Measuring the Invisible: What Tree Bark Teaches Us About Air Pollution in Paris

Source: French to English Tester   Published on: 2026-04-30

Source: The Conversation – France (in French)– By Yann Sivry, Professor at Universities, Institute of Earth Physics of Paris (IPGP)

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 scale. In Paris, a citizen science project shows how citizens, by sampling the bark of plane trees that populate the city, can complement official devices and produce data useful for public action.

---

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

This limit is particularly problematic for theinorganic fractionfine particles (with sizes less than 2.5 micrometers) and ultrafine particles (smaller than 0.1 micrometer). By “inorganic fraction,” we mean themineral particlesnot containing carbon. They originate either primarily (soil erosion, metallic particles linked to brake pad wear…) or secondarily, formed from other gaseous pollutants. These particles are closely linked to road traffic and associated withmajor health effectsCurrently, only one fixed measurement station is operational in Paris.

Yet, the measure should guide action: urban planning, cycling facilities, pedestrianization, or traffic regulation rely on data that is often too fragmented to inform local decisions.

Our research, published in the journalCommunity Science, is based on a simple observation: trees record pollution from their immediate environment. Particles from traffic 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.

The bark of plane trees, revealing pollution

As part of the projectEcorc’Air, volunteers collect, every spring, at the time of the annual exfoliation, fragments of plane tree bark, a tree omnipresent along the streets – notably within the Parisian capital, which hosts more than 40,000 plane trees.

These samples are then sent to the laboratory, where they are analyzed. The measurement of a particular physical property of the sample, magnetic susceptibility, allows for the estimation of the quantity of deposited metallic particles. These are directly related toautomobile traffic emissions.

---

Also to read:
Air pollution in the city: mapping, micro-sensors, and citizen science

---

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 prove toxic depending on their nature and the doses inhaled. The protocol implemented, very accessible even without prior knowledge, allows mapping pollution on a very fine scale, on the order of a few dozen meters.

Thanks to this massive sampling, made possible by citizen participation, several observations could be made:

• First, there are persistent “black spots.” Some areas in Paris have high and recurring levels of metallic particle pollution since the beginning of the monitoring: these include heavily trafficked quays (such as the drivable portion of the Georges-Pompidou road), the vicinity of the ring road, and congested arteries. Conversely, parks and spaces distant from traffic show relatively low levels. These maps allow the identification of intervention priorities where conventional monitoring stations are insufficient.

• Then, pollution decreases rapidly with distance. Our data show a clear reduction in particle contamination as soon as one moves away from the roadway, especially in the first few meters. This confirms the importance of choosing the location of sidewalks and bike lanes in relation to natural barriers (hedges or shrubs) and rest areas (spaces where benches, for example, can be found).

---

Also to read:
Why taking the metro exposes us to more fine particles

---

When cars act as screens

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

The observed differences are clear. The trees located closest to the roads with automobile traffic consistently show the highest values of magnetic susceptibility. Conversely, when an element (natural hedge, parked vehicle) separates the roadway from the sidewalk, the levels measured in the bark are significantly lower. This reduction is pronounced enough to be statistically robust across 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, and on the other hand, they act as a physical barrier to the direct projection of metal particles from traffic onto sidewalks. This “shield” effect reduces pedestrian exposure in a manner comparable to moving several meters away from the roadway.

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

These results, apparently intuitive, are nevertheless rarely supported by high spatial resolution data. They show that very concrete planning choices – parking plans, widening of sidewalks, actual separation of bike lanes, spatial separation of pedestrian zones and road traffic, projects ofgreening… – have measurable effects on the daily exposure of populations.

---

Also to read:
What is the assessment of Anne Hidalgo’s new “urban forests”?

---

Participatory science is changing the game

Such a level of detail would not have been possible without the massive participation of volunteers. Regulatory monitoring networks, essential for tracking long-term trends, rely on a limited number of fixed stations. In Paris, as in most large cities, these stations are too far apart to reflect 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 comparable measurement points over time. By mobilizing volunteers to collect plane tree bark samples at breathing height, it has been possible to build, year after year, an accessible database ofseveral thousand points, covering entire neighborhoods and allowing for temporal comparisons.

Results collected by the Ecorc’Air project between 2016 and 2025

This approach presents a second often underestimated advantage: it transforms data production into a subject of dialogue. Sampling sites are targeted not only by research teams but also by volunteers and communities, based on their expertise about their living areas, their perception of nuisances, their daily uses, or their questions about ongoing urban projects. This cross-fertilization between scientific knowledge and local experiences enriches the interpretation of data and strengthens their social legitimacy.

The interviews conducted by the scientific team within the framework of the project also show that the motivations to participate are diverse. Some people engage out of scientific curiosity, others out of concern for their living environment or a simple desire to improve their surroundings. From the perspective of local authorities, the interest lies as much in the production of environmental data as in the ability to establish a connection with the residents around major environmental and health issues. Participatory science is therefore not only a measurement tool: it becomes a device for intermediation between science, the population, and public action.

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

The results obtained in Paris show that certain areas remain persistently exposed, despite aoverall decrease in concentrations measured at the city scale. They also suggest that seemingly secondary planning choices—such as the location of bike lanes, parking arrangements, sidewalk widths—can have significant effects on the exposure of pedestrians to inorganic particles.

In a context where theinternational health recommendationsare becoming increasingly strict and where the social demand for environmental transparency is growing, this fine data provides valuable support for decision-making. They allow us to move beyond overly general debates on pollution to enter into a logic of concrete, targeted action, localized and discussed, in consultation with users.

In the long run, the issue is not only to measure better, but to decide better. Participatory sciences, integrated into public policies, can help fill a major blind spot in environmental governance: that of the daily, real, lived exposure at the street level. In Paris, but also in other European cities, the interestfor this type of approach grows. The challenge is no longer just to measure, but to transform this data into levers of action, at the neighborhood level.

Yann Sivry received funding from the National Research Agency, the CNRS, and the Université Paris Cité Foundation.

Aude Isambert received funding from the CPT (Centre for Earth Policies) at Université Paris Cité (StratEx-IdEx).

Claire Carvallo received funding from the CNRS.

Laure Turcati received funding through the Science Together network of the SAPS label of Sorbonne University and the State under France 2030 as part of the SOUND project led by the Sorbonne University Alliance.

Christine Franke does not work for, advise, hold shares in, or receive funds from any organization that could benefit from this article, and has declared no affiliation other than her research institution.

–ref. Measuring the invisible: what the bark of trees teaches us about air pollution in Paris –https://theconversation.com/measuring-the-invisible-what-tree-bark-teaches-us-about-air-pollution-in-paris-279149