Plants: Natural Air Purifiers?

Plants can help improve air quality and reduce air pollution. They can absorb carbon dioxide and release oxygen through photosynthesis, increase humidity by transpiring water vapour, and passively absorb pollutants on the external surfaces of leaves and the plant root-soil system. A 1989 NASA study found that plants can clean the air in a closed, limited environment. However, the effectiveness of plants in reducing air pollution depends on various factors such as species, types of soil, lighting, temperature, and size. In some cases, it may be more effective to reduce emissions of pollutants rather than relying solely on plants for air purification. Nevertheless, plants can play a role in improving air quality, especially in urban areas, and integrating them with smart sensor networks and other technologies can create more sustainable and cost-effective solutions.

Plants can reduce indoor air pollution

The idea that plants can improve indoor air quality can be traced back to a 1989 NASA study that found plants cleaned the air in a sealed, limited environment. The study also found that plants absorbed carbon dioxide and removed volatile organic compounds (VOCs) such as formaldehyde and benzene, which are known to cause respiratory problems and cancer. However, it is important to note that indoor environments are very different from space stations, and the sample sizes used in the NASA study were small, so the findings may not be applicable to real-world settings.

Subsequent studies have had mixed results. Some studies have found that plants can reduce pollutants, specifically VOCs, in the air. However, other studies have found that plants do not improve air quality. One challenge in studying the impact of plants on indoor air quality is controlling and measuring the air exchange rate, as it is difficult to determine whether the reduction in VOC concentrations is due to the plants or air exchange.

While the evidence on the effectiveness of plants in reducing indoor air pollution is inconclusive, plants can still be beneficial in other ways. They can reduce stress, increase work performance, and reduce symptoms of ill health. Additionally, plants can affect the moisture content of the air, which needs to be regulated to prevent mold growth.

To effectively reduce indoor air pollution, it is recommended to focus on source control and ventilation. Source control involves reducing or eliminating the use of products that contain high levels of VOCs, such as cleaning products, air fresheners, glues, permanent markers, cosmetics, and paints. Ventilation can be improved by opening doors and windows to bring in fresh outdoor air and dilute polluted indoor air.

Trees can reduce outdoor air pollution

Trees can play a vital role in directly removing pollutants from the air. They can remove air pollution through the interception of particulate matter on plant surfaces and the absorption of gaseous pollutants through the leaf stomata. Particulate matter is temporarily caught by trees and other plants, which filter the air between the street and your front door. A study from Lancaster University found that having trees between the road and a building's front door can reduce indoor particulate matter by half.

Trees can also improve air quality in indirect ways. They can help by shading surfaces and reducing temperatures. If buildings are shaded by trees, it reduces the need for conventional air conditioning, and the emissions of greenhouse gases that come with it. Plus, lower temperatures decrease the risk of harmful pollutants like ground-level ozone, which commonly spike on hot days in urban areas.

Trees have a remarkable range of traits that can help reduce urban air pollution. Urban forests can remove multiple tons of ozone, gaseous air pollution, and particulate matter each year either through direct uptake of gases or temporarily intercepting airborne particles. According to the USDA Forest Service, the combined positive impacts of an urban forest lead to a net reduction in urban ozone formation.

However, it's important to note that the effectiveness of trees in reducing air pollution depends on the context. The species, size, and structure of the tree, as well as local and environmental factors, all play a role in how well a tree can filter the air.

Phytoremediation technology can be used to clean air

Plants are natural air purifiers and can be used to clean the air in and around our homes. They can also clean toxic soil. In a 1989 NASA study, scientists discovered that plants absorb air pollutants while taking in carbon dioxide and releasing oxygen during photosynthesis.

Phytoremediation is an emerging technology that uses plants to clean the environment. It is a more cost-effective, environmentally friendly, and sustainable alternative to traditional remediation practices. Phytoremediation can be applied to clean air, soil, and water. It has been used successfully in the restoration of abandoned metal mine workings, dumping sites of polychlorinated biphenyls, and coal mine discharge sites.

Plants can extract, accumulate, and depollute the substrate (soil, air, and water) from contaminants through physical, chemical, or biological processes. Phytoextraction, a type of phytoremediation, has gained popularity over the last two decades, especially for heavy metals and other inorganic contaminants. The process involves growing plants in contaminated sites or providing them with contaminated water in a greenhouse. After harvest, a lower level of the contaminant remains in the soil, and the growth/harvest cycle may need to be repeated for effective cleanup.

Phytoremediation technology can be used to clean the air by employing specific plants that absorb pollutants. For example, the alligator flag (Thalia dealbata) is an effective phytostabilization plant for cadmium (Cd) cleanup. Cd levels in rice grains were significantly reduced when rice was intercropped with the alligator flag, as the fine roots of the flag absorbed Cd from the rice's rhizosphere soil, decreasing the rice's Cd intake.

In addition to phytostabilization plants, trees play a vital role in directly removing pollutants from the air. Urban trees can help reduce air pollution in cities worldwide, including New York and Beijing. Trees can improve air quality directly by removing pollutants and indirectly by shading surfaces and reducing temperatures, which lowers the need for air conditioning and the associated greenhouse gas emissions.

Plants can clean toxic soil

Plants can do more than just beautify our surroundings; they can also clean toxic soil. Phytoremediation plants, for instance, can reduce, degrade, or remove toxic residues from the soil. This natural process is progressive and sustainable, minimising the need for heavy machinery or additional contaminants.

Plants absorb and use nutrients from the soil, and this includes the uptake of toxins. Researchers have discovered that certain plants can absorb specific toxins without sustaining damage. For instance, sunflowers were used to absorb radiation at the site of the Chernobyl nuclear disaster. Mustard greens can absorb lead, and have been used on playgrounds in Boston to protect children from this toxin. Willow trees are excellent absorbers of heavy metals, storing them in their roots. Poplar trees can absorb water contaminated with hydrocarbons, as well as carbon tetrachloride, a well-known carcinogen. Other phytoremediation plants include alfalfa, corn, date palms, and certain types of grasses.

The process of using phytoremediation plants to clean contaminated soil is simple. The plants absorb the toxic material, storing it in their roots or other parts of the plant. Once the toxins are locked in, the plants can be burned, leaving behind a light and small ash that is easy to store. This method is particularly effective for toxic metals, which are not burned away when the plant is turned to ash.

While plants can be an effective and low-cost solution for cleaning toxic soil, it is important to note that not all plants can tolerate all toxic materials. The specific plant must be able to withstand the toxin it is absorbing. Therefore, it is crucial to select the appropriate plant species for the type of contamination present in the soil.

Plants can remove harmful gases

Plants can play a significant role in removing harmful gases and improving air quality. This process is known as phytoremediation, a potentially effective and low-cost method for addressing air pollution.

A 1989 NASA study revealed that during photosynthesis, when plants "inhale" carbon dioxide and "exhale" oxygen, they also absorb air pollutants. These include formaldehyde, benzene, trichloroethylene, and carbon monoxide. Certain plants are particularly effective at removing specific toxins. For example, the Areca palm, also known as the "Butterfly Palm," excels at eliminating xylene and toluene, while the Lady Palm is adept at reducing formaldehyde levels. Spider plants are highly efficient at removing formaldehyde molecules and carbon monoxide, with the ability to eliminate 96% of carbon monoxide in a closed chamber within 24 hours.

Outdoor trees and plants also contribute significantly to reducing indoor air pollution. They act as a natural barrier, filtering particulate matter that may enter homes through open windows or on the soles of shoes. A study from Lancaster University found that trees between the street and a building's entrance can reduce indoor particulate matter by half. Coniferous evergreens, like pines, are particularly effective at reducing particulate matter due to their year-round foliage.

In addition to direct pollutant removal, trees provide indirect benefits. They shade surfaces, reducing temperatures and the need for air conditioning, thereby lowering greenhouse gas emissions. Lower temperatures also decrease the risk of ground-level ozone, a harmful pollutant that often spikes in hot weather.

While plants can effectively remove harmful gases, it is important to note that their ability to control indoor air pollution is less well-established. The type of plant, its placement, and the context of its surroundings are crucial factors in determining its effectiveness.

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