There is “no compelling evidence” that widespread use of paints, sprays and materials designed to ‘trap’ air pollution on outdoor surfaces will reduce nitrogen dioxide (NO2) levels, according to a government body report.
Furthermore, the report warns that use of such photocatalytic materials could actually lead to emissions of other “undesirable” pollutants such as nitrous acid and formaldehyde “which can have wider impacts on atmospheric chemistry as well as adverse health impacts”.
The findings were published on Defra’s website on Wednesday (April 13) in a report produced by the Air Quality Expert Group (AQEG) – a committee which provides the government with scientific advice on air quality, although it does not advise on health effects.
The report assesses the effectiveness of photocatalytic material that can be used in such applications as paints, sprays, paving, roof tiles, self-cleaning glass and clothing to remove pollution from the atmosphere – mostly nitrogen oxides (NOx) and volatile organic compounds (VOCs).
Photocatalytic paints, for example, are marketed under various names by several commercial companies and there is increased interest in their use, having been trialled twice by the EU and by scientists in London.
In 2012, paving slabs using similar technology were installed in Kendal, Cumbria (see AirQualityNews.com story).
The technology utilises a chemical called titanium dioxide, which acts as a catalyst when exposed to ultraviolet (UV) light, leading to the removal of atmospheric pollutants by converting them into other species.
“It is not physically possible for large enough volumes of air to interact with the surface under normal atmospheric conditions and therefore this method will not remove sufficient molecules of NO2 to have a significant impact on ambient concentrations” -AQEG report
However, assessing the results of various trials undertaken into the using photocatalytic technology on outdoor surfaces, the report notes one particular trial of paint in London which may have reduced NOx concentrations by less than 0.7%, although it describes these findings as “uncertain”.
The AQEG – which includes eminent air quality scientists from universities such as Leicester, York, Birmingham and Kings College London – has therefore not recommended any further field trials are undertaken by Defra.
The report concludes: “Taken as a whole, there is little current evidence to suggest the widespread use of photocatalytic surfaces will reduce ambient concentrations of NO2. Furthermore, there is a risk that these materials will result in the production of other undesirable species such as nitrous acid and formaldehyde.”
The report notes that photocatalytic surfaces aim to reduce pollution already in the atmosphere and therefore “fundamentally differ” from the vast majority of pollution abatement technologies which seek to tackle the problem at the source of pollution, such as road transport.
This makes photocatalytic technologies an “attractive proposition in some respects” as it can be easily and cheaply applied to surfaces and only needs sunlight to function.
“Architectural coatings are not the optimal way to deliver photocatalytic solutions but fabrics comprising high surface area per unit area are” – Professor Anthony J Ryan, University of Sheffield
However, the report emphasises that it is much more difficult to remove pollutants once they have already been released into the air “simply because of the volume of the atmosphere compared to the surface area of any potential abatement technology”.
It is also difficult to assess and control how effective photocatalytic surfaces are in reducing pollution levels.
And, while the report concedes that such material can reduce concentrations of pollutants close to the treated surface, it states that this “will not result in significant reductions in NO2 concentrations in the surrounding air”.
It explains: “It is not physically possible for large enough volumes of air to interact with the surface under normal atmospheric conditions and therefore this method will not remove sufficient molecules of NO2 to have a significant impact on ambient concentrations.”
Case study – University of Sheffield poem
In 2014, a giant ‘catalytic’ Simon Armitage poem was draped over a University of Sheffield building with the aim of both raising awareness of and captuing air pollution (see AirQualityNews.com story).
The fabric poster uses similar technology to photocatalytic paints, but the University of Sheffield team explained that it designed its fabric poster in order to circumvent some of the problems associated with paints highlighted in the AQEG report.
The University of Sheffield’s professor Anthony J Ryan said: “The fabric was chosen because it was a way to get around the problems of limited surface area presented by a paint and the catalyst was applied directly to the fibres throughout the fabric. The effective surface area of the fabric poster is more than ten times the surface area of an equivalent solid poster or a paint and was chosen as a substrate specifically for this reason.
“The reason for this is that the fibers in the fabric are long and thin, and the fabric is not optically dense so that the whole fibre surface is exposed to the light. Architectural coatings are not the optimal way to deliver photocatalytic solutions but fabrics comprising high surface area per unit area are, and for this reason conform to some of the high surface area/high flow rate results of the studies dismissed in the report.”
-Defra AQEG report: ‘Paints and Surfaces for the Removal of Nitrogen Oxides’