🌳 What tree bark tells us about air pollution in Paris

Fine and ultrafine particles from road traffic are among the most harmful pollutants for health. They are also the most difficult to measure at the neighborhood level. In Paris, a citizen science project shows how residents, by collecting bark from the plane trees that populate the city, can complement official monitoring systems and produce data useful for public action.


In large cities, air pollution is monitored by fixed stations, such as those of Airparif in Île-de-France, which allow fairly detailed tracking of different types of pollutants and modeling of general trends. These stations are still too few to account for the actual exposure of populations, street by street.

This limitation is particularly problematic for the inorganic fraction of fine particles (smaller than 2.5 micrometers) and ultrafine particles (smaller than 0.1 micrometer). By "inorganic fraction," we mean mineral particles that do not contain carbon. They are either primary (soil erosion, metallic particles from brake pad wear...) or secondary, formed from other gaseous pollutants. These particles are closely linked to road traffic and associated with major health effects. Yet, currently, only one fixed monitoring station is operational in Paris.

Yet measurement should guide action: urban planning, bike lanes, pedestrianization, or traffic regulation rely on data that is often too sparse to inform local decisions.

Our research, published in the journal Community Science, is based on a simple observation: trees record the pollution of their immediate environment. Traffic particles settle on the bark, which acts as a passive sensor integrating pollution over several months. This makes it a relevant indicator for assessing chronic exposure.

Plane tree bark as a pollution indicator

As part of the Ecorc'Air project, volunteers collect fragments of plane tree bark each spring, during the annual exfoliation. Plane trees are ubiquitous along streets—especially in the French capital, which hosts more than 40,000 plane trees.

These samples are then sent to a laboratory for analysis. Measuring a particular physical property of the sample, magnetic susceptibility, makes it possible to estimate the amount of deposited metallic particles. These are directly linked to automobile traffic emissions.

From several thousand samples collected since 2016, we show that this magnetic signal is strongly correlated with the presence of metals, some of which can be toxic depending on their nature and inhaled doses. The protocol, which is very accessible even without prior knowledge, allows pollution to be mapped at a very fine scale, on the order of a few tens of meters (approximately 30–100 feet).


Thanks to this massive sampling, made possible by citizen participation, several observations have been made.

First, there are persistent "hotspots." Some areas of Paris have shown high and recurrent levels of metallic particle pollution since the beginning of the monitoring: these include heavily trafficked quays (such as the car portion of the Voie Georges-Pompidou), the outskirts of the ring road, and congested thoroughfares. Conversely, parks and areas far from traffic show relatively low levels. These maps help identify intervention priorities where conventional monitoring stations are insufficient.

Second, pollution decreases rapidly with distance. Our data show a clear drop in particle contamination as soon as one moves away from the roadway, especially within the first few meters (roughly 10 feet). This confirms the importance of where sidewalks and bike lanes are placed relative to natural barriers (hedges or shrubs) and rest areas (where benches are located, for example).

When cars act as a screen

One of the most striking results concerns the very concrete organization of public space. On several major Parisian avenues, especially Boulevard Saint-Germain, we compared pollution levels recorded by trees according to the configuration of the nearest lane: general automobile traffic (configuration A in the diagram below), bus-taxi lane (C), shared bus-bike-taxi lane (D), or the presence of a parking lane between the roadway and the sidewalk (B).


The observed differences are clear. Trees located closest to automobile traffic lanes consistently show the highest magnetic susceptibility values. Conversely, when an element (natural hedge, parked vehicle) separates the roadway from the sidewalk, the levels measured in the bark are significantly lower. This decrease is pronounced enough to be statistically robust over all data collected in 2020 and 2021.

This observation suggests that parked vehicles play a dual role. On one hand, they increase the distance between the emission source and pedestrians; on the other, they constitute a physical barrier to the direct projection of metallic particles from traffic onto sidewalks. This "screen" effect reduces pedestrian exposure to a degree comparable to moving several meters (roughly 10–20 feet) away from the roadway.

Our point here is not to promote the generalization of parking spaces along streets, which would encourage car travel, but to highlight the value of thinking about a real separation between the roadway and pedestrians. Conversely, shared lanes with buses and taxis, often presented as favorable to active mobility, remain associated with high levels of particulate pollution.

These results, seemingly intuitive, are nonetheless rarely objectified by high spatial resolution data. They show that very concrete planning choices—parking plans, sidewalk widening, real separation of bike lanes, spatial separation of pedestrian zones from road traffic, greening projects...—have measurable effects on the daily exposure of populations.

Citizen science changes the game

Such a level of detail would not have been possible without the massive participation of volunteers. Regulatory monitoring networks, essential for tracking broad trends, rely on a limited number of fixed stations. In Paris, as in most large cities, these are too far apart to capture the fine contrasts related to street morphology, local traffic intensity, or planning choices.

The Ecorc'Air project is based on a different logic: multiplying simple, robust, and temporally comparable measurement points. By mobilizing volunteers to collect plane tree bark samples at breathing height, it has been possible to build, year after year, an accessible database of several thousand points, covering entire neighborhoods and enabling temporal comparisons.

This approach has a second, often underestimated advantage: it turns data production into an object of dialogue. Sampling locations are not only chosen by research teams but also by volunteers and local authorities, based on their knowledge of living areas, their perception of nuisances, their daily practices, or their questions about ongoing urban projects. This cross-fertilization between scientific knowledge and local experiences enriches data interpretation and reinforces its social legitimacy.

Interviews conducted by the scientific team as part of the project show that motivations to participate are diverse. Some people get involved out of scientific curiosity, others out of concern for their living environment or a simple desire to improve their surroundings. For local authorities, interest lies both in producing environmental data and in building relationships with residents around major environmental and health issues. Citizen science is therefore not just a measurement tool: it becomes an intermediation device between science, the public, and public action.

For public authorities, the lesson is clear: there are now low-cost, proven complementary methods to document the actual exposure of populations to traffic-related pollution. Without replacing official networks, these approaches make it possible to identify high-risk areas, assess the impact of urban developments, and track changes over time at a scale relevant for local action.

Results obtained in Paris show that some areas remain persistently exposed, despite an overall decrease in concentrations measured citywide. They also suggest that seemingly secondary planning choices—location of bike lanes, parking organization, sidewalk width...—can have significant effects on passers-by's exposure to inorganic particles.

In a context where international health recommendations are becoming increasingly stringent and social demand for environmental transparency is growing, such fine-grained data provides valuable support for decision-making. They make it possible to move beyond overly general debates on pollution and enter into a logic of concrete, targeted, territorialized action, discussed in consultation with users.

Ultimately, the challenge is not only to measure better, but to decide better. Citizen science, integrated into public policies, can help fill a major blind spot in environmental governance: that of daily, real, lived exposure at the street level. In Paris, but also in other European cities, interest in such approaches is growing. The challenge is no longer just to measure, but to transform these data into levers for action at the neighborhood level.

This article is republished from The Conversation under a Creative Commons license. Read the original article.