Painting over the cracks – a short-term fix for air pollution?

Picture of the road A401 by Shepherd's Bush Green, flanked by tower blocks
A401, Shepherd’s Bush Green (c) Oxyman

The Grantham Institute’s Dr Andreas Kafizas blogs on how light-activated paint and coatings can help tackle air pollution, and whether or not they are effective.

Air pollution is a public health emergency. Imperial research shows that poor air quality in London leads to around 1,000 hospital admissions for asthma and serious lung conditions every year. For the UK as a whole, it is estimated that around 40,000 deaths per year are due to poor air quality. In many cities across the UK – such as Southampton, Sheffield, and London – the safety limit for air quality is breached daily.

Steps are being taken to reduce air pollution in cities by, for example, reducing the amount of traffic through congestion charging, encouraging active travel like walking and cycling, and investing in infrastructure to support the transition to electric vehicles. However, it will take time for these measures to take effect. So what can be done to reduce air pollution now?

Paint technology that cleans the air

One of the main components of air pollution are nitrogen oxide gases (NOx), which are formed when fuels are burnt (e.g. diesel in cars). These harmful pollutants can be removed from the air using new paint and coating technologies placed on surfaces in areas of high pollution. These technologies contain titanium dioxide (TiO2), a photocatalyst that, in the presence of sunlight, can convert NOx gases into less harmful nitrates. Applying these photocatalytic paints and coatings to the façade of buildings, pavements and road surfaces could be a low-cost way to help reduce air pollution in the short term.

The technology has already been used successfully. In 2014, scientists at the University of Sheffield’s Grantham Centre for Sustainable Futures coated a giant cloth with photocatalytic paint and draped it over a university building as part of an exhibition to raise awareness about air pollution. Over the course of the exhibition, the project team estimate that it removed over two tons of nitrogen oxide from the surrounding environment.

A university building in Sheffield covered with a big cloth with poetry written on it. A bus is in the foreground
(c) University of Sheffield

But are these technologies always effective?

Titanium dioxide is a versatile photocatalyst. It can be used to convert water into hydrogen fuel and carbon dioxide into carbon-based fuels. It can also purify polluted water streams, and kill bacteria. Photocatalytic technologies are already being applied in a range of products; including self-cleaning windows and tiles, as well as air purifiers. As a result, the market for these technologies is growing, and is now estimated to be is worth ~$3 billion worldwide.

A number of well-known companies are marketing photocatalytic paints and coatings to target NOx pollution in air; however, their effectiveness varies. There is no doubt that, in laboratory conditions, these paints and coatings can remove up to 60% of NOx from a polluted gas stream. However, when applied outdoors, their performance has been less consistent. Results range from a 23% reduction in levels of ambient NOx, to nothing at all. This inconsistency in performance is caused, in part, by differences in the conditions used to test paints and coatings in the laboratory, compared with the conditions found outside.

Photocatalytic products are also used in commercially available indoor air purifiers. In a recent study that compared the effectiveness of a wide range of models, some proved to be very effective in improving air quality; others however, actually made air quality worse.

In the UK, there is currently no requirement for photocatalytic products to be tested and certified. The International Organization for Standardization (ISO) have developed various tests for examining the effectiveness of photocatalytic products. Athough this is a step in the right direction, the ISO test for examining paints and coatings only uses nitric oxide (NO) gas as the pollution source. Polluted air contains a mixture of both NO and nitrous dioxide (NO2) gases. This is important as NO2 is around 25 times more toxic than NO.

An invitation to the Air Pollution Simulation Chamber

A picture of PhD student Megan Taylor using the Air Pollution Simulation Chamber
Using the Air Pollution Simulation Chamber, PhD student Megan Taylor will develop new photocatalytic coatings that can purify polluted air using ambient light. (c) Andreas Kafizas

Here at Imperial, we are addressing this issue. My Solar Coatings research group has built an Air Pollution Simulation Chamber that can accommodate both NO and NO2 gases. In this chamber, we can set the humidity, wind speed and light level, and therefore mimic the conditions found in a polluted environment. Using this Air Pollution Simulation Chamber, we will study how commercial photocatalytic paints and coatings perform under conditions that are more akin to their target area of application – like a busy road in London. This will give us a greater insight into the potential performance and impact of these coatings on air quality. Our findings will help inform a more rigorous testing process for photocatalytic products, to ensure that products on the market really are as effective as they say they are.

If you are interested in finding out more about the Air Pollution Simulation Chamber, or have any questions about the work of the Solar Coatings group, please contact Dr Andreas Kafizas – e-mail:; twitter: @CoatingsSolar

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