Introduction

Can growing houseplants really help turn our homes, schools and workplaces into better places to be? It is a question that numerous scientific studies have explored and results are now shedding light on the matter. Indoor plants offer two potential benefits for us: improved psychological (mental) well-being and improved physical human health (i.e. they support fitness and general health).

The psychological benefits of indoor plants have been shown as:

• An improved mood
• Reduced stress levels
• Increased worker productivity (adding plants to office environments in particular)
• Increased speed of reaction in a computer task
• Improved attention span (in some scientific studies, but not all)
• Increased pain tolerance (for example, where plants were used in hospital settings)

The physical health benefits of indoor plants have been shown as:

• Reduced blood pressure
• Reduced fatigue and headaches by 20-25 percent in one study
• Patients in hospital rooms with plants reported decreased post-operative pain

It is worth noting that the effect of plant species and cultivar differences has not been specifically investigated.

Are there any side effects for occupants to being exposed to indoor plants? Encouragingly, the presence of plants had very few negative effects when studies – i.e. very low level of reported skin or respiratory irritation. Also night-time release of carbon dioxide by houseplants is unlikely to be a problem, as emission levels are very low. 

Indoor plants and air quality

Data suggest that every year over 4 million people worldwide die prematurely due to indoor air pollution. Pollution levels are often higher indoors than outdoors as indoor air represents a mix of:

• Outdoor-derived compounds such as nitrous oxides, carbon monoxide and carbon dioxide and particulates (dust-like particles)
• Indoor-derived contaminants, predominantly Volatile Organic Compounds (VOC). VOCs are a large group of substances (including toluene, xylene, benzene etc.) which are emitted from furnishings, detergents, paints etc. and can have adverse health effects on humans. Additionally bio-aerosols (i.e. fungal spores and bacteria) can add to indoor pollution

All containments contribute to the so called Sick Building Syndrome (SBS). Symptoms associated with SBS include: eye, nose and throat irritation; headaches; fatigue and irritability; chest tightness and wheezing; and skin dryness / irritation.

Opening the windows and naturally ventilating our indoor spaces can remedy some of these problems. However, during winter months VOC concentrations indoors have been found to be at their highest, because air exchange rates are reduced (i.e. the windows/doors aren’t opened so much). People also spend more time inside during colder weather outdoors, therefore exposure is raised. 

Plant species and cultivars differ in the rate of removal of these chemical compounds. Recent research also suggests significant numbers of plants may be needed as well as additional light to get this benefit. Likewise, growing media used for plants can also significantly affect the rate of VOC uptake, with soil-based systems typically being superior to hydroponics.

How do I get the benefits from plants?

Based on the research, several approaches should be considered to get the benefits.

1. Rooms which look out on nature (including parks, gardens and wild spaces) have a head start as seeing this can provide psychological support.
2. Rooms with views of largely urban landscapes, would benefit most from including plants.
3. Regardless of the type of indoor space, including the right plants has the potential to boost health benefits.

Which plants to choose?

There is an ongoing discussion about the types and numbers of plants needed in homes to achieve health benefits. However, the greater the number of plants used, the more likely the benefit to air quality and overall well-being. 

Below is a list of easy to grow foliage houseplants, which could be grown in homes, schools and offices. They have attractive leaves and an ability to withstand the environmental conditions typically encountered indoors (tolerance of shade and fluctuating temperatures). In addition, they are usually inexpensive and easy to maintain.

Indoor foliage plants and the VOCs they have the potential to reduce

(data summarised from the review article by Dela Cruz et al., 2014) 

Benzene and formaldehyde (to varying degrees)
Chlorophytum comosum (spider plant)
Dracaena fragrans ‘Janet Craig’ (dragon plant)
Dracaena marginata (v) AGM (Madagascar dragon tree)
Epipremnum aureum AGM
Ficus elastica (India rubber tree, rubber plant)
Hedera helix (English ivy, common ivy)
Nephrolepis exaltata ‘Bostoniensis’ (Boston fern)
Sansevieria trifasciata (mother-in-law’s tongue)
Syngonium podophyllum AGM (Nephthytis)
Zamioculcas zamiifolia (fern arum)

Formaldehyde (to varying degrees)
Aglaonema ‘Silver Queen’ AGM (Chinese evergreen ‘Silver Queen’)
Rhapis excelsa AGM (bamboo palm, lady palm)
Spathiphyllum sp. (peace lily)

Benzene (to varying degrees)
Aspidistra elatior AGM (common aspidistra, cast iron plant)
Chamaedorea seifrizii (bamboo palm)
Crassula ovata (syn. Crassula portulacea) AGM (jade plant, jade tree)
Dieffenbachia ‘Tropic Snow’ (v) AGM (dumb cane)
Howea forsteriana AGM (flat palm, Kentia palm)

Limitations of our knowledge

It is worth noting that there are issues with interpreting this wide range of scientific studies for real-life-situations. The fact that they are conducted under different experimental conditions, and that the results of VOCs removal are expressed in differing terms, make direct comparisons a challenge.

The reported rates of removal even within species/cultivar vary greatly, and will depend on the starting concentrations of measured chemicals, environmental conditions within the experimental space etc. Further research is essential in order to replicate typical home and office environments to assess plants’ true impact on the quality of air indoors. Growing media, temperature and light intensity all have an effect on the rate and efficiency of VOCs removal. In a real-life setting, air exchange will vary compared to research undertaken in sealed chambers.

In addition, human well-being is a complex issue. Scientific research is beginning to evaluate the effect plants have on different aspects of this subject. Types of plants, their arrangement and factors such as scent and colour are also being investigated.