How clean is the air in your home or workplace? The smell of new furniture or carpets gives us a clue. Volatile compounds seep from furnishings, adhesives, electrical equipment and building materials and are augmented by those from outside. Not all of them are smelly but many are toxic to humans and their effects are boosted as the compounds circulate within a room.

It has been estimated that humans spend about 90% of their time indoors, so this non-occupational exposure to hazardous compounds makes an important contribution to the overall risk. It would be advisable to remove these vapours from indoor air to maintain a healthier atmosphere and houseplants have been proposed as one unusual vehicle.

It is known that outdoor plants remove gases from the atmosphere via two routes. Stomata on the outer surface of the leaf, which permit the transfer of carbon dioxide, water vapour and oxygen, can also absorb other gases. In addition, the leaf cuticles can adsorb gases. So, it seems reasonable to assume that indoor plants behave similarly.

Two scientists have published the results of an initial study to try and establish the uptake of volatile organic compounds (VOCs) by houseplants. Akira Tani from the University of Shizuoka, Japan, and Nicholas Hewitt from Lancaster University, UK, focused on low-molecular-weight aldehydes and ketones. They found very few reliable studies measuring their uptake by indoor houseplants and noted that most previous plant studies used non-representative VOC concentrations with mixing ratios of 10^-6 rather than typical indoor mixing ratios of 10^-8 to 10^-7 (10-100 ppbv).

This limitation was governed by the sensitivity of the measurement technique, such as GC. To counteract this and use "real" indoor VOC levels, the duo employed proton transfer mass spectrometry (PTR MS). It was linked to a flow-through exposure method to model more accurately indoor conditions.

Two common houseplants, peace lily (Spathiphyllum clevelandii) and golden pothos (Epipremnum aureum), were chosen for the experiments. Live, attached leaves from each plant were enclosed in a transparent Teflon bag and purified VOC-rich air was passed through. The VOC concentrations before and after plant exposure were measured by PTR MS and used with the leaf area to calculate the uptake rate. Seven aliphatic aldehydes, benzaldehyde and 6 methyl ketones were tested at concentrations of 14-650 ppv.

No VOC loss was detected from the bag when the plants were absent. However, when the plants were in the bag, the levels of aldehydes and ketones both decreased slowly but continuously, indicating removal by the plants. The uptake rates varied in the ranges 7-19 and 2-7 mmol/m²/s for aldehydes and ketones, respectively. The one exception was acetone, which was temporarily taken up but soon became saturated in the plant. VOC uptakes ceased in the dark and recommenced in the light.

The total concentrations of each VOC taken up by the plants were about 30-100 times greater than the amount dissolved in the leaf. The researchers proposed translocation to other parts of the plant or metabolism within the leaf as two possible mechanisms to account for this. The relative contribution of each process is difficult to determine due to the general lack of knowledge about the removal of contaminants in the phloem flow within the plants.

The calculated relative errors in the input and output VOC concentrations were 0.8-2.9%, so small changes in the levels were measurable. In addition, a mass spectrometric signal integration time of 10-20 s allows measurements to be made in real time. This level of performance would not be attainable, say Tani and Hewitt, by chromatographic methods.

This preliminary study, assisted by the sensitivity and accuracy of the PTR MS system, illustrates the potential of houseplants to act as indoor air cleaners for low-molecular-weight aldehydes and ketones. Further experiments will be needed using different houseplants and a wider range of typical indoor pollutants to examine the removal mechanisms and see if particular plants performed better than others before assessing the viability of a plant-based system.

• Environmental Science and Technology 2009, 43, 8338-8343: "Uptake of aldehydes and ketones at typical indoor concentrations by houseplants"

Article by Steve Down 

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.