Table 1.

Air disinfection and purification systems and techniques.

Technique		Description	Pros and cons	Ref.
Pressurization	Positive and Negative	Differential pressurization refers to measurable differences in air pressure that creates a directional airflow between adjacent spaces.	It requires detailed calculation and simulation to get its benefit and acute monitoring. It may be used alone or in combination with other techniques.	Memarzadeh et al. (2010).
Dilution	Natural, Mechanical and Hybrid	It is one of the easiest methods to remove pathogens. Dilution by ventilation strategies can improve IAQ, reduce energy, and control particles by removal through ventilation.	It is related to air distribution patterns, location of the ventilation inlet and outlet, the physical configuration of space, residents' thermal comfort, etc.	(Memarzadeh et al., 2010; Rackes and Waring, 2014).
Filtration	Mechanical filtration	A method widely used in HVAC systems to improve air quality with High-Efficiency Particulate Air (HEPA) filters in specific areas or through filters treated with antimicrobial agents.	It requires proper installation, maintenance, and monitoring. Due to the small size of the coronavirus, the virus can pass through most filters. However, HEPA filters catch larger particulates containing it.	(Horning and Davis, 2020; Memarzadeh et al., 2010).
	Biofiltration technology	A plant-based technology that can absorb CO2, NO2, SO2 to filter the air. The green wall and microalgae structure are the most common applications.	More research is needed, especially the required infrastructure and maintenance.	(Cummings and Waring, 2020; Packer, 2009).
Purification	Bipolar ionization	Integrated into HVAC systems, high voltage electrodes create reactive ions in the air that react with airborne contaminants, including viruses.	Although it enhances the filtration system, it may emit ozone, and more scientific studies are required.	(Horning and Davis, 2020), ASHRAE, 2015).
	UVGI technology	This technology based on the ability of UVGI to damage the DNA/RNA of pathogens and makes them harmless.	Still under development to avoid adverse health effects on the skin and eyes of humans.	(Bradley, 2020; Goel et al., 2020).
Nanotechnology	Silver nanoparticles	This method utilizes silver nanoparticles to control the viability of pathogens.	Practical but still needs further investigations regarding its possible adverse effects on health.	(Bolashikov and Melikov, 2009).
	Photocatalytic oxidation (PCO)	Recent studies present nanomaterials-based coatings for antibacterial applications. The most common Photocatalyst is TiO2.	It is still under exploration to avoid the potential impact of nanoparticles on human health and the environment.	(Chouirfa et al., 2019; Goel et al., 2020; Megahed, 2014).