Delhi residents beware: Not all green trees are safe; Study reveals which ones clean the air and which mak – The Economic Times

Report on Strategic Urban Afforestation for Sustainable Development Goals

Introduction: Aligning Urban Greening with SDG 3 and SDG 11

A recent study from the Indian Institute of Science Education and Research (IISER), Mohali, provides critical insights into urban afforestation strategies, directly impacting the achievement of Sustainable Development Goal 3 (Good Health and Well-being) and SDG 11 (Sustainable Cities and Communities). The research challenges the conventional wisdom that all green cover uniformly improves urban air quality, revealing that improper tree selection can exacerbate pollution in cities like Delhi. This report outlines the study’s findings, its proposed scientific framework, and its implications for creating healthier, more sustainable urban environments.

The Environmental and Health Paradox of Urban Flora

Biogenic Emissions and Their Impact on Urban Air Quality

The study highlights the role of Biogenic Volatile Organic Compounds (BVOCs), such as terpenes, which are naturally released by trees. In pristine environments, these compounds are harmless. However, in polluted urban settings, they react with anthropogenic pollutants like nitrogen oxides (NOx) from vehicular and industrial emissions. This chemical reaction contributes to the formation of harmful secondary pollutants, including:

• Ground-level ozone, which can impair lung function and trigger respiratory conditions like asthma.
• Secondary Organic Aerosols (SOA), which are fine particulate matter that degrades air quality.

This phenomenon directly undermines efforts to achieve SDG Target 3.9, which aims to reduce illnesses from air pollution, and SDG Target 11.6, focused on reducing the adverse environmental impact of cities by improving air quality. For instance, the high isoprene emissions from mango trees can elevate ozone levels, while native species like Ashoka absorb more pollutants than they emit, making them beneficial for urban health.

A Scientific Tool for Sustainable Urban Planning: The Air Quality Impact Index (AQII)

Methodology for Evidence-Based Tree Selection

To address this complexity, IISER scientists developed the Air Quality Impact Index (AQII), a novel framework designed to guide urban planners in selecting tree species that positively contribute to public health and environmental sustainability. The AQII provides a scientific basis for urban greening initiatives, aligning them with the goals of SDG 3 and SDG 11. The index evaluates species based on multiple factors:

• The tree’s capacity to absorb and remove pollutants from the atmosphere.
• The rate of emission of ozone and aerosol precursors (BVOCs).
• The potential for producing and spreading allergenic, windblown pollen, a significant public health concern.

Strategic Planting Recommendations for Achieving SDG Targets

Classification of Species Based on the Air Quality Impact Index

The study analyzed 149 tree species to generate AQII scores, providing a clear guide for urban planners. The recommendations are categorized to maximize positive health and environmental outcomes, contributing directly to safer and more inclusive green public spaces as envisioned in SDG Target 11.7.

1. Highly Recommended for Urban Plantations

   These species offer high pollution absorption and low BVOC emissions, making them ideal for improving air quality in residential and public areas.

   • Arjun (Terminalia arjuna)
   • Ashoka (Polyalthia longifolia)
   • Silver Oak (Grevillea robusta)

2. Suitable for Parks and Large Green Spaces

   While these trees emit higher levels of BVOCs, their pollen is not allergenic, making them appropriate for parks where they are not in immediate proximity to residences.

   • Teak (Tectona grandis)
   • Banyan (Ficus benghalensis)
   • Pilkhan (Ficus infectoria)

3. Recommended for Roadside and Highway Plantations

   These species have moderate-to-high emissions and produce allergenic pollen, restricting their use to non-residential transport corridors.

   • Gulmohar (Delonix regia)
   • Semal (Bombax ceiba)
   • Putranjiva (Putranjiva roxburghii)

4. Unsuitable for Urban Residential Areas

   Due to high isoprene emissions and allergenic pollen, these species can negatively impact public health and are recommended only for rural plantations.

   • Kachnar (Bauhinia variegata)
   • Poplar (Populus spp.)
   • Eucalyptus (Eucalyptus spp.)
   • Mahua (Madhuca longifolia)

Conclusion: Advancing Sustainable Urban Development Through Scientific Greening

The IISER Mohali study underscores that effective urban greening is not merely about increasing tree cover but about making strategic, science-informed choices. By adopting the Air Quality Impact Index (AQII), city planners can ensure that afforestation projects actively contribute to achieving Sustainable Development Goals, particularly SDG 3 (Good Health and Well-being) and SDG 11 (Sustainable Cities and Communities). This approach transforms urban green spaces from simple aesthetic assets into functional infrastructure that purifies the air, protects public health, and builds resilient, sustainable cities for the future.

Analysis of Sustainable Development Goals in the Article

1. SDG 3: Good Health and Well-being

   • The article directly connects urban air pollution to significant health problems. It states that pollutants formed from the interaction of Biogenic Volatile Organic Compounds (BVOCs) and urban emissions can “impair lung function, trigger asthma attacks, and worsen allergies.” It also mentions that allergenic pollen from certain trees contributes to “seasonal hospital admissions,” clearly linking the choice of urban flora to public health outcomes.

2. SDG 11: Sustainable Cities and Communities

   • The core theme of the article is improving the urban environment of Delhi. It addresses the challenge of “worsening air quality” in a major city and critiques the simplistic approach of just planting more trees. The development of the Air Quality Impact Index (AQII) is presented as a scientific tool for “urban planners” to create “healthier, sustainable urban environments” by making informed decisions about green spaces, which are described as the “lungs of our cities.”

3. SDG 15: Life on Land

   • The article touches upon the importance of biodiversity and ecosystem management, even within an urban context. It highlights the benefits of planting “native species like ashoka” which are well-suited to the local environment and have a high capacity for pollution uptake. This contrasts with other non-native or “exotic species” that may negatively impact the urban ecosystem by contributing to pollution, demonstrating a connection to managing terrestrial ecosystems sustainably.

Specific SDG Targets Identified

1. Target 3.9: By 2030, substantially reduce the number of deaths and illnesses from hazardous chemicals and air, water and soil pollution and contamination.

   • This target is central to the article’s argument. The research aims to mitigate the health impacts of urban air pollution, which includes ozone and secondary organic aerosols. The article explicitly mentions that these pollutants are “linked to asthma, lung inflammation, and reduced lung function,” and that the wrong choice of trees can “worsen urban pollution,” thereby increasing illnesses.

2. Target 11.6: By 2030, reduce the adverse per capita environmental impact of cities, including by paying special attention to air quality and municipal and other waste management.

   • The entire article is focused on improving Delhi’s urban air quality. The study provides a scientific framework (the AQII) to guide “smarter urban tree planting” specifically to reduce air pollution. It directly addresses the need to manage the “adverse per capita environmental impact” by ensuring that urban greening initiatives effectively combat pollutants like “nitrogen oxides (NOx),” ozone, and aerosols.

3. Target 11.7: By 2030, provide universal access to safe, inclusive and accessible, green and public spaces, in particular for women and children, older persons and persons with disabilities.

   • The article discusses the push for “more green spaces” such as “parks and residential areas.” However, it redefines what makes a green space “safe” by including air quality and allergenic potential as key factors. The AQII helps planners select trees to ensure these public spaces “genuinely clean the air” and do not expose residents, especially vulnerable groups, to harmful pollutants or allergens from “windblown allergenic pollen.”

Indicators for Measuring Progress

1. Health-Related Indicators (Implied for Target 3.9)

   • The article implies that progress could be measured by tracking health statistics related to air quality. Specific indicators would include the incidence rates of “asthma attacks,” “allergies,” “lung inflammation,” and “reduced lung function” in urban populations. A reduction in “seasonal hospital admissions” linked to allergenic pollen would also be a key indicator of successful tree selection.

2. Air Quality Measurement Indicators (Implied for Target 11.6)

   • Progress towards this target can be measured by monitoring the concentration of specific pollutants mentioned in the article. These include ambient levels of “nitrogen oxides (NOx),” ground-level “ozone,” and “Secondary Organic Aerosols (SOA).” The “Air Quality Impact Index (AQII)” itself serves as a novel, composite indicator developed to guide and measure the potential of urban greening projects to improve air quality.

3. Green Space Quality Indicators (Implied for Target 11.7)

   • Beyond simply measuring the area of green space, the article suggests qualitative indicators. Progress could be measured by the percentage of urban trees planted that have a positive AQII score. Another indicator would be the proportion of native, low-BVOC, and non-allergenic species (like Arjun and Ashoka) used in new “urban plantations,” “parks,” and “roadside” greening projects, ensuring these spaces are not only green but also healthy.

SDGs, Targets, and Indicators Summary

Source: m.economictimes.com