When Buildings Learn to Breathe : Towards a City That Cleans Its Own Air

The idea seems almost poetic: façades capable of capturing and breaking down pollutants simply thanks to daylight. Yet this technology already exists. Several researchers and engineers are developing photocatalytic coatings  “intelligent” materials that turn building surfaces into passive filters.

A scientific review published by Bai et al. in 2023 shows that materials based on titanium dioxide (TiO₂) can purify air, clean themselves, and even destroy certain bacteria. In other words, a wall becomes an active player in urban air quality.

How It Works

The principle is based on photocatalysis.When a material containing titanium dioxide (TiO₂) is exposed to light (mainly ultraviolet, sometimes visible light), its particles become activated. They then release hydroxyl radicals (OH·), highly reactive molecules capable of oxidizing pollutants.

Harmful gases such as nitrogen oxides (NOₓ) or volatile organic compounds (VOCs) settle on the material’s surface, then are transformed into harmless substances such as nitrates, soluble in rainwater.

It is as if the façade were “breathing”: it captures dirty air, treats it chemically, and releases it purified a natural reaction triggered simply by light and humidity. [1]

Walls That Truly Depollute?

The European project Light2CAT, carried out in Copenhagen, Valencia and on a Danish highway, tested photocatalytic concretes capable of acting even under visible light. Researchers observed a reduction in NOₓ between 5 and 20 %, depending on the test sites. In Valencia, pollution levels dropped from 84 ppb to 36.5 ppb  a reduction of more than 50 %. [2]

In France, a similar experiment conducted in a “street canyon” (a re-creation of a dense urban environment) observed a decrease of 62 % for NO and 46 % for NO₂ on walls coated with TiO₂. [3]These results vary according to sunlight, humidity, and air circulation, but they confirm that photocatalytic depollution truly works in real-life conditions.

The Shadows

Despite its promise, the technology is not magical.Several studies recall its physical and practical limitations.

• Dependence on light : Conventional TiO₂ is activated only by UV, which represents just 5 % of sunlight. Efficiency therefore falls in shade or in winter. To overcome this, researchers are testing functionalized variants (with iron, carbon, or graphene) capable of using visible light. [4]

• Durability : Dust, acid rain, or abrasion eventually clog the surface and reduce its efficiency. One study showed a 30 % loss of activity after 1,000 wear cycles. [3]

• Real impact on ambient air : Even if a wall locally breaks down 50 % of the NOₓ that reaches it, air dispersion in the city makes the overall effect more modest, often only a few percent. [5]

These observations remind us that photocatalysis is not a replacement solution but a valuable complement to other urban depollution strategies.

A Living Material in the City

One of the most fascinating aspects of these materials is their autonomy.Unlike a mechanical air purifier, they consume no electricity and require no complex maintenance. In the rain, reaction residues (nitrates, dust) are simply washed away.

Rabajczyk (2021) notes that these surfaces not only depollute but also clean themselves and have an antibacterial effect. Many modern façades or glazing systems already use this “self-cleaning” property, directly inherited from photocatalysis.

Cities thus become material ecosystems: their buildings, instead of suffering from pollution, actively participate in its mitigation.

What Researchers Say

In their 2023 review, Bai et al. analyzed more than 300 studies on photocatalytic materials.They conclude that TiO₂-based functionalized concretes or metal-oxide composites have a real and measurable effect on NOₓ concentration, particularly in high-traffic streets. However, they stress the need to develop visible-light-activated photocatalysts that are more stable and durable.

Other, more recent studies (Tang et al., 2024) explore multilayer nanocoatings capable of purifying air and filtering runoff water at the same time, paving the way for multifunctional, more efficient surfaces.

What makes this approach particularly interesting is its discretion.No visible machinery or energy input everything happens at the microscopic level.The impact is diffuse but cumulative. If thousands of square meters of façades, roofs, or pavements were covered with these materials, the effects could become significant.

A study shows that one kilometer of photocatalytic façade can neutralize up to 20 g of NO₂ per day under certain conditions [5]. That may seem small, but multiplied across an entire district it equals the depollution produced by hundreds of trees.

What Architecture Gains

For architects, this technology opens a new design field:

• Active façades interacting with their environment;

• Self-cleaning materials reducing maintenance costs;

• Functional surfaces providing concrete environmental benefits.

Manufacturers such as Italcementi (TX Active® concrete) are already testing these materials on bridges, tunnels, or public buildings exposed to road pollution. [2]These are tangible examples of buildings capable of playing an ecological role without aesthetic compromise.

Breathing for Tomorrow

It would be naïve to believe that photocatalytic façades will “save” the air of cities.But they belong to a broader trend: that of living materials, able to interact with their surroundings. Like green walls or porous concretes, these coatings express a new philosophy of architecture, designing buildings that contribute positively to their environment.

Research continues to make these surfaces more efficient, more resistant, and especially active under visible light  because that is where their democratization truly lies.

In the near future, walking down a street could mean breathing slightly cleaner air, simply because the buildings around us have learned… to breathe too.

Sources

[1] Bai X. et al., TiO₂-Based Photocatalytic Building Material for Air Purification in Sustainable and Low-Carbon Cities: A Review, Catalysts, 2023, MDPI.

[2] Light2CAT Project, Photocatalytic cementitious materials for air depollution under visible light, CORDIS, 2024.

[3] Tang S. et al., Enhanced Photocatalytic Building Materials: Applications in Exterior Façade and Runoff Purification, Environmental Technology & Innovation, 2024.

[4] Royal Society of Chemistry, Doped TiO₂ and Visible-Light Photocatalysis for Urban Pollution Mitigation, 2023.

[5] Removing NOx Pollution by Photocatalytic Building Materials in Real Life, Fotocatalisis.org / PMC, 2021.

[6] Rabajczyk A., Self-Cleaning Coatings and Surfaces of Modern Building Materials, Materials, 2021.