# A Promising Technological Approach to Improve Indoor Air Quality
## 4 Discussion  
The effect of photocatalytic paint in the improvement of IAQ has been studied in both laboratory and real scale tests. It is interesting to note that NO could be effectively converted by the photocatalytic paint, while toluene showed lower photocatalytic removal in the same photocatalytic process (lab and real scale). A possible explanation of the different photocatalytic performance of NO and toluene under the same application could be explained as follows: the dominant oxidants in a photocatalytic reaction are hydroxyl radicals (OH\({}_{\bullet}\)) and hydroperoxyl radicals (HO\({}_{2\bullet}\)), which are generated from the redox reactions of positive holes (h\({}^{+}\)) and electrons (e\({}^{-}\)) with O\({}_{2}\), H\({}_{2}\)O or OH\({}^{-}\). The kinetic coefficients of gas phase reactions for HO\({}_{\text{x}}\) radicals and targeted gas (NO and toluene in this case) have only minor differences [46; 47]. However, NO is better adsorbed by the alkaline constituents of the paint due to its acidic property. This significantly increases their reaction potential with HO\({}_{\text{x}}\) radicals. On the other hand, the adsorption capacity of toluene is lower than NO due to the extremely hydroxylated surface of the paint, which constrain its interaction with the active radicals. Toluene is absorbed more easily in a less hydrophilic TiO\({}_{2}\) surface than a more hydroxylated one [41]. It is obvious that different gaseous pollutants present significant variations in their photocatalytic activity due to the difference in their diffusion in the paint matrix. Efficient adsorption of the pollutant molecules at the catalyst surface could promote the photocatalytic reaction. Comparing lab versus real scale experiments results, it is observed that laboratory tests showed significant higher values of both pollutants (NO and toluene) degradation than in real scale application. The translation from the laboratory results to \"real\" site efficiency is difficult because of the great number of parameters involved such as traffic and environmental parameters (temperature, light intensity, relative humidity wind speed). For that purpose, the more complex and polyparametric environment of a real application lead to significant differences in the values, which characterize the photocatalytic efficiency of the paint. In any case, precaution has to be taken with the interpretation of data obtained from the real scale experiments since these results are limited over time. However, they provide an indicative picture of the efficiency of the photocatalytic paints to eliminate air pollutants under real world conditions and a basis on which to improve their photocatalytic capacity for future applications. The need for large scale applications is more imperative nowadays, as many photo-materials have indicated very promising results during lab scale tests, but their capacity under real world conditions has not been proven. The latter is critical and proven by the outcome of the current work, which showed significant differences between the photo-efficiency of the same material under two different scales. Additionally, the demonstration of the effectiveness of photocatalytic materials on site should also include negative effects, such as the formation and emission of by-products during the photocatalytic reactions, as well as the durability of the photo-paint mechanical properties. The latter is of high importance, as in most of the cases, TiO\({}_{2}\) oxidizes not only the air pollutants on the photo-material surface, but also their organic and inorganic components. For that purpose, special attention is given in the formulation of such materials in order to avoid it. As it is not in the scope of the current manuscript to study the durability of the photo-paint mechanical properties, measurements to characterize the mechanical durability of the paint were not performed. However, it is worth mentioning that almost two years after the photo-paint application, cracks or other surface damages have not been observed.