UVGI — ultraviolet germicidal irradiation, commonly called UV-C — uses ultraviolet light near 254 nanometers to damage the DNA and RNA of bacteria, mold, and viruses so they can't reproduce. In a home it shows up in two forms: a coil-mounted lamp that keeps the AC evaporator coil and condensate pan free of mold and biofilm, and an in-duct air-disinfection system aimed at microbes in the moving airstream.
A useful safety detail: the 254 nm germicidal wavelength does not generate ozone — ozone forms only below about 200 nm — so a properly specified UV-C lamp avoids the byproduct problem that dogs some 'active' air cleaners. The coil-sterilization application is the most consistently worthwhile: a clean coil drains better, moves air more efficiently, and doesn't seed the supply air with mold spores.
Where homeowners get oversold is on what UV-C can do for the air. Disinfecting microbes in a fast-moving duct depends on lamp strength and exposure time, and even an effective system does nothing about particles — dust, pollen, smoke, PM2.5. UV is a complement to filtration and source control, never a replacement. Treat a coil-protection lamp as a reasonable add-on and any whole-air germicidal claim with the same skepticism you'd apply to other active technologies — and confirm the install targets the coil or a properly sized duct section, not just a bulb stuck in the plenum.