HVAC,Vivarium Design

UV installed in vivarium HVAC systems can act as a secondary barrier against the introduction of biological contamination.

In a vivarium, there are multiple sources of biological contamination, some of which may become airborne and transported in the air currents to another susceptible species, namely a laboratory specimen. The average size of individual bacterium ranges from 0.5 to 5 microns in diameter. Mold spores are much larger, with diameters of 1 to 50 microns. On the other hand, viruses are much smaller, typically less than 0.1 micron in diameter and difficult to filter out of the air. Since these viable particles are so small, they readily carry with the flowing air currents, whether thermally or pressure-driven. Generally, each room in a vivarium is designed with supply and return paths to permit efficient purging of contaminated air, as well as developing a pressure gradient, i.e., positive pressure in the clean room to keep contaminants from leaking in. A large percentage of the air in the facility is returned to the suction of the air handler for pre-heating, cooling, humidification and filtration, and recirculated back to the rooms and areas.

Over the past decades, there has been a growing experience base in applying ultraviolet (UV) technology to disinfect the recirculating air, or the air in a single room, particularly for hospitals, food manufacturing, cleanrooms, and even office buildings to remedy Sick Building Syndrome (SBS).

UV light fixtures have been installed in air handlers and ductwork to disinfect biogrowth on cooling coils, filters, and duct surfaces, and also to provide inline air disinfection to supplement the filters in reducing airborne biological contamination. A secondary, yet cost-recovering, effect of controlling biogrowth on cooling coil surfaces is the restoration of lost energy due to the highly insulating property of the biofilm.

Making the Case for Ultraviolet Light
Commercial UV light has been in existence for decades as a germicidal tool. Pioneering experiments in disinfecting air were performed in the 1920s and 30s. In 1940, Dr. William F. Fells studied its use for reducing the spread of measles in schools. In 1989, Dr. Richard Riley and Dr. Edward Nardell prepared a comprehensive article on applying UV to air disinfection.¹ By installing ceiling-mount UV fixtures in a room, the authors compared the impact of upper air UV equivalent to 20 air changes per hour (ACH). The rule was that one 30-watt UV fixture equaled 20 ACH in a 200 ft² room. From a biological contamination perspective, this installation can significantly reduce the amount of air turned over in a facility.

Research conducted at the National Institutes of Health (NIH) by Farhad Memarzadeh, MD, with assistance from Andrew Manning, MD, indicates that UVGI, used in the proper configuration with adequate ventilation flow rate, can have a significant impact on reducing the number of viable Mycobacterium tuberculosis (TB) bacteria in a patient room.² The researchers also validated the fact that UV can reduce the total airflow required, resulting in a cost savings in the ventilation system, as well as an overall operational cost savings because of the reduced air change requirement. The study evaluated 40 different room configurations and three different combinations of lamp intensity and location. The study produced the following conclusions as related to this paper;

• UV is very effective in killing or inactivating TB bacteria. Even at the lowest UV intensity levels modeled in this study,UV killed more than 50% of the TB bacteria in the room after 5 minutes.
• Use of UV is significantly more effective than increasing the airflow rate (ACH) in the room. Increasing the airflow rate from 6 ACH to 16 ACH results in a 30% reduction in the viable TB bacteria in the room if UV is not utilized. In comparison, utilizing UV with a turnover of 6 ACH results in a 68% decrease in the number of viable bacteria.