Beneficial influences of different evergreen species on the air quality: pollution mitigation and carbon uptake.

Air pollutants and their effects on human health

In 2012, 51% of the worldwide population was living in cities and this percentage is expected to increase up to 60% by 2030 (Population Reference Bureau, 2012). Urban ecosystem is strongly unbalanced in terms of energy and matter consumption, and production of pollutant emissions and carbon dioxide (CO₂). CO₂, although not considered a pollutant, is the main greenhouse gas, responsible of the greenhouse effect with the consequent increase of air temperature (Kiran and Kinnary, 2011). Air pollution continues to pose a significant threat to health worldwide. The negative effects on human health are mainly related to the respiratory and circulatory systems (WHO, 2006; COMEAP, 2010; EPA, 2011) and they were estimated to cause more than 3.7 million deaths per year (WHO, 2012). Traffic, building heating and industrial activities are the main sources of air pollution, releasing in the atmosphere high quantities of particulate matter (PM), (Suzuki, 2006; Oliva and Espinosa, 2007; Sawidis et al., 2011). Particulate matter is a mixture of solid and liquid substances, heavy metals, black carbon, polycyclic aromatic hydrocarbons (PAH) and other substances suspended in the atmosphere (Bell et al., 2011). Usually, PM is classified based on the diameter of the particles as PM10 or coarse (2.5– 10 μm), PM2.5 or fine (0.1–2.5 μm), and PM0.1-0.2 or ultrafine (≤0.1 μm) (Beckett et al., 1998). This classification is based on the different effects on human health, which are caused by different size particles . In particular, PM10 is also defined “thoracic fraction” because it is able to penetrate until the first part of the respiratory system, whereas PM2.5 presents a higher hazard because it can reach the deeper parts of the respiratory system as the non-ciliated ones (Di Menno di Buccianico et al., 2006). Particles are generated from primary or secondary sources. Primary sources are the incomplete combustion of fuel, mining, quarrying, brake and tire wear in motors and natural dusts. Combustion produces mainly PM2.5 whilst the mechanical processes generally release larger particles (PM10). Natural sources as volcanic eruptions, wind‑blown dust, sea salt and biological particles such as pollen and fungal spores are also considered as primary sources. These particles often exceed 2.5 µm in size. On the other hand, chemical reactions, gas condensation, and sulphate and nitrate aerosols are responsible of secondary particles formation in the atmosphere. Particles from these sources are usually smaller than 2.5 µm (Bealey et al., 2007). Constituents and sources of PM10 and PM2.5 are reported in Table 1 and 2.

http://hdl.handle.net/2067/2833