Antimicrobial air filter coating with plant extracts against airborne microbes

Antimicrobial air filters are required to protect humans from the risk of secondary bioaerosol pollution as well as airborne particles. Three plant extracts (tea-tree oil, rosemary, and garlic) were selected to replace antimicrobial chemicals in air filters. The antimicrobial activity of plant extracts was investigated using Micrococcus luteus and Escherichia coli. Phytochemicals present in the three plant extracts were identified using a gas chromatograph coupled with a mass spectrometer. The extracts were spray-coated on polyethylene terephthalate filter surfaces using silicate polymeric coating and evaluated via X-ray photoelectron spectroscopy and a scanning electron microscope with energy dispersive spectroscopy. After coating, an increase of 9.1% in the pressure drop was observed. The strain Micrococcus luteus was used to evaluate the antimicrobial activity of the air filter. After bioaerosol exposure, the tea-tree oil-coated filters immediately induced M. luteus cell inactivation (40–55%), whereas the rosemary and garlic coated filters did not. However, 48 h after exposure, a significant M. luteus inactivation of 99.99%, 99.0%, and 99.9% was recorded for concentrations of 2.89, 6.73, and 11.51 mg/cm² for the tea-tree, rosemary, and garlic extracts, respectively. The coated filters exhibited high antimicrobial activity, thereby indicating significant potential for application as self-cleaning air filters.