**3. Titanium dioxide**

Titanium dioxide (TiO2) is a photocatalytic inorganic chemistry that can be applied to a wide variety of surface types to provide antiviral protection. It does not inactivate viruses directly, but acts as a catalyst in the presence of UVA light (wavelength 315 to 400 nm) to generate reactive oxygen species that cause structural damage to viruses. The presence of moisture (in the air or on the surface) and oxygen are necessary for TiO2 to be an effective antiviral agent. Light intensity is also key in driving the photocatalytic reaction. Residual photocatalytic activity may also occur in the dark after exposure to UV light, but is dependent on the prior exposure intensity.

Most of the studies evaluating the antimicrobial effectiveness of TiO2 have focused on bacteria, and data on viruses remains scant in the literature [16]. TiO2 has demonstrated >3 log10 reduction against influenza A within 4 hours, and > 1 log10 inactivation of feline calicivirus within 8 hours [24]. TiO2 coatings have also been modified with fluorine to increase the production of reactive oxygen species under the low UVA-intensity fluorescent lighting that is typically found within indoor settings. Bacteriophage MS2, feline calicivirus, and murine norovirus infectivity levels were reduced by 2.6, 2.0, and 2.6 log10, respectively, on fluorinated TiO2 surfaces [25]. The antiviral action of TiO2 can be further enhanced within indoor environments by the addition of metals [26, 27]. A 1% silver-amended TiO2 formulation yielded >4.00 log10 reduction of influenza A and enterovirus following a 20-minute exposure in the presence of a low intensity (15 W) UVA lamp [28]. More recently, infectious SARS-CoV-2 was reduced to levels below detection on TiO2 and TiO2-Silver (Ag) ceramic-coated tiles within 5 hours of exposure [15].
