How many pot plants do you have in your room? If it is less than five in a medium-sized 4mx5m room you may want to take a trip to the local nursery.

A new analysis of decades of past research suggests that if you add a few more plants you'll have enough to strip out the toxic organic compounds coming off your walls, carpets and furniture and could be making you feel sick. And even if the air is already clean, just looking at your plants can be enough to make you feel good, energised and more productive.

Researchers from the University of Melbourne and RMIT have crunched the data on just over 100 peer-reviewed research papers going back 50 years to establish some general guidelines on using plants to make our indoor spaces healthier and more satisfying. And with urbanites spending 90 per cent of their lives indoors it is clearly time to think harder about how we bring nature inside.

The research forms the basis of a new virtual reality app, Plant Life Balance, that provides guidelines and ideas for adding pot plants to your home by rating the implied air quality and wellbeing of your rooms based on the number of plants you have or don't have. It is part of a campaign by Horticulture Innovation Australia to promote the health benefits of plants.

"We needed to look across all the major research and synthesise that into a scale of the benefits that come from plants, both in terms of air quality and wellbeing," says Dr Dominque Hes, director of the Thrive Research Hub at the Melbourne School of Design, University of Melbourne. Dr Hes, along with RMIT's Associate Professor Marco Amati and Thrive researcher Cristina Hernàndez have driven the research as part of the Clean Air and Urban Landscapes Hub collaboration.

Based on their analysis and expert consultations they developed a rule-of-thumb that in order to remove indoor pollutants you need to have 1 medium sized plant per 2.2 square metres.

Research studies suggest that indoor plants are effective in reducing airborne chemicals and particles in indoor spaces by as much as 75-to-90 per cent, depending on the pollutant and the plant. These pollutants include Volatile Organic Compounds that are emitted by things like glues and modern furnishings that can cause maladies like headaches, nausea, irritation, sleeplessness or fatigue. These VOCs include toxins such as formaldehyde, benzene and toluene.

But surprisingly the survey of the research suggests that it is the plant root system, or more accurately the bacteria living in them, that do the heavy lifting in removing VOCs, rather than the leaves of the plant. These bacteria, called gram-negative bacteria, effectively eat the chemicals. The leaves help to absorb chemicals through their pores (stomata) and their waxy covering.

"In terms of removing chemicals from the air, 80 per cent of the reduction comes from these bacteria in the roots. So it means a bigger pot is better," says Ms Hernàndez.

Plants are also effective at filtering out small particle by absorbing or collecting the particles.

Research also suggests that just having enough plants around can enhance our wellbeing. Various studies have found that when we experience plants and nature, our concentration improves, we take less sick leave, we recover better from stress, we perform better at shape recognition tasks, and we are more likely to consider long-term goals. For instance, indoor plants within a school classroom have been associated with an 11-14 per cent improvement in student performance in maths and science performance.

There is more to be done on understanding how plants enhance our wellbeing, but some theories suggest that the patterns found in nature help to restore our attention and that we have an evolutionary connection to nature that relaxes us.

"The biophilia hypothesis is that we have an innate need to be connected to nature," says Dr Hes. "We evolved in nature so our brains are more relaxed when we are connected to nature and that is all connected to wellbeing. It's like the difference between how you are after a good night's sleep compared to having an okay but not great night."

To formulate equations to run the app, the researchers analysed 101 published studies on the effectiveness of plants in improving air quality and improving wellbeing. This information was then synthesized using statistical analysis to produce a scale of plant benefits to inform the equations.

A panel of five independent experts provided advice throughout the process to navigate the gaps in the existing evidence by using a structured consensus-making technique that was first developed by the US military during the Cold War. The Delphi process is a way to speed decision making in the absence of full information by having the experts consider assumptions, conclusions and each other's judgements.

Dr Hes says the role of the experts was particularly important because the gaps in the research meant that the final equations had to be simplified to make them workable within the app.

A key gap in the existing research is that not enough work has been done on determining how the characteristics of different species of plants influence their effectiveness in improving air quality and enhancing wellbeing. As a result, the equation used in the app is simplified by using the number of generic plants as a proxy for effectiveness on the basis that larger plants will have more stomata and root organisms to metabolise toxins, and have larger surface areas to capture particulates.

The other big gap in the existing research is little of testing has been done in actual homes. Instead it has focused on laboratories, schools, offices and hospitals.

"The app is based on the existing research, but because there are large gaps in the research it is only giving us about 20-30 per cent of the full picture," says Dr Hes. "We will only get the full picture if all 200 plants in the app were uniformly researched and tested in home settings, but that would cost millions of dollars and years of work."

But such work can be done over time and the researchers are hoping that their analysis will encourage more research into the relative effectiveness of plant species in making our indoor spaces healthier.

"This is really a shout out to other researchers to look into how we can unify the different approaches for testing plant types so that we can close these gaps in our knowledge and create profiles of different plant species," says Ms Hernàndez.

Indeed, Ms Hernàndez is planning to do a trial on herself to try and test out how best to structure a larger research project using air sensors and possibly keeping some form of wellbeing diary. And for her, it is a personal mission. Having grown up in the outskirts of Mexico City in a house surrounded by trees, Ms Hernàndez now lives in an apartment in Melbourne.

"In Mexico, I was surrounded by nature, and now living in an apartment I really miss not being to simply look anywhere and see greenery."

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