Can Urban Trees Clean the Air? Benefits, Limits and the Green Paradox
Air pollution has been a major global public-health issue for decades. According to the World Health Organization (WHO), ambient outdoor air pollution was associated with an estimated 4.2 million premature deaths worldwide in 2019.
Air pollution has many sources, including fossil-fuel combustion in transport, energy, heating and industry. So can urban trees genuinely help?
Can trees clean outdoor urban air?
In a study published in Environmental Pollution in 2014, researchers from the US Forest Service examined the question: do trees really remove meaningful amounts of air pollution?
Tree leaves and needles can intercept some airborne particles through deposition on their surfaces. Some gaseous pollutants can also be taken up by leaves, including through their stomata. Heavy metals are often carried on particles that can deposit on vegetation. There is no universal 300-metre “clean-air radius”, however. Depending on location, form and urban context, trees and shrubs may modify local concentrations of some pollutants.
Air pollution can also damage leaves and affect physiological processes such as photosynthesis, potentially reducing the capacity of stressed trees to deliver ecosystem services. In other words, trees themselves are living organisms exposed to the same polluted environment, and their capacity to influence air quality has limits.
The expression “pollution-cleaning trees” is therefore too simplistic. Recent research shows that urban vegetation can either improve or worsen local air quality depending on its configuration. In a busy street canyon, a dense canopy can slow ventilation and locally increase some pollutant concentrations; in more open settings, well-placed trees or hedges can promote particle deposition or form a barrier between an emission source and pedestrians. Similar caution applies to exaggerated claims about indoor plants, as explained in a detailed report from the French ecological transition agency (ADEME).
So the picture is more complex than the slogan suggests.
Are some tree species better than others?
Recent research confirms that species choice, tree form and above all placement in the urban environment all matter. Some species can be useful for soil phytoremediation, while birches and other species with rough or hairy leaf surfaces may intercept particles effectively. Other species can also have useful traits, including some alders, oaks, beeches and elms. There is no universal ranking: leaf area and texture, evergreen foliage, biogenic volatile organic compounds, pollen, drought tolerance and street geometry all need to be considered together.
Many of these species are familiar in Loire-Atlantique, where I lead forest therapy walks and training.
Conversely, a large canopy does not automatically mean better air quality. In narrow streets enclosed by buildings, dense crowns — regardless of species — can reduce the dispersion of traffic pollution. Street geometry, wind, foliage density and tree placement can therefore matter as much as species choice.
Evergreen conifers offer another feature: cedars, pines, thujas and spruces retain needles through winter and can therefore intercept particles over a larger part of the year. Again, selection must also consider pollen, biogenic VOC emissions and adaptation to the urban climate.
In some ecosystem-service assessments, models, including examples cited by the city of Salaberry-de-Valleyfield in Quebec, estimate that urban trees can intercept measurable amounts of particles containing heavy metals. Such estimates depend strongly on the model, species, tree size and local pollution levels and should not be extrapolated to every tree.
Are trees a real solution to urban air pollution?
Urban trees are valuable allies for making cities more liveable, through shade, cooling, biodiversity and, in some settings, pollutant deposition. But their direct effect on air pollution is complex: they may reduce some pollutants through deposition or uptake while sometimes slowing street ventilation.
Recent evidence therefore confirms that it is misleading to present trees’ “air-purifying capacity” as
a stand-alone solution to air pollution.
The essential priority remains to reduce the main sources of air pollution in urban areas. Cleaner transport, efficient buildings and industry, cleaner energy systems and better waste management all address pollution at its source.
Nature-based solutions have an important place in cities, but they should never distract from the priority of reducing emissions at source. Tree planting, mobility, urban design, heating, industry and waste management need to be considered together.
In my own small way, whenever you buy a forest therapy gift card on the website, 1% is contributed to decarbonisation projects through Stripe Climate. It is a small contribution — my own hummingbird’s share. ;)
For fine particles and heavy metals, source reduction remains essential, although innovation continues. One example is the European Re-Breath project, which explores ways to reduce particulate emissions from brake wear in urban public transport.
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