On a smoggy morning in Delhi, commuters often cough their way through streets clouded with dust and exhaust fumes. But few realise that the silent banyan trees and neem plants lining those same streets are recording this pollution in their leaves. Beneath their green surfaces lies a tale of toxic metals, seasonal shifts, and the city’s battle to breathe.
A recent study led by Dr. Durga Prasad Tripathi, a researcher at the Indian Institute of Technology Delhi (IIT Delhi), published in the journal Environmental Pollution under the guidance of Prof Arvind Kumar Nema, uncovers how plants in one of the world’s most polluted megacities are capturing airborne metals in ways that directly reflect human exposure risks.
The hidden metals in Delhi’s air
Air pollution in Delhi is not just about fine particles or smog visible to the naked eye. Suspended particulate matter (SPM), which includes PM2.5 and PM10, carries with it a cocktail of metals and metalloids such as lead, chromium, zinc, nickel, arsenic, and cadmium. These pollutants come from vehicle exhaust, industrial emissions, road dust, and waste burning.
The World Health Organization estimates that prolonged exposure to particulate matter contributes to millions of premature deaths globally each year. In Delhi, approximately 30 percent of residents suffer from respiratory problems, a figure twelve times higher than the national average. While discussions often focus on the sheer concentration of particles, the presence of metals bound to these particles poses even more severe threats, including cardiovascular diseases, neurological disorders, and cancer.
The IIT Delhi study set out to quantify exactly how much of these metals plants absorb in different seasons, and what that means for air quality and human health.
Plants as bioindicators of toxic air
The study investigated six common plant species across Delhi: Ficus benghalensis (banyan tree), Ficus religiosa (peepal tree), Polyalthia longifolia (ashoka tree), Azadirachta indica (neem), Ficus benjamina, and Bougainvillea glabra. These species are widely found in both green belts and polluted roadside areas.
By analysing more than 2500 leaf samples from traffic intersections, industrial zones, residential neighbourhoods, educational hubs, and commercial centres, researchers found that plants act as natural monitors of air pollution. The leaves not only trap metals on their surfaces but also absorb them into their tissues.
The results were sobering. Before the monsoon, the commercial hub Anand Vihar recorded the highest metal loads, with iron concentrations on leaves reaching over 2700 milligrams per kilogram and zinc levels nearing 300 milligrams per kilogram. Other toxic metals like chromium and copper were also alarmingly high.
Seasons change but metals linger
Delhi’s seasonal cycle, defined by scorching summers, intense monsoons, and polluted winters, influences how pollutants behave. The study found that rainfall during the monsoon helps wash away some of the metals from leaf surfaces, leading to lower concentrations in the post-monsoon period. However, inside plant tissues, levels of metals like cadmium, lead, and zinc often remained higher after the rains.
This seasonal shift matters because while rains may temporarily clear the air, the accumulation of toxic substances inside plant tissues shows that these metals persist in the environment. Their long-term presence raises concerns about bioaccumulation in food chains, soil contamination, and chronic human exposure.
A city of hotspots
Not all parts of Delhi are equally polluted. The study identified Anand Vihar and ITO as hotspots, with higher concentrations of chromium, nickel, copper, and lead compared to other areas like Dwarka or Siri Fort. Traffic-heavy areas and industrial belts showed clear signatures of specific pollutants, reflecting their local emission sources.
This spatial mapping is critical for policymakers. By knowing which neighbourhoods accumulate more toxic metals, targeted interventions such as stricter traffic regulations, dust control measures, or strategic greenbelt development can be prioritised.
The banyan tree leads the defence
Among the six species studied, the banyan tree (Ficus benghalensis) emerged as the most effective at capturing metals, with the highest Metal Accumulation Index during both pre-monsoon and post-monsoon periods. Its large leaves and complex surface structure allow it to trap more particles than other species.
In contrast, neem (Azadirachta indica) showed the lowest capacity for trapping metals. This does not make neem useless in urban landscapes, but highlights that different plants play different roles. For air pollution mitigation, species like banyan and ashoka may serve as better natural barriers along busy roads or industrial corridors.
What this means for human health
The presence of metals in leaves is not just an ecological curiosity. The same metals trapped by plants are the ones people breathe in daily. Lead inhalation is linked to neurological and cardiovascular problems. Chromium, nickel, and cadmium are known carcinogens. Zinc and copper, while essential in trace amounts, become toxic at elevated levels.
The study even found that certain metal concentrations inside leaves exceeded WHO permissible limits, underscoring the intensity of exposure in Delhi. If plants, which are stationary, show such high absorption levels, humans moving through the same environment are inevitably at risk.
Delhi in the global context
Delhi is often described as one of the world’s most polluted cities, but it is far from unique. Other megacities such as Beijing, Lahore, and Mexico City face similar challenges with particulate matter and heavy metals. The Indo-Gangetic Plain, stretching across northern India, Pakistan, and Bangladesh, is particularly vulnerable due to a combination of meteorological conditions and high emissions.
Linking Delhi’s situation to global urban health makes the findings of this research relevant beyond India. As climate change alters rainfall patterns and urbanisation accelerates, the dynamics of airborne metals may shift in ways scientists are only beginning to understand.
From scientific findings to policy action
The implications of this study go beyond laboratory results. It offers evidence that could shape urban planning and environmental policy. Incorporating plant species with high metal-trapping potential into city greenbelts is not a luxury, but a public health necessity.
Environmentalists argue that vegetation should be treated as living infrastructure. Just as cities invest in flyovers or sewage systems, they should equally invest in strategic planting of species like banyan and ashoka to create pollution barriers.
At the same time, the study highlights the limits of relying on nature alone. Plants can capture and record pollution, but they cannot replace stronger regulations on emissions from vehicles, industries, and construction. Without systemic changes, even the most tolerant trees will eventually reach saturation.
A moment of urgency
Delhi’s toxic air has repeatedly made global headlines, from smog emergencies forcing school closures to alarming AQI spikes during festivals and crop burning seasons. The IIT Delhi findings add a crucial layer of understanding by showing that it is not just the visible haze but the invisible metals that pose long-term risks.
For citizens, this raises unsettling questions. If the trees around us are accumulating dangerous levels of metals, what are our lungs accumulating? If seasonal rains only offer temporary relief, what is the permanent cost of breathing Delhi’s air?
A call to rethink cities and health
The study by Dr. Tripathi and colleagues is more than a scientific report. It is a wake-up call for cities across the globe. The evidence suggests that plants can help mitigate pollution, but the ultimate solution lies in reducing emissions at the source.
The question remains: should cities treat trees not just as ornaments or shade providers, but as frontline defenders of human health? If the banyan tree is silently absorbing toxic metals every day, perhaps it is time for urban planners and policymakers to listen to what nature is telling us.
Reference
Tripathi, D. P., & Nema, A. K. (2024). Assessment of metals and metalloids agglutinated to airborne suspended particulate matter in selected plant species during the pre- and post-monsoon in the urban area. Environmental Pollution, 356, 124300. https://doi.org/10.1016/j.envpol.2024.124300
