Aims: Indoor air quality (IAQ) has attracted increased attention with the emergence of COVID-19. Ventilation is perhaps the area in which the most changes have been proposed in response to the emergency caused by this virus. However, other strategies are possible, such as source control and the extraction of pollutants. The latter incorporates clean technologies, an emergent area with respect to IAQ. Method: Various air treatment technologies can be used to control contaminants, which are reviewed and discussed in this work, including physicochemical technologies (e.g., filtration, adsorption, UV-photocatalytic oxidation, ultraviolet disinfection and ionization) and biological technologies (e.g., plant purification methods and microalgae-based methods). Results and interpretation: This work reviews currently available solutions and technologies for “cleaning” indoor air, with a focus on their advantages and disadvantages. One of the most common problems in this area is the emission of pollutants that are sometimes more dangerous to human health than those that the technologies were developed to remove. Another aspect to consider is the limitation of each technology in relation to the type of pollutants that need to be removed. Each of the investigated technologies works well for a family of pollutants with similar characteristics, but it is not applicable to all pollutant types. Thus, the optimal solution may involve the use of a combination of technologies to extend the scope of application, in addition to the development of new materials, for example, through the use of nanotechnology.

An approach for detecting hazardous volatile organic compounds (VOCs) in ppb and sub-ppb concentrations is presented. Using three types of metal oxide semiconductor (MOS) gas sensors in temperature cycled operation, formaldehyde, benzene and naphthalene in trace concentrations, reflecting threshold limit values as proposed by the WHO and European national health institutions, are successfully identified against a varying ethanol background of up to 2 ppm. For signal processing, linear discriminant analysis is applied to single sensor data and sensor fusion data. Integrated field test sensor systems for monitoring of indoor air quality (IAQ) using the same types of gas sensors were characterized using the same gas measurement setup and data processing. Performance of the systems is reduced due to gas emissions from the hardware components. These contaminations have been investigated using analytical methods. Despite the reduced sensitivity, concentrations of the target VOCs in the ppb range (100 ppb of formaldehyde; 5 ppb of benzene; 20 ppb of naphthalene) are still clearly detectable with the systems, especially when using the sensor fusion method for combining data of the different MOS sensor types.

The world population is ageing, in particular in the developed world, with a significant increase in the percentage of people above 60 years old. They represent a segment of the population that is more vulnerable to adverse environmental conditions. Among them, indoor air quality is one of the most relevant, as elders spend comparatively more time indoors than younger generations. Furthermore, the recent COVID-19 pandemic contributed immensely to raising awareness of the importance of breathing air quality for human health and of the fact that indoor air is a vector for airborne infections and poisoning. Hence, this work reviews the state of the art regarding indoor air quality in elderly centers, considering the type of pollutants involved, their emission sources, and their health effects. Moreover, the influence of ventilation on air quality is also addressed. Notwithstanding the potential health problems with the corresponding costs and morbidity effects, only a few studies have considered explicitly indoor air quality and its impacts on elderly health. More studies are, therefore, necessary to objectively identify what are the impacts on the health of elderly people due to the quality of indoor air and how it can be improved, either by reducing the pollutants emission sources or by more adequate ventilation and thermal comfort strategies.