Air pollution is a considerable environmental hazard in urban and sub-urban areas that affects human health mainly through the respiratory and cardiovascular systems, leading to increased hospitalisations and deaths worldwide. Exposure to high levels of air pollution can cause a variety of adverse health outcomes and increase the risk of respiratory infections, heart disease, and lung cancer. Air pollution has caused 22–53 percent of global deaths from cardiovascular-related diseases and lung cancer.
Figure shows the cycle of airborne PM. A: airborne PM is comprised of dust from ground activities, biomass from ground-level combustion, and the emissions originated from underground fossil-fuel; B: Urban vegetation as well as urban buildings intercept and retain the airborne PM; C: PM captured by plants as well as building surfaces resuspended by winds, or washed off by rainfall; D, PM back to the ground and deposited into the soil.
CAUSES OF AIR POLLUTION
In urban areas, air pollution is mainly caused by road traffic emissions, construction, rapid urbanisation, industrialisation, and open burning. Air pollution is characterised by particulate matter (PM) and various toxic gases, including nitrogen oxides, carbon monoxide, sulphur dioxide, volatile organic compounds, and ozone. Particulate matter is of greater concern due to its serious negative effects on human health than those of other air pollutants. PM consists of several solid or liquid particles that suspend in the atmosphere. The most health-harmful pollutants—closely associated with excessive premature mortality—are fine PM2.5 particles that penetrate deep into lung passageways. Due to rapid development in urban and sub-urban areas, PM is being continuously released into the urban atmosphere, and there are, in fact, little or no measures to control air pollution.
AIR POLLUTION IN MAJOR CITIES IN BANGLADESH AND HUMAN HEALTH
In Bangladesh, industrialisation and urbanisation focusing on rapid economic growth have resulted in massive environmental costs, which have aggravated the outlook for sustainable economic development. Bangladesh is the world’s most polluted country, where the annual average particulate pollution (PM2.5) level exceeds both the World Health Organisation guideline of 5 µg/m³ and the country’s own national standard of 15 µg/m³. Even in the least polluted district of Sylhet, particulate pollution is 9.7 times the WHO PM2.5 guideline and 3.2 times the national standard. Dhaka, Chattogram, Narayanganj, and Khulna are the major cities in Bangladesh that have the highest PM concentrations among some cities in the world. The main sources of fine PM (PM2.5) and coarse PM (PM10) are brick kilns, motor vehicles, construction activities, and road transport. Air pollution is one of the most important risk factors for mortality (123,000 deaths in 2017) in Bangladesh, of which more than 47,000 deaths have been attributed to exposure to outdoor PM2.5. Particulate pollution is the second-greatest threat to human health in Bangladesh (closely following cardiovascular diseases), taking 6.8 years off the life of the average Bangladeshi.
AIR POLLUTANTS REMOVED BY VEGETATION
Research found that urban trees and vegetation have the capacity to reduce atmospheric PM concentration by depositing on leaves, while the physical properties of the tree cover and planting pattern in urban areas may improve the microclimate of cities. Urban vegetation can reduce 40 percent of NO2 and 60 percent of PM concentrations in street canyons. Research in Australia reported that trees were found to remove a higher amount of air pollutants compared to the green wall and green roofs in city areas, and areas with less greenspace coverage were found to have higher concentrations of PM than the areas with high greenspace coverage, even taking vehicular traffic into account. Deposition of PM on leaves depends on quantity and sources, tree leaf morphology, leaf orientation, weather conditions, and wind speed. In the winter season, when rainfall is limited, the concentration of PM is usually high. The surface and shape of the leaves also affect the PM deposition on them. Oblong and ovate-shaped leaves, which are widest in the middle, accumulated the highest amount of PM. On the other hand, obovate-shaped leaves, which are the slenderest below the middle, accumulated the least amount of PM. Our research in Chattogram City found that the hairy, rough-surfaced leaves of Psidium guajava (guava) and Tectona grandis (segun) deposit a higher amount of PM because they can intercept more PM than leaves with a smooth surface. We also documented that roadside had lower relative greenspace but the highest concentration of atmospheric PM, which was up to 56% higher than that of residential, park, and commercial areas where relative greenspaces were higher. Our results show that tree species such as Psidium guajava (guava), Artocarpus heterophyllus (kanthal), Swietenia mahagoni (mahagoni), Tectona grandis (segun), and Mangifera indica (aam) deposit a higher amount of PM on leaves, and PM deposition was higher at lower tree heights.
Rapid urbanisation, particularly in developing countries, coupled with infrastructure development and industrialization has had a serious impact on urban air quality, and ambient air quality has been causing many life-threatening diseases. Therefore, it is essential and sensible to find out economically and ecologically feasible and effective ways that can remove atmospheric PM and protect humans from exposure to polluted air. Research has shown that urban trees can remove PM through deposition on leaves. The concentration and deposition of PM on leaves depend on leaf surface, shape, and tree height. Therefore, for better removal of PM, planting trees with oblong, ovate-shaped, and rough-surfaced leaves that are lower in height is recommended because these attributes of trees have a higher potential for deposition of PM. The expansion of urban greenspaces would be a low-cost solution to reduce PM pollution, and these urban greenspaces have many other health and ecosystem services for sustainable cities.
[Tapan Kumar Nath is a professor at the School of Environmental and Geographical Sciences, University of Nottingham Malaysia.
Tarit Kumar Baul is a professor at the Institute of Forestry and Environmental Sciences, University of Chittagong.]