Tree Leaves Reveal Hidden Metal Pollution Hotspots Across Pune’s Streets

In the rapidly growing city of Pune, India, an unassuming scientific instrument has been quietly collecting data on air pollution for years: the leaves of roadside trees. A new study published in Environmental Monitoring and Assessment has harnessed this natural sampling network to map heavy metal contamination across 27 locations spanning busy roadsides, industrial zones, peri-urban fringes, highways, and rural outskirts. By analysing the particulate matter that settles on plant leaves, researchers led by Akshay Paygude of the Indian Institute of Remote Sensing, together with colleagues at the Indian Institute of Tropical Meteorology, have produced one of the most detailed pictures yet of how metals from traffic, construction, and industry are distributed across a mixed-use Indian city.

The technique at the heart of the study, known as leaf-deposited dust analysis, exploits a simple physical process. As air moves past vegetation, particles suspended in it collide with leaf surfaces and stick there, trapped by waxy cuticles, hairs, and the rough microtopography of the leaf itself. Over time, these deposits accumulate a chemical fingerprint of everything the surrounding air has carried: mineral dust from construction sites, exhaust-derived metals from vehicles, particles resuspended from road surfaces by passing traffic, and emissions from nearby factories. Because leaves integrate deposition over weeks or months, they act as passive, low-cost samplers that require no power supply, no calibration, and no maintenance, making them especially valuable in cities where dense networks of electronic monitors are impractical.

The researchers collected leaf-deposited dust from their 27 sites and analysed it for ten elements: calcium, potassium, iron, sodium, magnesium, manganese, zinc, copper, chromium, and lead. The concentrations followed a strikingly consistent order, with calcium the most abundant, followed by potassium, iron, sodium, magnesium, manganese, zinc, copper, chromium, and finally lead. This hierarchy tells a story in itself. Calcium and potassium at the top of the list point to the dominance of crustal material, the mineral dust generated by construction activity and the constant resuspension of road dust, rather than to combustion sources, which would elevate different elements.

Correlation analysis reinforced this interpretation. Strong statistical relationships among calcium, iron, magnesium, manganese, lead, copper, and chromium suggested that these elements share common origins, arising from a combination of natural geological sources and human activities. In an urban environment like Pune, where building booms and expanding road networks generate enormous quantities of mineral dust, the boundary between natural and anthropogenic contributions is often blurred. The correlations indicate that a single parcel of deposited dust can carry metals from several overlapping sources at once, which is precisely why the team turned to more sophisticated statistical tools to disentangle them.

One of those tools, enrichment factor analysis, compares the concentration of each metal in the deposited dust against its expected abundance in the Earth’s crust, normalised to a reference element. When a metal’s enrichment factor is substantially elevated above crustal levels, it signals an anthropogenic source. In the Pune study, copper, zinc, and chromium emerged as substantially enriched at select locations, particularly those influenced by traffic and industry. This finding matters because these three metals are classic markers of human activity: copper is associated with brake wear and industrial processes, zinc with tyre wear and galvanised materials, and chromium with metal plating, tanning, and various manufacturing operations.

To go beyond simple enrichment and identify coherent source groups, the team applied principal component analysis, a statistical technique that reduces many correlated variables into a smaller number of underlying factors. The analysis distinguished four distinct source signatures. The first linked construction activity and road dust, dominated by the crustal elements. The second captured vehicular emissions, carrying the traffic-related metals. The third pointed to biomass burning or food-related activity, a reminder that in Indian cities, cooking fires, street food stalls, and the burning of organic waste all contribute particles to the urban atmosphere. The fourth identified localised industrial inputs, confined to specific sites rather than spread across the city.

The spatial patterns that emerged from combining these analyses were equally revealing. Central urban sites were influenced mainly by construction, manufacturing, and vehicular dust, consistent with the intense building activity and dense traffic of Pune’s core. Outer-zone sites, by contrast, reflected additional contributions from industrial operations, agricultural activity, and resuspended dust. This gradient illustrates a fundamental principle of urban air quality: pollution is not a single blanket draped uniformly over a city but a patchwork of microenvironments, each with its own chemical signature shaped by the land uses immediately surrounding it. A tree on a highway median and a tree in a peri-urban field, only a few kilometres apart, can record entirely different exposure histories.

The health dimension of the study focused on inhalation-related non-carcinogenic risk. The researchers calculated hazard quotients for the metals, which ranked in the order chromium, lead, copper, and zinc, meaning chromium contributed the greatest potential concern. When these quotients were summed into hazard indices for both adults and children, the values remained below the threshold generally used to flag non-carcinogenic risk. On its face, this is reassuring news for Pune’s residents. However, the authors are careful to note an important caveat: the aggregate risk estimates can mask spatial variability, and the localised enrichment of copper, zinc, and chromium shows that exposure potential is unevenly distributed across the city even when citywide averages look acceptable.

This tension between average safety and local hotspots is perhaps the study’s most consequential message. A hazard index computed from citywide mean concentrations can fall comfortably below regulatory thresholds while individual neighbourhoods, particularly those near traffic corridors or industrial clusters, experience substantially higher exposure. Children, who breathe more air per unit of body weight and often play close to the ground where resuspended dust concentrates, are especially sensitive to such spatial disparities. The findings therefore argue for zone-specific monitoring rather than one-size-fits-all assessment, and for control measures targeted at the sources that dominate each zone, whether that means suppressing road dust and regulating construction practices in the urban core or tightening industrial emissions standards at the periphery.

There is also a broader lesson here about the tools available for environmental surveillance in rapidly urbanising regions. Leaf-deposited particulate matter offers a practical, inexpensive complement to instrumental monitoring, capable of filling spatial gaps that fixed stations cannot cover. As cities across South Asia and the developing world expand, the combination of botanical sampling, enrichment factor analysis, principal component analysis, and health risk assessment provides a template for identifying where pollution originates, where it accumulates, and who is most exposed. For Pune, the immediate implications are clear: road dust, construction activity, traffic emissions, and industrial inputs each demand attention in the zones where they dominate. For the wider field of urban air quality science, the study demonstrates that sometimes the most informative sensors are the ones already growing along the roadside, quietly recording the chemical history of the air we all share.

Subject of Research: Heavy metal contamination and source profiling of leaf-deposited particulate matter in Pune city

Article Title: Heavy metal exposure and source profiling based on leaf-deposited particulate matter in Pune city

Article References: Paygude, A., Sharma, R., Radhadevi, L., & Bandaru, M. (2026). Heavy metal exposure and source profiling based on leaf-deposited particulate matter in Pune city. Environmental Monitoring and Assessment, 198(10), Article 1080. https://doi.org/10.1007/s10661-026-15906-w

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15906-w

Keywords: air pollution, heavy metals, leaf-deposited dust, Pune, enrichment factor, principal component analysis, health risk assessment, road dust, vehicular emissions, construction activity, industrial pollution, biomonitoring