The problem with ultraviolet (UV) air purifiers, or combination units that include UV light bulbs, is that the science isn't conclusive. Studies do show that UV lights installed in hospitals or workplace ventilation ducts can kill germs and reduce illness. In one six-year study by the U.S. Centers for Disease Control and Prevention published in 2003, UV lamps installed in the ceiling of a hospital room (not in an air cleaner) inactivated tuberculosis-like bacteria in the room. In a 2003 study published in the British medical journal, The Lancet, germicidal UV lights installed in an office ventilation system significantly reduced workers' breathing problems and overall sickness.
However, most home versions are ineffective, according to both the U.S. Environmental Protection Agency (EPA) and Federal Trade Commission (FTC). "Typical UVGI [ultraviolet germicidal irradiation] cleaners used in homes have limited effectiveness in killing bacteria and molds," the EPA says. "Effective destruction of some viruses and most mold and bacterial spores usually requires much higher UV exposures than a typical home unit provides." For now, it's not clear whether UV air purifiers are worth the extra expense over a good HEPA unit -- and none of our sources highly recommends them.