Delhi residents beware: Not all green trees are safe; Study reveals which ones clean the air and which make pollution worse

Delhi’s worsening air quality has been a growing concern for years, prompting city planners and environmentalists to push for more green spaces. But a recent study by scientists at the Indian Institute of Science Education and Research (IISER), Mohali reveals a surprising truth: not every tree that looks green on paper is actually beneficial for urban air quality.

Why Some Trees Like Arjun and Ashoka Improve Delhi’s Air While Mango Can Make Pollution Worse

Many city planners have assumed that all trees improve air quality. However, the IISER study found that certain species release volatile organic compounds that react with urban pollutants, worsening the air rather than cleansing it.

Dr Vinayak Sinha, Associate Professor in Earth and Environmental Sciences at IISER Mohali, told TOI, “When we walk through a forest, the refreshing smell comes from hydrocarbons like terpenes released by trees. In clean air, these compounds, called biogenic volatile organic compounds (BVOCs), are harmless. But in urban environments, they react with nitrogen oxides (NOx) and other pollutants from vehicle exhausts, industries, and burning sources to form ozone and secondary pollutants. This can impair lung function, trigger asthma attacks, and worsen allergies.”

For instance, mango trees, which emit high levels of isoprene and monoterpenes, can increase ozone levels in polluted cities. On the other hand, native species like ashoka release fewer harmful compounds and can absorb more pollutants than they emit, making them better suited for urban plantations.

Introducing the Air Quality Impact Index (AQII)

To tackle the complex interactions between trees and air pollution, IISER scientists developed the Air Quality Impact Index (AQII). Unlike traditional scoring systems such as the Anticipated Performance Index (API), which primarily measures a plant’s growth and adaptability, AQII considers:

• Pollution uptake capacity of the tree
• Emissions of ozone and aerosol precursors
• Potential for allergenic windblown pollen

Dr Savita Datta, a lead author of the study published in Urban Climate, told TOI, “Green spaces are the lungs of our cities, they cool the air, calm the mind, and connect communities. But the choice of species determines whether they improve or deteriorate urban air quality. Some trees have windblown allergenic pollen, contributing to seasonal hospital admissions. AQII gives urban planners a scientific guide to select trees that genuinely clean the air.”

Which Trees Are Ideal for Delhi’s Parks and Residential Areas According to AQII

The study analysed 149 tree species and calculated AQII scores for 98 of them, ranking them based on their potential to improve urban air quality. Key recommendations include:

Highly Recommended for Urban Plantations:

• Arjun – Excellent pollutant absorption, low BVOC emissions
• Ashoka – Native species with high pollution uptake and minimal ozone formation
• Silver Oak – Positive AQII due to crown structure, non-windblown pollen, and low BVOC emissions

Suitable for Parks but Not Close to Residences:
Teak, Banyan, Pilkhan – Emit high isoprene and monoterpenes, but pollen is not allergenic, making them ideal for city parks

Best for Roadside or Highway Plantations:
Gulmohar, Semal, Kapok, Putranjiva, Kamini, Ullu (Ailanthus excelsa) – Moderate-to-high monoterpene emissions, but windblown allergenic pollen restricts them to non-residential areas

Unsuitable for Residential Areas:
Kachnar, Yellow Gulmohar, She Oak, Seesam, Poplar, Eucalyptus, Willow, Mahua – High isoprene emissions and allergenic windblown pollen; recommended only for rural plantations

How Trees Affect Urban Air Pollution: Understanding the Science

Trees in urban environments interact differently with air pollution compared to rural forests. BVOCs, which are harmless in forests, react with nitrogen oxides (NOx) and other human-made pollutants to form:

• Ozone – Linked to asthma, lung inflammation, and reduced lung function
• Secondary Organic Aerosols (SOA) – Fine particles that degrade air quality further
• Allergenic Pollen Spread – Increasing hospital visits during seasonal pollen surges

Dr Sinha added to TOI, “Even though trees are essential for city life, some inadvertently contribute to pollution. Using AQII, we can ensure the green spaces we create truly benefit urban air quality.”

Why Selecting the Right Trees Is Crucial for India’s Polluted Cities

As cities like Delhi face record air pollution episodes every year, simply planting more trees is not enough. “Urban planners must consider a tree’s structure, leaf aerodynamics, pollution tolerance, BVOC emissions, and pollen allergy potential,” Dr Datta emphasised to TOI.

This research highlights that while native trees like arjun, ashoka, and silver oak can genuinely clean the air, some fruiting trees or exotic species may worsen urban pollution. Careful selection can ensure city greenery not only beautifies but also protects public health.

Not All Green Is Clean, Science Guides Smarter Urban Tree Planting

The IISER Mohali study underlines a key message: urban greening must be smart and scientific. By using the Air Quality Impact Index (AQII), planners can choose species that reduce air pollution, protect residents from allergens, and create healthier, sustainable urban environments.

In a city battling smog and respiratory illnesses, planting the wrong trees could do more harm than good. Science, it seems, has finally given urban India the tools to make every tree count.

Inputs from TOI