# Improvement of Buildings' Air Quality and Energy Consumption Using Air Purifying Paints
## 4 Discussion  
The improvement of IAQ using photocatalytic paints (organic and/or inorganic) under visible light with parallel energy saving is feasible. Based on the outcome of real-scale applications, it was proven that NO and VOCs (toluene) could be effectively removed by the photocatalytic paint, although toluene is shown with less photocatalytic sensitivity than NO under the same photocatalytic process. The latter could be explained by the higher adsorption of NO by the alkaline constituents of the paint due to its acidic property, which significantly increases NO reaction potential with HOx radicals. It is worthwhile to note that HOx radicals are the dominant oxidants in a photocatalytic reaction [50; 51]. On the other hand, toluene is absorbed better in less hydrophilic TiO\({}_{2}\) surfaces than a hydroxylated one [52]. It is clear that the diffusion degree of each pollutant in the paint matrix significantly affects its photocatalytic activity. A photocatalytic reaction is promoted by a more efficient adsorption of the pollutant molecules at the catalyst surface. As far as the energy consumption is concerned, it was calculated that the reduced usage of the ventilation system due to the improved IAQ results in energy saving of more than 7% in the current case study of Demo Houses, promising more optimistic rates of reduction in real-world applications. The simulations proved the strong correlation of the ventilation process and the reduction in the ventilation rate because of the photocatalysis. Laboratory tests of the powder showed a higher degradation rate of pollutants compared with the corresponding values of the real-scale application of the paint. The latter is expected for two reasons. Mixing photo-powder in a paint matrix decreases its active catalytic surface. The paint matrix itself might impede the diffusion of pollutants to the embedded photo-powder particles, affecting the efficiency of the catalytic process by reducing the contact between the pollutants and the active sites. In addition, the paint matrix might encapsulate the photo-powder particles, limiting their exposure to the environment, and preventing the interaction between the pollutants and the active catalytic sites on the powder's surface. Furthermore, factors like temperature, humidity, airflow, and other environmental variables can significantly impact the performance of the photo-powder in degrading pollutants. Although the polyparametric and complex real-world environment results in lower photocatalytic efficiency values of the paint compared to lab-scale tests, it provides a more realistic characterization of the photocatalytic paints' efficiency to improve IAQ. There are many cases in the literature of photocatalytic building materials, which showed very promising results during lab-scale tests, but the latter were not proven when applied to real-world conditions. To that end, it is very important to prove the efficiency of a material under real-scale conditions, due to the elimination of the photocatalytic character when upscaling in a more complex environment. Additionally, in most cases, TiO\({}_{2}\), under UV irradiance, oxidizes not only the air pollutants on the photo-paint surface, but also their organic components. To that end, it is crucial to give special attention to the formulation of the paints (especially inorganic), which will be activated under Vis light. In the current study, the inorganic paint was produced with the same ingredients as the organic formulation except the binder. Instead of using organic polymer emulsion, a water-glass solution was used (potassium silicate). To that end, the formation and emission of by-products during the photocatalysis are eliminated while the mechanical properties of the photo-paint remain constant. More specifically, almost one year after the photo-paintapplication, surface damages have not been observed.