Particulate Matter (PM) is a class of dangerous airborne compounds composed of a mixture of natural sources and the byproduct of combustion engines, like cars, airplanes and industrial activity.
PM is the most harmful form of air pollution, as they are very small and can penetrate deeply into the lungs, bloodstream and brain. When its particles are very small (< 10 thousandth of a millimeter), they can cause serious issues to the respiratory and cardiovascular system.
Trees act as air purifiers, removing these pollutants from the air by intercepting its particles that are retained on the plant’s leaves surface and not inhaled by us.
Carbon Dioxide (CO₂) is a natural gas produced by natural sources, like volcanoes and all air-breathing animals in the sea and on land, like humans. CO₂ is also produced by decaying organisms and fermentation.
However, CO₂ is also produced by the combustion of wood and fossil fuels such as coal, petroleum and natural gas. In nature, the amount of carbon dioxide varies in a dynamic equilibrium with photosynthesis of land plants, but an increase in atmospheric carbon dioxide means an increase in global temperature, that is global warming. The atmospheric concentration of CO₂ has increased by about 50% between the 1800 and 2020, since the beginning of the industrialization age.
Plants help reduce the concentration of carbon dioxide in the atmosphere, absorbing CO₂ and transforming it into nutrients for growing. CO₂ is also “converted” in plant biomass or stored in the soil (carbon sink): removing it from the atmosphere helps reduce global warming and climate change. However, not all the CO₂ absorbed by plants is stored in the plant and removed from the atmosphere: fruits, flowers, leaves and fallen branches are decomposed or digested by other living organisms and the CO₂ produced by these processes returns to the atmosphere.
Under their shade and in the surrounding areas, trees and vegetation act as natural “air conditioners”. Trees therefore contribute to decrease the urban “heat island” effect, in which city areas are significantly warmer than the rural areas around. They absorb a significant proportion of the available heat energy in the atmosphere and use this energy to convert water stored in the leaves into water vapour, which is then transpired through the trees (evapotranspiration).
The results can lead to a mean temperature reduction of 2.5°C in buildings surrounded by trees and a maximum reduction of 0.9/1.5 °C on tree-lined streets. This means that in buildings surrounded by trees the power consumption to cool indoor environments can be diminished up to 30-50% during summer, and also around 20-50% for heating in winter.