# Improvement of Buildings' Air Quality and Energy Consumption Using Air Purifying Paints
## 1 Introduction  
Air quality in indoor spaces has become an increasingly studied aspect of environmental and public health research. To that end, the focus of the building sector and construction industries is to develop innovative technologies to improve indoor air quality (IAQ), given its importance for human health and comfort. One of the technological approaches that were developed is the photocatalytic oxidation (PCO). More specifically, the integration of semiconductor materials like titanium dioxide (TiO\({}_{2}\)) or zinc oxide (ZnO) into paints, coatings, or other building materials results in the absorption of photons from light sources, exciting electrons and generating electron-hole pairs. Subsequently, these electron-hole pairs initiate redox reactions on the material's surface when in contact with water or oxygen,leading to the breakdown of inorganic and organic pollutants [1; 2; 3]. To that end, a new-generation paint with photocatalytic properties has been development based on the PCO technique. Regarding indoor microenvironments where air quality is a growing concern, photocatalytic surfaces actively purify the air by decomposing harmful pollutants such as nitrogen oxides (NOx) and volatile organic compounds (VOCs) into harmless substances. This dual functionality of enhancing esthetics while mitigating IAQ impact positions photocatalytic pains as a cornerstone in sustainable architectural practices. This technique has been hailed as a promising new zero-emission technology that improves indoor air quality by transforming conventional pigments to innovative pains with self-cleaning properties, air purification capabilities, and potential energy harvesting attributes [4]. Although the motivation behind the development of photocatalytic pains is to improve IAQ, the effect in the energy consumption of the buildings could be considered as a significant added value. This interest is particularly pronounced within the stringent regulatory environment of the European Union (EU), which places a vast importance on conserving energy and enhancing living conditions. Energy consumption is a matter of immense environmental concern as the energy usage of buildings, especially through Heating, Ventilation, and Air Conditioning (HVAC) systems, constitutes a considerable share of the total energy usage, contributing to global warming and climate change [5]. Due to these reasons, any strategies that can provide avenues to improve IAQ while also being energy-efficient are deemed of high value and become target areas for regulatory policy-making. Therefore, there is a strong correlation between indoor air quality, thermal comfort, and HVAC systems, which are actually connected on energy consumption as well as energy efficiency. One significant parameter to define thermal comfort is the air temperature within the indoor environment. Nevertheless, apart from thermal comfort, there are also important aspects to evaluate in order to achieve energy efficiency in the built environment. For instance, indoor air quality is an aspect correlated to human health and well-being that is a key issue parameter to energy efficiency as well as the overall energy management of existing buildings. More specifically, and mainly in relation to the ventilation process, poor indoor air quality levels can increase discomfort and also cause serious health problems for users. No-one can easily forget in 2012 based on the World Health Organization survey the 99,000 deaths in Europe because of bad indoor air conditions. As a result and also taking into consideration the European policy [6] for newly constructed and nearly zero refurbished buildings, issues regarding indoor air quality should be placed high on the agenda of directives, standards, and legislation not only as a general term of thermal comfort but separately expressed by different aspects such as daylight, acoustic comfort, and indoor air quality. One significant parameter to define thermal comfort is the air temperature within the indoor environment.