5. Leopold Park, behind EU buildings

Air pollution and lichens

Below you will find the audio and text of the walk ! 
Have a good adventure !

What do you think is the link between lichens and air pollution?

As we said earlier an organism is affected by its environment and in turn alters its habitat. This is what ecologists study, the relationship between organisms and their environment. Thus, by recording the absence or presence of certain lichen species, we can get an idea of the state of the habitat.

Lichens, “like mosses, incorporate the material effects of urban ecologies across time and space and therefore form a process of bioindication in the city, capturing pollutants and transforming them into resources accessible to other organisms.”

Jennifer Gabrys, 2012 in “Becoming urban: sitework from a moss eye view“.

But what is bioindication?

Bio-indication is a process whereby environmental pollution becomes embedded in the bodies of and the relationships between organisms. These organisms express the accumulation of pollution through physiological or other observable changes.

Lichens are interesting for assessing pollution levels because they are sensitive to air composition. The reasons are as follows : 

  • Lichens do not have roots – they have rhizines that allow them to attach to substrates – and therefore absorb the water and the solute substances from the air. 
  • Lichens do not have a protective layer – a cuticle – which means that pollutants can easily enter into the fungal and algae cells.  In fact, unlike plants that have stomata, lichens do not have a system for regulating the entry and exit of gases and water in the thallus. As a result, lichens indiscriminately absorb all substances that reach them, both toxic and nutritious.
  • Lichens are continuously active. All year round, from summer to winter, they are able to photosynthesise, even when temperatures are below -10°C. In winter, lichens are subject to increased pollution (e.g. increased use of electric heating) and remain active despite this.
  • Lichens grow very slowly so that any injury cannot be easily repaired. To give you an extreme example, the growth rate of lichens in harsh environments, such as continental Antarctica, is 1 cm per 1000 years. More information on lichens in Antartica here.

Lichens are affected by pollution and these changes can be seen in their morphologies. We can therefore assess the quality and composition of the habitat by observing the presence and/or absence of certain lichen species. For example, in an old forest in Scotland where the air is clean, you will find many lichens of the genus Usnea (see photo) because these lichens are extremely sensitive to air pollutants such as nitrogen dioxide (mainly emitted by fuel combustion).

Lichen of the genus Usnea, a species sensitive to air pollution. Credit: Usnea filipendula by Bernd Haynold on Wikimedia. CC BY-SA 3.0. 

In cities, on the other hand, we find lichen species that are more tolerant to air pollution. The species that we have seen up until now are all tolerant to nitrogen dioxide (Xanthoria parietinaPhyscia adscendens and Physcia tenellaPhysconia griseaPunctelia subrudecta). What’s more, these species have varied tolerance to pollutants. For example, when Physconia grisea or Melanohalea lichens are present, the air quality will be intermediate. When Evernia prunastri or, generally, foliose species are present, the air quality is good.

The assessment of air quality through studying changes in the distribution and diversity of lichens is increasingly used in cities. If this is done regularly, one can even observe the potential changes in the air composition in the city over time. 

For example, before 1990, sulphur dioxide (SO2), a pollutant emitted mainly by industries, was prevalent in many cities. In 1990, the Clean Air Act and an international treaty – similar to the Kyoto Protocol, a trade in carbon dioxide – were created to decrease the concentration of sulphur dioxide in the air. It worked, seeing the change in the lichen distribution in cities around the world. Lichens that thrived in an environment with SO2 gradually disappeared and other more sensitive lichens repopulated the cities. It is also interesting to think about the impact that reduced pollution has on more-than-human species and the legitimacy of valuing these non-polluted environments at the expense of other species.

Lichens can be affected by changes in their habitat on several levels. For example, temperature change, storms, heavy rainfall can affect lichen communities, as can the surface of a tree trunk. If a crack is created in certain places, it can change the way water flows down the trunk and affect the distribution of lichens. Small details matter. 

For example, a study in England showed that lichen communities are different at the top of a church compared to its bottom, such as in the cemetery. This is partly due to the different air currents. Closer to the ground, the atmospheric pollution is captured by the buildings increasing the air pollution levels. The open flow of air at the top of the church creates a greater dilution of the atmospheric pollutants which is thus less localised. 

Lichens and citizen science 

In the UK, the Natural History Museum in London created a project called OPAL (Open Air Laboratory based on Lichens). The project was based on citizen science. This means that citizens were involved in the data collection and were invited to record and observe lichen species in the area where they lived. 

One of the benefits of citizen science is that it creates a huge database and a better understanding of the dynamics of our surrounding environment. Citizen science also allows citizens to be involved in understanding and observing their environment and learning from it.

This study, OPAL (Open Air Laboratory based on Lichens), allowed us to observe several changes. Firstly, the fact that lichen species tolerant to NO2 (nitrogen dioxide) have become widespread throughout the UK, in cities and in the countryside. This could be caused by traffic, but also by the fertilisers used in fields for intensive agriculture. Some intermediate lichens – which can live in more than one environment – have also been found in towns and cities and therefore have a widespread distribution.

And it’s not just happening in the UK, here in Belgium, Science Infuse has launched a project with the UCL (Université Catholique de Louvain) and suggests the following hypothesis : “the type of lichen association and their frequency can give us a first information about the air quality“. To test this hypothesis, the project is looking for classes of students to start the study… More citizen participation ! For more information, visit the website

Environmental pollution and its effects on humans

Air pollution has an impact on lichens but also on our health. Human exposure to pollution leads to respiratory and cardiovascular diseases. Pollutant particles have also been found lodged in some human’s heart and brain (Loxham et al., 2019). In Europe, 800,000 people die each year from pollution-related causes (Lelieveld et al., 2019). Pollution is a problem of environmental health as much as it is a problem of the health of our bodies (Gabrys, 2020).

This can lead us to rethink urban spaces and their infrastructure to make room for other organisms. We know that lichens, among other organisms, are impacted by changes in the infrastructure and management of the city.

Hence, how can we, as urban dwellers, recognise the presence of other organisms in the city?

It takes learning to recognise and be interrupted by the presence, strengths and forms of organisms of other species than humans and to notice the role they play in the construction and disruption of public places.

Jennifer Gabrys, 2018 


Are you motivated to try and explore the environment which you live in ?
Let’s get started… 
There are plenty of tools at your disposal even if you know nothing about lichens. 

  1. Lichens GO is a great French project that is based on a simple but scientifically sound method. The flyer is available here and contains an identification key on lichens in the city (we’ll talk about what an identification key is later, here) and the method to follow. The creators of the project have even created a video with more information on air pollution and its impact on lichens. If you do this research, don’t forget to share your data collection on the site !
    Unfortunately, the content is only accessible in French. I will share more information in English in the next months.

2. There is Science Infuse, which, as part of the Science Spring festival (printemps des sciences) organised by UCLouvain, invites school pupils to participate in the data collection to examine the following hypothesis: “the type of lichen association and their frequency can give us a first information on air quality“. They follow the methods proposed by Lichen GO but they also have more interesting information

3. Are you curious and would like to learn more about lichens as a bioindication tool ? There are plenty of resources online, including this document from the Encyclopedia of the Environment. Don’t hesitate to contact me, I’m writing my thesis on this subject ! 

Let’s rethink the city together… How can we create cities that are inspired by and responsive to wildlife ? This is a challenge for all of us, city dwellers ! 

Check out the identification guide I have created for urban lichens. It contains the description of 28 urban lichens found on the tree bark. For each species, there is a description of its sensitivity to pollutants in the city !


  • Gabrys, J. (2018). Sensing Lichens: From Ecological Microcosms to Environmental Subjects. Third Text32(2-3), 350-367.
  • Loxham, M., Davies, D. E., & Holgate, S. T. (2019). The health effects of fine particulate air pollution.
  • Lelieveld, J., Klingmüller, K., Pozzer, A., Pöschl, U., Fnais, M., Daiber, A., & Münzel, T. (2019). Cardiovascular disease burden from ambient air pollution in Europe reassessed using novel hazard ratio functions. European heart journal40(20), 1590-1596.

Let’s meet on the Place du Luxembourg at the foot of a charm next to the restaurant Vesterbar👇🏾👇🏾

If you don’t continue the ride, can you give us feedback on your experience here ? It will help us improve!

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