KEY CONCEPTS

Air stream disinfection happens in the ductwork and “on-the-fly” as air passes through. Bulbs are placed inside ducts and sanitizes the air as it flows through. Air stream disinfection has been used in hospitals and operating rooms to ensure safe spaces. A few key parameters to consider when using UV-C lights in ducts are airflow, air temperature and relative humidity, light exposure time, duct dimensions, reflectivity, and lamp output. Pathogens absorb UV-C light at different rates meaning one system may not be effective for all contaminants. The changing factors are important to consider when installing UV-C in order to effectively eradicate the pathogen.

Upper-air and room disinfection happens in the open room space and has been used in schools, hospitals, and other places of large gatherings. The system is a wall-mounted lamp around seven feet high and a foot below the ceiling. The space allows for mixing of the air. Another design includes a shield fixture to direct the light upwards. Keeping the light in the upper part of the room cleans the circulating air without being in the way of room occupants. The air exchange between the upper part of the room and lower patient area is essential to effective germ eradication.

HVAC coil & surface cleaning UV-C lighting works to accomplish multiple disinfections at the same time. Similar to air stream disinfection, the lighting would be placed inside the duct, but differs with fewer bulbs. The goal of this system is to maintain the HVAC coils and filters, minimizing pathogens and maintenance. When coils are cleaned, the maintenance and replacement rate is lowered and the system can work more efficiently.

Environmental impacts of mitigation efforts

Energy use

    From a sustainability and longevity standpoint, UV lights raise some concerns. The bulbs contain mercury and need to be carefully handled. Strict and safe disposal measures should be implemented. Additionally, the UV-C bulbs are shown to last about a year leading to higher replacement costs and time than other bulbs. According to an ASHRAE journal, the coil cleaning and improved efficiency of HVAC systems can result in 10-15% energy savings. The image below shows dirty coils in week 1 before UV-C lights are introduced. In week 5, the image shows coils that have been cleaned by the UV-C lights. In addition to coil longevity, a National Institute of Standards and Technology workshop on UVGI presents the idea that annual costs for air stream UVGI would be much lower than mechanical ventilation. Continuing to use UVGI but moving away from conventional bulbs towards LEDs may be a tactic to achieve sustainable UVGI. 

UV-LED

The LED version of UV bulbs are a way of not using mercury while benefiting from many other features. UV-LED bulbs can be made much smaller than the traditional mercury bulb. The small design allows the lamp to be placed in more specific areas. Within the selected area, the LED bulbs start working quicker than the conventional bulb. There is no “warm-up” time for LEDs. Furthermore, while conventional bulbs only emit one wavelength, the LED bulbs can be adjusted to target specific pathogens. With a consistent output regardless of temperature, the LED bulbs are effectively used in various environments, something not possible with conventional bulbs. UV LEDs are still in younger phases. More research and testing is needed to fully ensure the effectiveness. 

Considerations 

When using UV-C light near filters, the filter material needs to be carefully chosen. Typical filters use polypropylene and acrylic fibers, both of which are degraded by the use of UV light. Filters made of glass media should be used instead. The education on this is essential for maintenance workers to avoid any unintended air stream particulate contamination. 

ASHRAE addresses two studies involving S. marcescens, a naturally occurring bacteria. When UVGI was coupled with increased humidity, the effectiveness was decreased. Before making changes to the room environment, it is advisable to check cross reactivity.