Urban tree plantation efforts, while well-intentioned, can inadvertently worsen air pollution if not carefully planned.
Certain tree species emit volatile organic compounds (VOC) that react with vehicular emissions, forming harmful ground-level ozone.
The 'Right Tree, Right Place' approach advocates for selecting low-VOC-emitting trees in high-traffic areas to improve air quality and public health.
Tree plantation drives are often seen as a simple yet powerful way to help the planet. From enhancing human well-being to mitigating climate change and curbing urban air pollution, trees are viewed as green allies. But did you know that not all trees are equal when it comes to air quality?
Some species release chemicals called volatile organic compounds (VOC) in significant amounts, which can react in the atmosphere and worsen air pollution under certain conditions, particularly in urban areas with high vehicular emissions. Therefore, cities already battling severe air pollution should follow the ‘Right Tree, Right Place’ approach so that these well-intentioned efforts do not do more harm than good.
Let’s explore why thoughtful tree selection matters for cleaner, healthier air.
VOC emissions are a natural part of tree metabolism, particularly under abiotic stress factors (or non-living environmental factors) such as high temperatures, drought, and nutrient deficiency. VOCs play a vital role in enhancing tree resilience and protection by helping them manage oxidative stress, regulate leaf temperatures, and communicate with nearby plants.
VOC emissions are generally higher in warmer summer months and may peak during flowering and rapid growth periods or under environmental stress. While VOC emissions can disperse harmlessly under cleaner environments, they can react with vehicular emissions (such as nitrogen oxides or NOx) in the presence of sunlight. This reaction can lead to the formation of tropospheric (ground-level) ozone, a major pollutant, especially in urban areas with high-traffic zones.
Ozone levels can worsen in cities with high NOx pollution from vehicles, especially when VOC levels are also high, as these conditions enhance ozone formation. Thus, in cities like Delhi (with abundant NOx and low VOC levels in some areas), ground-level ozone formation is highly sensitive to even small increases in VOC emissions.
According to the US Environmental Protection Agency, even relatively low concentrations of ozone can negatively affect human health, causing respiratory diseases and eye irritation and aggravating asthma. A Health Effects Institute report also showed that 237,700 deaths in India were attributable to ozone in 2021. Thus, for effective ozone management, reducing VOC emissions must be prioritised alongside NOx to improve air quality and protect public health.
Beyond air quality concerns, VOCs also influence the climate. They may compete with atmospheric oxidants and prevent them from decomposing methane, therefore increasing the lifetime of this very potent greenhouse gas and indirectly contributing to global warming.
Rather than blanket tree plantation drives, a science-based approach considering local pollution levels and ecosystem needs (climate regulation, carbon sequestration, and habitat and food provision) can help effectively expand greenbelt coverage in Indian cities. Because VOCs are inherent to plant physiology and their levels vary with tree species, the primary focus should be on controlling NOx pollution.
However, addressing NOx emissions from transport and industry is a long-term challenge requiring systemic interventions. A practical intermediate step to minimise added air pollution is to avoid planting high-VOC-emitting tree species in traffic-heavy areas. For instance, among common tree species catalogued by the Department of Forests and Wildlife, Government of the National Capital Territory of Delhi, in the city, many — including peepal (Ficus religiosa), mango (Mangifera indica) and banyan (Ficus virens) — are high VOC emitters, releasing compounds such as isoprenes and monoterpenes.
More suitable alternatives with lower VOC emissions and air quality benefits include neem (Azadirachta indica), tamarind (Tamarindus indica), golden shower (Cassia fistula), arjuna (Terminalia arjuna), ashoka (Polyathia longifolia), blackboard (Alstonia scholaris), silk cotton (Ceiba petendra) and silk floss (Chorisia speciosa).
Alongside choosing the right tree, selecting the appropriate location is important. Areas where trees (especially those with large leaf area) can serve as natural pollutant traps, such as areas with high dust and particulate pollution, should be prioritised. Further, regular pruning and active leaf litter management can help reduce VOC emissions from decaying biomass.
While tree plantations are crucial for urban resilience, their impact depends on planning and maintenance. Being a tropical country with an environment conducive to growing a wide variety of trees, India requires a balanced strategy to ensure that tree plantations truly contribute to healthier cities and cleaner air.
Urban forestry plans should be integrated into broader air quality management efforts, allowing tree plantations to complement other pollution control strategies. The National Clean Air Programme recommends urban greening as a strategy to improve air quality, making it essential to factor in ozone chemistry while planning new plantations.
Horticulture departments in urban local bodies, which currently undertake tree plantation drives, should be trained in the nuances of air pollution and VOC-related considerations. Guidelines aligning ecological suitability and air quality goals are warranted, accounting for local species, water availability, VOC emissions, and other factors that vary significantly across locations. Moreover, creating public awareness about the science behind tree selection and its air pollution impact is crucial.
The ‘Right Tree, Right Place’ approach can pave the path towards truly green cities. By aligning afforestation efforts with air quality goals, cities can build an urban green cover that benefits both the environment and public health, without unintended trade-offs, ultimately leading to sustainable cities.
Nidhi Malik is a senior associate and Emil Varghese is a senior analyst in the air quality research team at the Center for Study of Science, Technology and Policy (CSTEP), a research-based think tank. Views expressed are the authors’ own and don’t necessarily reflect those of Down To Earth.