Recent advances have been made in light-emitting diode (LED) technology that emit radiation in the ultraviolet-C (UVC) range, which is used for disinfection of microorganisms. The application of LEDs for air disinfection is interesting, because it allows additional flexibility. The solid state construction is advantageous, Radiation wavelengths can be chosen, the small size allows for creative placement, and they use less power. They allow flexibility in selection of pulse characteristics as well.
I looked into LEDs back in 2008, and published a paper on our results in 2010: Ryan, Kevin, et al. “Inactivation of airborne microorganisms using novel ultraviolet radiation sources in reflective flow-through control devices.” Aerosol Science and Technology 44.7 (2010): 541-550. What we found was that the LED was outrageously expensive, and produced so much waste heat, that the efficiency was too low for any meaningful application (0.3%). We had to operate our LED on a dry ice bath to keep it from over heating! We also found that the LED performance at 265 nm was comparable to mercury vapor lamps at 254 nm.
Another recent paper was published in 2013 exploring the use of LEDs for air disinfection: Wengraitis, Stephen, et al. “Pulsed UV‐C Disinfection of Escherichia coli With Light‐Emitting Diodes, Emitted at Various Repetition Rates and Duty Cycles.”Photochemistry and photobiology 89.1 (2013): 127-131. This paper looked at disinfection of E. coli, which is typically very sensitive to UVC radiation and does not like being aerosolized or airborne. The LED that was used in this study is the same as the one we used, but at 272 nm. Our study exposed airborne organisms to LED radiation, while this study exposed E. coli on agar plates, not exactly an air disinfection scenario. The authors did not estimate the UVC efficiency of their LED, but report that the device consumed 204 mW of energy and produced an irradiance of 5.5 μW/sq-cm. If we estimate the area of the agar plate at 21.5 sq-cm, then the efficiency is ~0.01%. The authors predict that LED pulsing at 10% duty cycle would provide the most energy-efficient disinfection.
More work needs to be done to move this UVC technology along for air applications, but I expect that is will happen, given the past experience with visible light LEDs.