Role of trees in protecting environment, helping animals to survive by releasing oxygen into the air and providing shade to keep earth surface cooler is well-known. These are too familiar truths to repeat. But humans often forget some familiar truths because that is expedient. Indiscriminate cutting down of trees to create concrete jungles for the great cause of urbanisation, which is a synonym for development, is driven by a madness born of the philosophy of controlling nature. And Dhaka, the capital of Bangladesh, though a latecomer in the race for urbanisation has about destroyed the canopy of green top cover that the city was once famous for. Left with a few patches of greenery like the National Botanical Garden, Ramna Park, Suhrawardy Uddyan etc., of late, Dhaka's position as a megacity with its air beating its other global competitors in pollution level is often the stuff of the news. And the most alarming part of the news is that a component of the pollution, the particulate matter (PM), especially its finest varieties including those measuring 2.5 micrometres (PM2.5s) can cause huge damage to public health, if inhaled. That is because through bloodstream the PMs enter vital organs of the body like lungs, heart, lever, etc. But other than describing the threat those fine material particles that include organic, chemical and metallic ones pose to human health in detail, there was not much information in media reports on how to rid the air of the PMs. Spraying water droplets into the air can be a way of removing the PMs from the air.
Also, during the rains, particles suspended in the air fall on earth. But what about those particles in the air during the winter when construction activities start in full swing emiting enormous quantities of dust in the air? So, there should be a long-term strategy to keep the air clean from dust particles which include deadly PM2.5s. Here comes the use of trees in removing dust, especially PMs from the air.
But in what ways do trees reduce air pollution? According to the US National Park Service, trees improve air quality in three ways. First, they (trees) cool the surroundings that alter concentration of pollutants in the air. Second, trees in urban setting reduce energy consumption in buildings (for temperature control), which in turn reduces consumption of energy from polluting sources. Third, they directly remove pollutants from air.
In the case of gaseous pollutants, the minuscule pores on the leaves of a tree, called stomata, absorb gases containing toxic pollutants like sulphur dioxide, nitrogen dioxide, carbon monoxide and ozone. And then the gases are diffused within the inner surface of the leaves where they (the gases) are then broken down thereby doing away with their toxicity. And when it comes to the particulate matters, PMs, the waxy, hairy surfaces of tree leaves catch some of them (PMs) which are later washed away by rainwater into the drains.
According to a BBC report, the mayor of London in 2019 announced that 7000 trees would be planted before the end of the following year. Why? Because trees clean air from pollution. Likewise, French capital Paris, too, is planning to create an urban forest that will encompass its most iconic landmarks. China's Hebei province, which cradles Beijing, the country's capital, on the other hand, is working to make what it calls a 'green necklace' of plants to reduce air pollution. But does Dhaka have any such plans? Our city mayors should know.
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