The science behind smog-eating surfaces

Air pollution levels in Delhi have spiked to 20–30 times above the levels recommended by the World Health Organisation, prompting widespread public concern. To address it, the government is pursuing “smog-eating” surfaces, among other measures, such as cloud seeding and the installation of anti-smog guns on high-rise buildings.

Smog contains a mixture of volatile hydrocarbons, ozone, oxides of nitrogen and sulphur, and particulates. Here, air pollution is caused by both photochemical reactions involving sunlight, unburned hydrocarbons, and nitrogen oxides, and high concentrations of particulate matter, primarily from car exhaust, construction dust, and biomass burning. It creates a dense haze that reduces visibility and can cause eye, nose, and throat irritation, respiratory problems, and cardiovascular disease.

Earlier this month, Environment Minister Manjinder Singh Sirsa announced several pollution control plans, including a focus on “smog-eating” surfaces, for which the government has signed an MoU with IIT Madras. These will be public surfaces coated with photocatalytic materials, such as titanium dioxide (TiO2), that degrade pollutants.

How does it work?

“Photocatalytic materials are basically semiconductors,” says Dr Pardeep Singh, professor and head at the School of Advanced Chemical Sciences, Shoolini University. “Substances like TiO2₂or zinc oxide are typically used as photocatalysts. When light falls on these semiconductor catalysts, electrons within them jump from one energy level to another, creating a hole. These photocatalytic holes and electrons subsequently form free radicals, which have strong oxidising properties and can break down certain air pollutants with which they interact,” he explains.

The main constituents of smog include nitric oxide, nitrogen dioxide, sulphur dioxide, ozone, and particulate matter (PM). When these interact with photocatalytic materials, some gaseous pollutants — especially nitrogen oxides — can be converted into less harmful compounds, such as nitrates, or decompose into ions, a process known as pollutant mineralisation.

This is new to India, but not to the world. In the Netherlands, photocatalytic concrete coatings were used to reduce local nitrogen oxide (NO) concentrations in the air, rather than greenhouse gas concentrations. In 2017, scientists in Italy developed a type of photocatalytic cement that absorbs pollutants and converts them into harmless salts. Palazzo Italia in Milan, which opened in 2015, is one of the first buildings to use this cement.

“Photocatalytic materials get activated under primarily UV light (and to a lesser extent visible light), so in places where there is less sunlight, there might be an issue, but otherwise they could play a limited, localised role in reducing smog in Delhi,” Singh says.

Impact on health

He adds that researchers typically test these materials under very harsh conditions. “And if they stand up to those conditions, only then do we claim that the photocatalyst is ready for use in the real world.

How efficient are photocatalytic coatings? Dr Ashish Moon, from Smt. Radhikatai Pandav College of Engineering, Nagpur, estimates that, although this depends on environmental conditions, TiO2₂ can locally reduce certain gaseous pollutants such as nitrogen oxides under ideal conditions, rather than absorbing “up to 50%” of all harmful gases. He adds that, when used properly, titanium dioxide does not release harmful gases. He co-authored a 2021 paper titled Detailed Case Study on Smog-Eating Tile.

“It’s better to use it in liquid form than as a powder. I haven’t come across any harmful interactions between TiO2 and particles or gases in the air,” he adds.

However, he cautions that the efficiency of this method depends on atmospheric conditions. “How it works in Delhi depends on the weather conditions and the atmosphere. There should be enough UV light for the nanoparticles to activate,” he says.

However, Sasho Gligorovski of the Chinese Academy of Sciences and his team found that photocatalytic paints can release nanoparticles and volatile organic compounds (VOCs) over time due to surface degradation. Their study showed that while indoor photocatalytic paints containing TiO2₂ decreased nitrogen oxides, they also generated high levels of nitrous oxide, a harmful indoor pollutant.

Singh acknowledges the risk associated with photocatalysts and says it can be reduced by immobilising them on another surface. “Organic or inorganic supports such as activated carbon, graphene, and silicates can help reduce the release of harmful byproducts,” he says. He adds that ZnO or TiO2₂ photocatalytic materials are generally less toxic in bulk form and are therefore used in products such as sunscreen lotions.

However, activists and experts have expressed concerns about this, arguing that the reliance on chemical solutions may exacerbate pollution if not carefully regulated.

Not an immediate solution

Both Singh and Moon feel that photocatalytic coatings should not be seen as a standalone solution. “We have to focus on both prevention and cure. The attention needs to be on tackling vehicular and industrial emissions,” Singh says.

Moreover, the impact may not be immediate, he adds. “You can’t expect results within one or two days. After identifying the surfaces and applying the treatment, we will need to analyse the resulting changes. This will take at least one to two years,” he explains.

He notes that photocatalytic materials, such as TiO2, may not be able to degrade airborne dust. “If particulate matter contains oxides of nitrogen and sulphur adsorbed on its surface, then the photocatalytic material may work on them. But if particulate matter is only made up of sand or inert dust, then photocatalytic materials won’t be able to degrade it.”

Photocatalytic coatings have not shown significant improvements in air quality in some previous studies. A 2023 study from a Spanish city found that although this measure showed potential under controlled conditions, it yielded only a modest reduction in ambient NO2₂ when implemented in real outdoor urban settings. The authors also added that the implementation costs were much higher than the damage costs saved.