Table 4.

Analysis of air purifiers.

Filter types	Specifications	Applications	Key findings	References
Activated carbon	Obtained from carbonaceous materials, have a large surface area and porosity.	Find its application in adsorbing NO2 and VOCs.	Cost friendly, possess good chemical, thermal and mechanical stability, cannot completely remove contaminants.	(Molina-Sabio & Rodríguez-Reinoso, 2004; Mondal & Saha, 2019; Nowicki et al., 2009)
HEPA filter	Removes up to 99.97% of particles that pass through.	Used worldwide in cleaning medical rooms and are placed in AHU for filtration of dust.	Costly, very efficient, Regular filter replacements, cannot remove particles size 200−300 nm, require pre-filters.	(Bolashikov & Melikov, 2007; Dey et al., 2017; Vijayan et al., 2015; Yang, 2012)
PCO	Uses metal-oxide semiconductors having high photocatalytic activity.	For the combined treatment of diverse pollutants	Complete reaction, ease of operation and maintenance, release ozone as a by-product	(Bolashikov & Melikov, 2007; Destaillats et al., 2012; Xu et al., 2018; Zhao & Yang, 2003)
Germicidal UV	Uses UV-C band wavelength radiation.	Widely used in hospitals for disinfecting surfaces.	Directly damages DNA of pathogens, releases ozone gas, require pre-filters	(Chang et al., 1985; Kierat et al., 2020; Nakpan et al., 2019; Wielick et al., 2021)
Ionization filter	Use high dc voltage through corona discharge	In industries and as portable purifiers in residential buildings	Low energy costs, reduced formation of harmful emissions, potential health benefits, excessive electrostatic discharge charges objects, O3 emissions	(Dong et al., 2019; Grabarczyk, 2001; Shiue et al., 2011)