Upper-room Air UVGI

What is Upper-room UVGI
Upper-room UVGI is a safe means of air disinfection that is possible in rooms with high ceilings. In this method, specially designed and installed UV-C fixtures that irradiate only the air above 7 feet constantly disinfect the upper air volume. This is most effective when there is constantly mixed air by fans and HVAC ventilation, but even without strong ventilation or fans, air constantly mixes by movements and normal convective currents.

 Why is upper-room UVGI more effective than UV in ventilation ducts or in room air cleaners?
Upper-room UVGI  disinfects large volumes of room air (above occupants’ heads) at once, resulting in high "equivalent" air changes per hour (ACH) in terms of air disinfection only—UVGI does not dilute odors or CO2, the main functions of building ventilation. Odor control and CO2 removal are accomplished by relatively low levels of ventilation (1 to 2 ACH), but air disinfection requires much higher rates of ventilation (6 to 12 ACH), or the equivalent produced by upper-room UVGI.

Two hospital-controlled studies (1,2) have shown upper-room UVGI to be about 80% effective against tuberculosis (TB) spread. Even when UVGI is confined to the upper room, good air mixing (ideally with low-velocity ceiling fans but easily accomplished by other types of forced-air ventilation) results in very high equivalent ACH in the lower, occupied space—estimated to be an additional 24 ACH in a South African study.1

Is UVGI inside air ducts sufficient for room-air disinfection?
When UV is used in ducts, although it ensures that recirculated air does not have viable pathogens, it unfortunately does relatively little to prevent person-to-person transmission in a room where both an infectious source and other susceptible persons share the same air.

For effective interruption of transmission, air disinfection has to occur in the same room where transmission is occurring. Portable air cleaners can be placed in rooms where there is a risk of transmission but moving large volumes of air through any device is difficult, limited by the clean-air delivery rate of the portable air cleaner.

References
1 Mphaphlele M, Dharmadhikari AS, Jensen PA, Rudnick SN, van Reenen TH, Pagano MA, Leuschner W, Sears TA, Milonova SP, van der Walt M, et al. Institutional tuberculosis transmission. Controlled trial of upper room ultraviolet air disinfection: A basis for new dosing guidelines. Amer J Respir Crit Care Med. 2015;192(4):477-84.
 2 Miller SL. Upper room germicidal ultraviolet systems for air disinfection are ready for wide implementation (editorial). Am J Respir Crit Care Med. 2015;192(4):407-9.