# A Promising Technological Approach to Improve Indoor Air Quality
## 1 Introduction  
Indoor air quality (IAQ) is an important determinant of human health, comfort and productivity. For that purpose, high quality indoor air is desirable. Indoor air pollution can be addressed through the two approaches of prevention and removal. The latter includes the use of air cleaning technologies especially in buildings where ventilation rates are being reduced in order to save energy. Indicative air cleaning technologies which have been developed during the recent years are: filtration and adsorption, electrostatic air purification, air filtration and gas adsorption filtration [1; 2; 3], ozonation [4; 5; 6; 7], non-thermal plasma [8; 9] and photocatalytic oxidation (PCO) [10; 11; 12; 13]. PCO is a general air cleaning technology, which is able to degrade Volatile Organic Compounds (VOCs), such as aromatics, alkanes, odor compounds etc. Air cleaning photocatalytic technology is based on the principle that radiation of suitable wave-lengths can be absorbed by semiconductors, which leads to the creation of reactive oxygen species (ROS) that can degrade air pollutants. TiO\({}_{2}\) is the most commonly-used semiconductor in PCO research. However, over the last years, scientists have combined TiO\({}_{2}\) with other materials such as activated carbon and zeolite hybrid catalysts in order to enhance the PCO degradation of air pollutants [14; 15; 16]. Furthermore, numerous TiO\({}_{2}\) photocatalysts with different morphological designshave been developed: nanoparticles, nanotubes, hollow fibers and mesoporous. To this end, the need to evaluate the photocatalytic performance of the above materials in a common methodological approach has been raised and extensive research efforts have been devoted to it [17; 18]. However, in most of these studies, only a single compound was tested, using a photocatalytic reactor and a methodological approach, which have been developed by the same lab that produced the material. It is well known that indoor air contains numerous contaminants; thus, tests of only one compound may be misleading. Furthermore, many studies proved the generation of by-products during the photocatalytic processes, such as formaldehyde, ozone, benzaldehyde, acetaldehyde etc. It is obvious that in some cases, by-products could be more harmful than the target pollutant [19; 20; 21; 22]. For that purpose, and although photocatalytic technology is promising, the synthesis route of a photocatalytic material should be designed carefully in order to avoid contaminants which could lead to the formation and emission in the gas phase of intermediate products as they can be more hazardous than the target pollutant. The latter is clearly demonstrated during the evaluation of two photocatalytic air-purification in a mock-up air cabin. Although two symptoms, dizziness and claustrophobia, were reported to decrease when either one of the photocatalytic air-purification devices was operated, intermediates (acetaldehyde and formaldehyde) were detected as a result of ethanol photodegradation [23; 24]. The present research addresses the indoor air purification study using a photocatalytic paint, which was tested under both laboratory and real world indoor conditions (application in building walls). A modified TiO\({}_{2}\) was chosen as photocatalyst in order to be activated by visible light which is the dominant spectrum in indoor environments. Traditional TiO\({}_{2}\) is activated only under UV light. More specifically a Mn-doped TiO\({}_{2}\) photocatalyst (powder) was used in the synthetic route of a photocatalytic paint production. The powder named TCM-1 [25] has already been successfully tested for the oxidation of air pollutants under indoor-like illumination conditions and when mixed either with calcareous or cementitious base matrices demonstrated a unique ability to efficiently degrade volatile organic compounds (VOCs) such as BTX (Benzene, Toluene, Xylene), formaldehyde and nitrogen oxides (NOx) [26; 27; 28]. Additionally, it has been proven effective on eliminating bacteria such as _E.