Plants' Resilience: Surviving Air Pollution

Plants are the first to be affected by air pollution as they are fixed in place and are the basis for the functioning of ecosystems. While plants are known to improve air quality by absorbing carbon dioxide and releasing oxygen through photosynthesis, air pollution can cause oxidative stress in plants, leading to the production of free radicals and reactive oxygen species that can damage cells. The impact of air pollution on plants varies across regions and can have consequences for humans, especially through the consumption of contaminated plant products.

Plants absorb carbon dioxide and release oxygen

Plants are at the forefront of air pollution as they are fixed in place and are the basis for the functioning of ecosystems. They react to air pollution by producing reactive oxygen derivatives. While plants are absorbing more carbon dioxide and helping to fight climate change, they can only sequester a fraction of human emissions.

During the day, plants are both respiring and photosynthesising, so oxygen and carbon dioxide are diffusing in and out of the leaves. But overnight, without sunlight, photosynthesis stops and only respiration takes place. Therefore, only oxygen diffuses into the leaves and only carbon dioxide diffuses out. The amount of oxygen that plants absorb overnight is too small to have any impact on humans.

Most plants release only one gas at a time, but there are exceptions. For example, cacti, bromeliads, and certain succulents rely on an alternative photosynthetic pathway called crassulacean acid metabolism, or CAM, which allows them to keep their leaf stomata closed during the day and open at night.

Plants increase humidity by transpiring water vapour

Plants play a crucial role in the functioning of ecosystems, and they are at the forefront of air pollution. They are also a source of many food chains. Therefore, it is important to understand how plants survive air pollution and the role they play in increasing humidity through transpiration.

Transpiration is the physiological loss of water in the form of water vapour, mainly from the stomata in leaves, but also through evaporation from the surfaces of leaves, flowers, and stems. The stomata are the pores in the leaf that allow gas exchange, where water vapour leaves the plant and carbon dioxide enters. The rate of transpiration is influenced by various factors, including the thickness of the cuticle layer on the leaf surface, leaf size, environmental conditions, and relative humidity.

Relative humidity (RH) is a critical factor in determining transpiration rates. RH is the amount of water vapour in the air compared to the maximum amount of water vapour that the air can hold at a given temperature. When RH is high, the atmosphere contains more moisture, reducing the driving force for transpiration. Conversely, when RH is low, the atmosphere is drier, and the driving force for transpiration increases. Temperature also plays a significant role, as warmer air can hold more water, leading to increased transpiration rates.

Plants can regulate their transpiration rates through various mechanisms. For example, they can open or close their stomata to control the exchange of gases. When stomata are open, transpiration rates increase, while closing them leads to a decrease in transpiration. Additionally, plants can alter the size of their boundary layers, which are thin layers of still air hugging the leaf surface. A larger boundary layer slows down the diffusion of water vapour, reducing transpiration rates. Plants from hot, dry climates tend to have thicker cuticles, which act as a barrier to water movement, resulting in lower transpiration rates.

By transpiring water vapour, plants increase the humidity in their surroundings. This process is essential for maintaining plant water balance and facilitating the uptake of nutrients. Optimal transpiration rates vary by plant type, age, and season, making climate control a crucial aspect of plant growth and management. While transpiration is necessary for plant survival, excessive water loss can lead to dehydration, impacting their productivity and yield. Therefore, growers often use climate control strategies, such as plastic tents or propagation chambers, to maintain optimal relative humidity levels and ensure proper air circulation for healthy plant growth.

Plants passively absorb pollutants on external surfaces

Plants passively absorb pollutants on their external leaf surfaces, improving air quality. The leaves of plants act as biofilters, trapping pollutants such as particulate matter (PMs) and gaseous toxins like nitrogen oxides, ammonia, and sulfur dioxide. The leaf surface is filled with trichomes, which are epidermal outgrowths in various forms. The physical properties of trichomes play a role in mechanical defence, while the secretion of secondary metabolites contributes to biochemical defence.

The waxy cuticle on the upper surface of leaves also aids in pollutant absorption, acting as the first barrier against xenobiotics. The composition and amount of surface waxes on leaves vary among plant species, influencing their capacity to retain and absorb pollutants. For example, a study by Sbø et al. found that the Mediterranean Hackberry tree, which can grow 25-30 metres relatively quickly, is effective at absorbing carbon dioxide.

In addition to leaves, the root-soil system of plants can also passively absorb pollutants. This absorption occurs through the plant's root-soil interface, enhancing the removal of harmful substances from the air. The specific characteristics of the root-soil system, such as its morphology and physiology, influence its ability to absorb and process pollutants effectively.

While plants are known to passively absorb pollutants, it is important to note that they can also be negatively affected by air pollution. Air pollution can induce oxidative stress in plants, leading to the production of free radicals and reactive oxygen species (ROS) that can cause damage at the cellular level. Additionally, certain pollutants, such as hydrofluoric acid (HF), can disrupt the cellular metabolism of calcium, resulting in the precipitation of calcium in the form of CaF2.

Plants can reduce indoor air pollution

The idea that plants can reduce indoor air pollution is a topic of ongoing scientific debate. While some studies, including a 1989 NASA experiment, have found that plants can indeed reduce indoor air pollution, other research suggests that the effects of plants on indoor air quality are negligible.

The NASA study, conducted to research ways to clean the air in space stations, found that plants could absorb carbon dioxide and remove volatile organic compounds (VOCs) such as formaldehyde and benzene. Formaldehyde, for instance, is a known volatile organic compound with a long history of health impacts. These compounds are linked to several health issues, including respiratory problems and cancer. However, it is important to note that the NASA study was performed in a sealed, controlled environment, and real-world settings may not yield the same results.

Subsequent studies have faced challenges, including inaccurate measuring equipment and difficulties controlling or measuring the air exchange rate. The small sample sizes used in many studies, including the NASA study, may also contribute to the difficulty of translating the findings to real-world settings.

While plants may have a limited impact on reducing indoor air pollution, they are not a cure-all solution. Overwatering plants, for example, can lead to mould growth, which can trigger allergies and asthma symptoms. Additionally, dust can accumulate on leaves and distribute into the air, causing respiratory issues and exacerbating existing conditions such as asthma and allergies. Certain plants with flowers can also release pollen, spores, or fragrances that can cause allergic reactions in some individuals.

To effectively reduce indoor air pollution, it is essential to address the source of the pollution and increase ventilation. This can be achieved by opening doors and windows to allow fresh air to circulate, reducing the use of products that contain high levels of VOCs, and ensuring proper drainage and air circulation for plants to prevent mould growth.

Planting near schools can protect children from air pollution

Plants are at the forefront of air pollution as they are fixed in place and are the basis for the functioning of ecosystems. They react to air pollution by producing reactive oxygen derivatives, which can cause damage at the cell level. However, they can also help to reduce air pollution and protect humans from its negative health effects.

Planting Near Schools to Protect Children from Air Pollution

Planting vegetation near schools can protect children from air pollution, particularly from traffic-related air pollution, which has been linked to a range of health risks, including asthma attacks, reduced lung function, and cardiovascular, respiratory, and neurological issues. A study conducted by Professor Barbara Maher of Lancaster University found that planting "tredges" (trees that serve as head-height hedges) around school playgrounds can significantly reduce air pollution. The study, conducted in Manchester during the 2019 summer break, showed that the most effective species for reducing air pollution was the western red cedar, which captured around 46% and 26% of fine particulates, PM2.5 and PM1, emitted by passing traffic.

The benefits of such planting are not limited to schools, as green infrastructure has been shown to be effective in reducing air pollution in residential areas, along highways, and near industrial sites. It can also help to improve soil quality, prevent erosion, and provide habitats for local wildlife, creating more pleasant environments that promote mental and physical health.

However, it is important to note that reducing exposure to traffic-related air pollution can be complex due to cost and location constraints. While vegetative buffers offer a potential solution by physically blocking and filtering polluted air, other options such as refitting HVAC systems or building solid noise barriers may be too costly or impractical for schools. Nevertheless, with rising concerns over climate change and the need for sustainable practices, integrating natural elements into urban planning can create more resilient and sustainable communities that benefit both people and the environment.

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