Background There is a clear need for action at national and EU levels to improve urban air quality. Although major efforts have been made, an EC analysis showed that significant negative impacts will persist despite current legislation. Therefore, depollution methods are required for achieving levels of air quality that do not pose risks to human health and the environment. Recently, photocatalytic self-cleaning and depolluting materials have been suggested as a remediation technology, mainly for nitrogen oxides (NOx) and aromatic Volatile Organic Compounds (VOCs) in the urban environment. The associated technologies are based on the photocatalytic properties of a thin layer of titanium dioxide (TiO2) on the surface of a material (such as glass, walls or pavements) or embedded in paints or concrete. However, to date, the take-up of this technology has been low. Objectives The PhotoPaq project aimed to demonstrate the ability of photocatalytic materials for air pollution reduction in urban environments. The feasibility of using TiO2-based products to alleviate air pollution problems was evaluated in simulation laboratory chambers and outdoors in Belgium and Italy under real atmospheric conditions, in combination with numerical modelling. PhotoPaq also sought to increase the acceptance and take-up of this emerging technology. Results The PhotoPaq project generated a massive dataset on the photocatalytic reactivity of a variety of compounds. This was organised within a large database, developed from data obtained from laboratory testing and pilot campaigns. The performance of several types of photocatalytic material was successfully determined, while the project generated realistic estimates of photocatalytic air remediation possibilities in urban environments. Although the experimental methods of atmospheric depollution were very effective in laboratory conditions, the findings of PhotoPAQ showed that they cannot be applied to outdoor situations where environmental parameters are highly fluctuating. The low reductions in pollution in field measurement campaigns were mostly attributed to transport limitations of the pollutants on the active surfaces. The project is a step towards filling the gap between R&D and the industrial application of photocatalytic environmental technology. The numerous environmental benefits arising from PhotoPaq are therefore indirect, because the project mostly discarded the photocatalytic potential of a variety of materials for particular conditions. These indirect benefits include an in-depth understanding of the heterogeneous reactivity of some major pollutants, the invalidation of lower-reactivity photocatalytic materials for urban tunnel use, the determination of a reactivity threshold for outdoor applications, the development of an easy-to-use test procedure prior to large-scale implementation, and the development of a numerical simulation tool to assess the NOx reduction effects of photocatalytic coatings in tunnel environments. As the unique PhotoPAQ dataset comes into the public domain, it can be used in the long-term prior to any new implementation of photocatalytic materials in European cities. The project represents a successful model of international collaboration, with CNRS partnered with six major organisations from France, Germany, Belgium, Italy and Greece. It achieved international recognition thanks to its effective communication and dissemination strategy, which included three PhotoPaq workshops. The project published recommendations on the testing of photocatalytic material prior to their installation. The demonstration value of the PhotoPAQ project resides in the application of photocatalysis in specific urban conditions, namely a city centre tunnel with extreme traffic density (Brussel's Léopold II tunnel, Belgium) and a street canyon in an industrial area (a factory complex in Bergamo, Italy). Measurements from these campaigns were compiled in a digital model, which can be used to generate a set of parameters and compute levels of pollution abatement by photocatalysis in other environmental conditions. This will be of value to other European cities facing problems of excessive NOx and VOCs concentrations. Once photocatalytic technology has evolved into an economically-viable solution, to which PhotoPaq has contributed, it will initiate the rise of a specialised branch of the building industry with significant socio-economic impacts. Further information on the project can be found in the project's layman report and After-LIFE Communication Plan (see "Read more" section).