What is Air Pollution
According to WHO, air pollution is contamination of the indoor or outdoor environment by any chemical, physical or biological agent that modifies the natural characteristics of the atmosphere. It can harm the health of humans, animals and plants and damage the buildings as well. Some common sources of air pollution include motor vehicles, industrial facilities, forest fires and so on. Pollutants of major public health concern include particulate matter, carbon monoxide, nitrogen dioxide, sulfur dioxide and ozone. Outdoor and indoor pollution cause respiratory and other diseases and are important sources of morbidity and mortality.
Role of trees
Degrading air quality in urban areas is a matter of concern for public health. Massive tree plantations are usually done in order to provide oxygen to choking cities. However, while designing green belts in urban areas, it is very important to have the knowledge of trees which are capable of withstanding heavily polluted air and environment. Tolerance and pollution response of the trees is regulated by its different characteristics such as height, canopy size, leaf form, texture and nature of the tree.
Various studies have shown that deciduous trees with compound leaves, small-to-medium canopy and round-to-oval shape are more tolerant to air pollution. This is because as compared to simple leaves, compound leaves are less exposed to air pollutants making trees bearing such leaves more tolerant.
Related: The best trees to reduce air pollution
10 trees to beat Air Pollution
1. Indian Redwood (Caesalpinia Sappan)
Indian Redwood is a huge tree bearing deciduous foliage with a tough bark exfoliating in plates or scales. The compound leaves are crowded at the ends of the branches. The greenish white flowers are borne in large clusters on the tree. Whole fruits of the Indian-redwood are widely sold as “wild lily flowers”, the columellas and the central interior columns are sold as “lily pods”. This tree has various medicinal uses as well.
2. Shisham (Dalbergia sissoo)
Shisham is a fast growing, hardy, medium to large deciduous tree with a light crown, rough bark and compounded leaves. It can grow up to 25 m height and 2 to 3 m in diameter. Flowers are yellowish white in colour, fragrant and dense in clusters. They have a long taproot and numerous surface roots that produce suckers. Young shoots are downy and drooping, established stems have light brown to dark grey bark, up to 2.5 cm thick, shed in narrow strips; larger upper branches support a spreading crown.
3. Shirish (Albbizia lebbeck)
Shirish is a deciduous erect spreading tree commonly growing at roadside throughout India from the plains upto 900 m in Himalaya. It is a tall tree of 18-30 m height with a trunk of 50 cm to 1m in diameter. The flowers are yellowish-white, fragrant growing in clusters of 15-40 pedicellate flowers. These plants are able to succeed in most soil types, including saline but cracking clay, as long as they are well drained.
4. Neem (Azadirachta indica)
Neem is ecologically very special. It can tolerate very high levels of pollution and has the capacity to recover even if most of its foliage is dropped. Plants with a large leaf area such as neem, accumulate relatively higher quantities of lead. It can reach 15-30 m high and have attractive rounded crowns and thick furrowed bark. The compound leaves have toothed leaflets and are typically evergreen but do drop during periods of extreme drought.
5. Gulmohar tree (Delonix regia)
Delonix regia is a leguminous plant which is widely grown as an ornamental or agroforestry tree. These are large, deciduous, and flowering plants with a subtle fragrance and a lovely appearance. The leaves of the tree are arranged in the way of individual leaflets supporting the clusters of flowers, The tree grows to a height of 30 to 40 feet and the spread of the tree forms a flat-topped umbrella-like crown.
6. Guava tree (Psidium guajava)
Guava is a very common tropical fruit tree that is known for its delicious flavour of the fruit. The tree can grow up to 3 metres to 10 metres and is a fast-growing evergreen shrub. The tree has a shallow root, and the trunk can be up to 20 cm in diameter. According to various bio-monitoring studies, guava plants are shown to be effective accumulators of sulfur and fluoride and sensible to ozone in semi-controlled experiments.
7. Blackboard tree (Alstonia scholaris)
Blackboard tree, also known as Scholar tree, Milkwood or devil’s tree is a tropical tree native to India and surrounding countries. It is an elegant evergreen tree, with greyish rough bark found in most parts of India. It gets its name from the fact that its timber was traditionally used to make wooden slates for school children. It can reach a height of over 130 feet and its flowers are very fragrant.
8. Banyan tree (Ficus benghalensis)
Banyan, also called banyan fig or Indian banyan are unusually shaped tree of the mulberry family native to Indian subcontinent. This tree reaches a height up to 30 m and spreads laterally indefinitely. Aerial roots that develop from its branches descend and take root in the soil to become new trunks. Researchers say that tree species such as banyan has proved effective in controlling air pollution.
9. Ashoka tree (Polyalthia longifolia)
Ashoka is an evergreen tree with a straight trunk, conical crown and slender drooping branches, about 10-25 m tall. Leaves are simple, alternate, with wavy margins. According to research, Polyalthia longifolia or False Ashoka is an ideal choice for better air quality at kerbside locations. Due to its height, straight shape and dense canopy, the tree has high leaf area index.
10. Mango tree (Mangifera indica)
The tree is evergreen and attains great age, often reaching 90 feet in height and 80 feet in width. The leaves are leathery, lanceolate and found in simple-alternate arrangement on the branches. Mango has the longest life-cycle as compared to other fruit-bearing trees including guava, papaya and citrus. As mango trees have a larger surface area compared to other fruit-bearing trees, they absorb more carbon dioxide than other trees.