Gardens: Natural Air Purifiers Or Just Another Pretty Space?

Air pollution is a pressing issue, causing serious health problems and environmental degradation. As people spend more time indoors, the air they breathe is often a mix of outdoor and indoor-derived compounds, including carbon dioxide, dust particles, and toxins emitted from paints and detergents. Luckily, plants can help combat this issue. Through photosynthesis, plants absorb carbon dioxide and release oxygen, improving air quality. They also increase humidity by transpiring water vapour through microscopic leaf pores and passively absorb pollutants on leaf surfaces and root systems. While all plants absorb carbon dioxide, some are better at filtering toxins and particulate matter, such as trees, which are very effective at removing microscopic dust, chemicals, and other toxic particles.

Plants absorb carbon dioxide and release oxygen

Plants are an essential part of our planet's ecosystem, and their role in absorbing carbon dioxide and releasing oxygen is vital for maintaining air quality. This process, known as photosynthesis, is a superhero act that keeps the air we breathe healthy.

Plants, like all living things, need food to survive. However, they can't just go to the store and buy their ingredients. Instead, they make their food through photosynthesis. This process involves plants taking in water from the soil through their roots and absorbing carbon dioxide, a gas found in the air, through tiny holes in their leaves. With these two ingredients and sunlight, plants can make their food. The leftover by-product of this process is oxygen, which is released from the leaves into the air.

This oxygen is crucial for humans and other animals to breathe and survive. When we exhale, we release carbon dioxide, which plants then absorb and use to start the cycle all over again. This exchange of gases is known as the oxygen cycle. It is a delicate balance that ensures the well-being of all living organisms on Earth.

Trees, in particular, are champions of this process. Through photosynthesis, they pull in carbon dioxide and water, using the sun's energy to convert them into food for themselves. As a result of this chemical reaction, they produce and release oxygen, providing a day's supply of oxygen for up to four people. Additionally, trees store carbon dioxide in their fibers, helping to clean the air and mitigate the negative effects of this greenhouse gas on our environment. According to the Arbor Day Foundation, a mature tree can absorb more than 48 pounds of carbon dioxide from the atmosphere in a year while releasing oxygen in return.

While most plants release oxygen during the day when sunlight powers photosynthesis, there are exceptions. Certain plants, like cacti, bromeliads, and some succulents, rely on a different pathway called crassulacean acid metabolism (CAM). This mechanism allows them to keep their leaf stomata closed during the day, reducing water loss. As a result, these plants release oxygen at night when the stomata open and the oxygen can escape.

Plants passively absorb pollutants on leaf surfaces and root-soil systems

Plants passively absorb pollutants on their leaf surfaces and root-soil systems. The aerial surfaces of plants, particularly leaves, are estimated to sum up to 4 × 10^8 km^2 on Earth. These leaves are also home to up to 10^26 bacterial cells.

Leaves are the primary photosynthetic organs with distinctive upper and lower surfaces. The upper surface has a layer of waxy cover called cuticle, which prevents water evaporation and acts as the first barrier for the penetration of xenobiotics. The leaf surface is filled with trichomes, which play a role in mechanical defence due to their physical properties and also in biochemical defence due to the secretion of secondary metabolites.

The underside of leaves also has a layer of epidermal cells where most stomata are located. Stomata regulate the flow of gases in and out of leaves and can also adsorb or absorb other chemicals.

Plants absorb gaseous pollutants through their leaves, mainly via the stomata. Some of the volatile organic compounds (VOCs) are recognised as xenobiotics by plants and are detoxified through xenobiotic metabolism.

Plants can directly absorb formaldehyde and transform it into organic acids, sugars, carbon dioxide, and water. Formaldehyde-responsive genes have been identified from golden pothos (Epipremnum aureum). Glutathione-dependent formaldehyde dehydrogenase and formate dehydrogenase can detoxify formaldehyde to formate and further to carbon dioxide.

Plants can also absorb benzene, toluene, and xylene (BTX) mainly through the stomata. These compounds are converted to phenol or pyrocatechol and subsequently to muconic acid and fumaric acid.

In addition to leaves, the root-soil system also plays a crucial role in absorbing and accumulating pollutants. Scientific evidence associates prolonged prenatal exposure to polycyclic aromatic hydrocarbons (PAHs), a highly carcinogenic contaminant commonly found in soil, with low birth weight, premature delivery, heart malformations, lower IQ, and childhood asthma. Long-term exposure of an adult can cause damage to the lungs, kidneys, liver, and skin.

Trees are very effective at removing tiny particulate matter

Trees: Nature's Air Purifiers

Trees are nature's air purifiers. They are very effective at removing tiny particulate matter, which can be irritating to the lungs and can cause serious health issues. These particulates, often in the form of dust, chemicals, acid, and other toxic particles, come from factories, construction sites, and other pollution-emitting areas. Let's explore the role of trees in combating this issue.

The Power of Trees in Removing Particulate Matter

Trees act as a natural filter, trapping and reducing the presence of particulate matter in the air we breathe. Their leaves, with their unique structures, are key to this process. The shape and texture of tree leaves play a crucial role in capturing and retaining these harmful particles. Coniferous trees, with their needle-like leaves, are particularly effective due to their dense canopy structure. The rough and hairy surfaces of certain tree leaves, such as those found on silver birch, yew, and elder trees, act as "best filters," trapping up to 70% of particulates.

The Benefits of Urban Greening

The inclusion of trees in urban areas offers multiple advantages. They not only improve air quality by dispersing and removing pollutants but also provide shade, reducing the need for air conditioning and lowering greenhouse gas emissions. This, in turn, minimizes the production of harmful ground-level ozone pollutants. Additionally, trees contribute to the well-being of city dwellers, offering aesthetic appeal and mental health benefits.

The Right Tree for the Job

When it comes to selecting trees for air pollution reduction, it's important to consider the species and their specific traits. Conifers, such as pines and cypresses, are excellent choices due to their evergreen nature, providing year-round filtration. However, their sensitivity to salt levels in the soil and their potential to block sunlight in colder climates are considerations to keep in mind. Native tree species are also ideal as they are well-adapted to the local environment and more resistant to insects and diseases.

A Holistic Approach to Air Quality

While trees play a vital role in improving air quality, it's important to remember that they are just one part of the solution. Reducing emissions at the source is crucial, and urban planning must take into account factors like wind direction and landscape structure to maximize the benefits of trees in combating air pollution. Additionally, a diverse range of plant species, including shrubs and smaller plants, can contribute to a comprehensive approach to enhancing air quality and promoting biodiversity.

In conclusion, trees are nature's unsung heroes in the fight against air pollution, especially when it comes to removing tiny particulate matter. By understanding their unique abilities and incorporating them into our urban landscapes, we can create healthier and more sustainable environments for all.

Native plants are more manageable and resistant to insects and diseases

Plants are a great way to combat air pollution. They take in carbon dioxide and release clean oxygen, and they can also filter toxins and other particulate matter from the air.

Native plants are well-adapted to local conditions, including the local soil and weather, and they are also more resistant to many insects and diseases. Native plants have evolved alongside local pests and have developed ways to deter them. They often produce chemical compounds that make them unappealing or toxic to these pests. For example, milkweed produces latex sap that deters herbivores, and some oaks release tannins that inhibit the growth of insect larvae.

Native plants also have a coevolutionary relationship with local insects, leading to a delicate ecological balance. Some insects have become specialized feeders of specific native plants, and these plants have mechanisms to withstand the impact of herbivory. Native plants are also more likely to attract beneficial insects, such as ladybugs, lacewings, and ground beetles, which are natural enemies of pests and can help keep their populations under control.

In addition, native plants are better for biodiversity. They attract local wildlife, such as butterflies and birds, and provide food and habitat for insects, birds, and mammals. They can also create a more diverse ecosystem, which makes it more difficult for pests to establish themselves.

Native plants are a more manageable choice for gardeners as they require less management than traditional lawns or non-native plants. They are also less likely to upset the local environment if they escape from your garden, as they will meld into the existing ecosystem.

The Phyto-sensor toolkit details how to build an air quality garden

The Citizen Sense research group, led by Jennifer Gabrys from Goldsmiths, University of London, has developed the Phyto-sensor toolkit. The toolkit was tested and refined through a public workshop and walk held at the Museum of London in March 2018. It provides resources for creating air-quality gardens and monitoring air quality with low-cost sensors.

The toolkit includes instructions for identifying the best locations for air quality gardens, different planting strategies, and maintenance techniques. It also contains information on plants that are best equipped to combat harmful pollutants. These include plants that reduce particulate matter, such as the wallflower, and common ivy, which is ideal for air purification due to its extensive leaf surface area.

Some plants highlighted in the toolkit also combat nitrogen dioxide, including Alchemilla mollis, or Lady's Mantle, which has hairy leaves that trap harmful particles and can reduce nitrogen dioxide levels by up to 40%. Aster, which reveals high ozone levels by yellowing leaves, and Osmanthus, which displays chronic leaf damage when exposed to sulphur dioxide, are among the best bioindicating plants.

The toolkit also recommends planting particular trees to improve local air quality and the installation of green walls and screens as barriers to busy roads. It provides examples of successful community projects that have utilised these planting strategies.

• Location: Select the best location for your air quality garden to ensure maximum impact. Consider planting near schools, residential areas, hospitals, or parks to minimise the effects of air pollution on certain populations.
• Collaboration: Collaborate with existing community or gardening groups, or engage with volunteers and neighbours to work together on the garden.
• Resources: Research funding sources from local governments or councils to support the development of green spaces. Also, look into in-kind donations or donated supplies.
• Design and plant selection: Visit existing air quality gardens and planting schemes to get ideas for your own design. Consider the site requirements, emission sources, and planting strategies that best suit the context. Select plants that are suitable for the site conditions, such as trees, green screens, herbaceous plants, or shrubs.
• Implementation: Decide on the approach for building the garden, whether through volunteer efforts or hiring a contractor. Prepare the necessary tools and supplies, including compost, fertiliser, and irrigation systems.
• Launch event: Once the garden is established, host an event to bring together contributors and raise awareness about the benefits of the air quality garden.
• Maintenance: Develop a plan for ongoing maintenance, including watering, pruning, and weeding. Determine whether this will be a volunteer effort or handled by a contractor.

Frequently asked questions