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    Green walls are growing in popularity.

Ordinary potted house plants can potentially make a significant contribution to reducing air pollution in homes and offices, according to new research led by the University of Birmingham in partnership with the Royal Horticultural Society (RHS).

During a series of experiments monitoring common houseplants exposed to nitrogen dioxide (NO2) -- a common pollutant -- researchers calculated that in some conditions, the plants could be able to reduce NO2 by as much as 20 per cent.

 researchers tested three houseplants commonly found in UK homes, easy to maintain and not overly expensive to buy. They included Peace lily (Spathiphyllum wallisii), Corn plant (Dracaena fragrans) and fern arum (Zamioculcas zamiifolia).

Each plant was put, by itself, into a test chamber containing levels of NO2 comparable to an office situated next to a busy road.

Over a period of one hour, the team calculated that all the plants, regardless of species, were able to remove around half the NO2 in the chamber. The performance of the plants was not dependent on the plants' environment, for example whether it was in light or dark conditions, and whether the soil was wet or dry.

Lead researcher Dr Christian Pfrang said the plants used were all very different from each other, yet they all showed strikingly similar abilities to remove NO2 from the atmosphere.

“This is very different from the way indoor plants take up CO2 in our earlier work, which is strongly dependent on environmental factors such as night time or daytime, or soil water content,” Pfang said.

The team also calculated what these results might mean for a small office (15 m3) and a medium-sized office (100 m3) with different levels of ventilation.

In a poorly ventilated small office with high levels of air pollution, they calculated that five houseplants would reduce NO2levels by around 20 per cent. In the larger space, the effect would be smaller -- 3.5 per cent, though this effect would be increased by adding more plants.

While the effects of the plants in reducing NO2 are clear, the precise mechanism by which they do this remain a mystery.

"We don't think the plants are using the same process as they do for CO2 uptake, in which the gas is absorbed through stomata -- tiny holes -- in the leaves,” Pfang said.

“There was no indication, even during longer experiments, that our plants released the NO2 back into the atmosphere, so there is likely a biological process taking place also involving the soil the plant grows in, but we don't yet know what that is."

In the next phase of the research, the team will be designing sophisticated tools for modelling air quality indoors encompassing a much wider range of variables.

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