at
least 1,400 people p.a. in Sydney alone (which is about equal
to the total number of road deaths per annum in the whole of
Australia). Overseas research has shown that indoor plants can
remove a number of urban air pollutants, including nitrogen
and sulfur oxides, ‘air toxics’ and fine particulates (organic
dust).
We have previously reported to NIPA on our UTS testchamber studies of VOC removal with seven plant species,
and our ‘real-world’ study in 60 UTS offices with two of the
species. The studies all showed that potted-plants can reliably
reduce total loads of volatile organic compounds (VOCs; the
major class of indoor-sourced air pollutants) to below 100 ppb,
and maintain levels there. Our laboratory studies showed that
microorganisms of the potting mix are the main VOC removal
agents, the plant’s role here being to nourish and maintain
their root-zone microbial community. That is, it is a symbiotic
microcosm at work. Our test-chamber studies also showed
that, with every plant species tested, VOC removal rates were
stimulated to an accelerated level (ie ‘induced’ to respond and
break down the substance) by an initial dose of the pollutant
(eg benzene, toluene, etc). Maximum removal rates for a
given dose were usually achieved after three doses, i.e., within
about a week from initial exposure. The office study showed
the microcosm (with the amounts of plant material we trialed)
worked equally well with or without air-conditioning, and in
light or dark (ie, 24/7).
TWO MORE SPECIES LABORATORY-TESTED
We reported on our most recent research findings,
summarized here, in October, both at the NIPA Training Day
session held in Sydney, and at the international Indoor Air
Quality conference in Japan. This year we have completed
a test-chamber comparison of VOC removal with two species
untested previously by us - Aglaonema modestum and
Zamioculcas zamiifolia (Zanzibar; ZZ plant), with materials
(in 200 mm pots). Our thanks to Stuart and Sue Swaddling,
Tropical Plant Rentals, for the donation of the plants. We used
three repeated high doses (25ppm) benzene as the test VOC
(this is equal to five times the WorkSafe Australia allowable
occupational maximum 8-hr averaged exposure limits). We
also examined whether removal rates of either species could be
related to a plant or potting-mix attribute.
The benzene removal rates for the two species are shown
below in graph below. The two species showed similar patterns
of induction of a stimulated VOC removal with the first dose,
and, by the third dose, each removed the VOC effectively to zero
in less than 48 h. The results show a similar pattern to those of
the seven species we previously tested. We then re-calculated
VOC removal rates on several different bases - on leaf area,
and on wet or oven-dried (at 60oC) weights of shoots, roots,
or potting mix. We found that removal rates of the two species
were equal on the basis of per kg wet weight of potting-mix,
which is consistent with the potting-mix microorganisms being
the main VOC removal agents.
Removal of benzene from test-chamber air with potted
Zamioculcas zamiifolia and Aglaonema modestum, with
three consecutive doses of 25 ppm benzene (means ± SE; 6
replicates).
EFFECTS OF INDOOR PLANTS ON OFFICE LEVELS
OF CO2 AND CO
In our office study, mentioned above, as we measured
weekly TVOC levels we also measured levels of carbon dioxide
(CO2) and carbon monoxide (CO). However, we have only
recently analysed the results for these two gases, for offices with
or without three floor specimens of Dracaena ‘Janet Craig’. But
first, some background on the two compounds, both of which
are produced, among other means, by fossil fuel combustion
emissions again.
Carbon dioxide (CO2)
Global warming /climate change is discussed daily in
the press, and is generally regarded as being caused, or at
least aggrevated, by too much fossil fuel combustion, which
produces too much CO2 emission, which prevents heat loss
from the planet, which resuts in global warming, etc.. It is
also well known that, with adequate lighting, green plants
will photosynthesise and, in the process, refresh air in two
ways – by absorbing CO2 (used to synthesise carbohydrate)
and releasing O2 (molecule for molecule) as a by-product.
Hence re-afforestation offered as a means of combatting global
warming.
There has been very little research on the ability of indoor
plants to refresh air via this gas exchange. However, for
building occupants, the main purpose of ventilation is not so
much to replenish O2 (21% of the atmosphere) as to remove
CO2. (still only about 370 ppm outdoors. But indoor levels are
often higher, because we are all in there, breathing in oxygen
and exhaling CO2). Studies have shown that work performance
and productivity decline with increasing CO2 levels (which lead
to drowsiness and a ‘dull head’ – sound familiar?).
Carbon monoxide (CO)
To humans, CO is a very far more toxic fuel combustion
product than CO2 (CO latches tightly on to haemoglobin,
stopping it from picking up oxygen. Think exhaust-pipe fumes
in confined spaces). Fortunately, however, in some plant
species CO can be utilized, stimulating root growth and seed
germination, or alleviating salt stress. And some bacteria can
use CO as a nutrient or in normal metabolic enzyme processes.
There has been no previous research on the effects of indoor
plants on CO levels in the air.
Findings
In unplanted offices CO2 levels were, as expected, much
higher than levels of CO. Potted-plant presence was associated
with significant reductions in both CO2 and CO concentrations
(Table below). CO2 levels in planted offices were reduced by
about 10% in the air-conditioned building, and by about 25%
in the building without air-conditioning. CO concentrations were
very greatly reduced with plant presence, down to 8-14% of
those in unplanted offices, with or without air-conditioning.
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