The environmental benefits of interior plants

Interior plant displays can offer several positive environmental effects in a building.  Some of these may even help a developer to gain 'Green building' status in those areas where such standards exist.

The effects of plants on the interior environment are not going to be enormous, but they measurable and should not be ignored.

Additional information on the environmental benefits of interior plants can be found in module 1.

Air quality improvements

Chemical pollutants

The work of Bill Wolverton, both during his time with the National Aeronautics and Space Administration (NASA) and afterwards, is probably the most notorious in plant benefits research. He showed that plants can absorb pollutant gases such as formaldehyde, benzene and trichlorethane, which are released in small quantities by a whole range of materials and human activities. Most of his research on these so-called volatile organic compounds (VOCs) was carried out on a small laboratory scale and until recently had little support from the scientific community, who wanted to see data taken inside real buildings with sensible planting densities and air exchange rates.

Such data are still thin on the ground, but support for the concept and mechanism of pollutant removal, including reduction in high carbon dioxide levels, has come from research carried out in Australia and India.  (Click here to download a paper that shows how plants can reduce the amount of hexane and benzene in the atmosphere, given by Dr Ron Wood and his colleagues at a recent symposium.)  Taking this, and the fact that gas exchange is fundamental to plant growth, there can now be little doubt that plants do filter out some of the pollutant gases found in buildings. The very fact that people believe this to be true may also have a placebo effect far greater than any physical effect.

Humidity

Plants can help to keep the air in buildings fresh and at the optimum humidity level of between 40% and 60% although the effect is small and localised.  They achieve this in two ways.

First, transpiration of water through the plant, from root to leaf and out into the atmosphere.  Transpiration is governed by species, light levels, humidity and temperature. The more active the plant is, the greater the transpiration rate. Conversely, plants that are under stress, e.g. under attack by pests; or too dry, often reduce transpiration by closing the pores (stomata) in their leaves.  The greatest benefit is therefore derived from healthy, well-maintained plants. (N.B. Some plants, especially those from arid zones, e.g. Crassula species only transpire at night. This is an adaptation to allow them to conserve water in their natural habitat. These plants will offer no benefit during daylight hours.)

The second mechanism is evaporation from the soil, which has a localised cooling and humidifying effect.  Dry air tends to be a bigger problem in the winter months, when windows are shut and heating turned up, and in offices with lots of computers and other electrical equipment.  Air conditioning also dries the air.

Although plants won’t completely solve a dry air problem they can make a useful contribution, especially if sited near to where people work.  However, the amount of moisture that enters the atmosphere from a plant display is limited by the amount of water added to the display to begin with.  If a plant only receives 1 litre of water a week, then the most that can be transpired or evaporated will be 1 litre a week.

Dust reduction

Research in the USA has shown that planted buildings have less airborne dust than unplanted ones. The effect is mainly due to the increased humidity, but static electricity also plays its part. Plants are slightly negatively charged compared with their surroundings and so will attract positively charged dust particles, reducing the number in the air and on surrounding surfaces. This keeps offices cleaner, reduces the risk of allergies and helps to protect sensitive electronic equipment.   It does, of course, mean that the plants will need cleaning regularly.  As with the removal of pollutants, the ability of plants to remove dust is limited, and will depend on the number of plants installed and the level of dust present.

Cooling

Plants can have a useful effect on the temperature regulation of buildings, and it is one reason why plants are often put into atriums.

Many building atriums are designed to help manage the thermal regulation of a building.  Ventilation ducts often extract air from atriums and exhaust air back into them.  The large mass of air and the temperature gradients that occur in the space will affect the way that air circulates.

The air in atriums can be cooled by plant displays.  The heat in the air can evaporate moisture from the soil and allow transpiration to take place.  The soil also has a high heat capacity and can buffer against extremes of temperature.  Many modern buildings are being designed to incorporate plants to aid temperature regulation and improve the efficiency of heating and air conditioning systems.  Decorative water features, often installed with plant displays, will also have a significant benefit.

Noise reduction

The acoustic benefits of plants are explained in detail elsewhere on this web site.  Noise in buildings is sometimes a serious issue that needs to be dealt with.  Plants can provide a low-cost, and visually pleasing, solution to some problems associated with noise and can do so without the need for expensive engineering methods.

What plants can't do

The publicity that still surrounds the claims made for the reduction in pollutants by plants has lead, in some areas, to the promotion of interior plants as a panacea for all indoor air quality problems.  Whilst it is clear that plants do have a positive effect on the indoor climate, their sole use cannot be recommended as the answer to air quality problems.  Instead, they can be used as part of an integrated approach to maintaining a pleasant and healthy indoor environment that also includes the use of well designed heating and ventilation systems.

Click here for a reading list of papers and articles on the environmental benefits of plants in buildings.