Practical Applications of Ultraviolet Germicidal Irradiation Systems

a report by
William F. Carey

Chief Executive Officer, NQ Industries, Inc.

The use of ultraviolet germicidal irradiation (UVGI) for the neutralisation of micro-organisms has been studied and documented since the 1930s. Sunlight, which has the entire spectrum of light, emits a broad band of wavelengths from 2,000 angstroms (�) to 40,000�. An angstrom is one hundred millionth of a centimetre and is the measure of the length of the wave (the distance between peaks) to define the energy of radiation. In understandable terms, a sheet of paper is one million angstroms thick. The range between the low end of the spectrum and 900� to 3,000� is the UV range and the upper end of the spectrum, 8,000 to 40,000 is the infrared. The visible range is 4,000 to 8,000�.

Microbes are specifically sensitive to the effects of invisible light at wavelengths of 2,537� due to the resonance of this wavelength with molecular structures. In other words, the microbes have a specific sensitivity to this light and therefore suffer a damaging effect to their molecular bond. The DNA structures of these micro-organisms are damaged and are not able to replicate. The UV band of sunlight is the main cause why microbes and pathogens die in the outside air. The rate of morbidity varies from organism to organism but the total effect remains the same. Moulds and spores are very large in size and have a much longer rate of survival in exposed light while viruses and bacteria have a relatively brief survivability in that same light. The components of commercial grade UVGI systems are utilising a much more concentrated level of UV light than that found in natural sunlight.

A system utilising sufficient levels of UVGI light has been found to be highly effective in preventing the growth of certain organisms in hospitals and studies performed in schools. The Center for Disease Control (CDC) has created guidelines recommended by their Morbidity and Mortality Weekly Report that utilise UVGI in conjunction with high-efficiency particulate air (HEPA) filtration as the best alternative to central heating, ventilation and air-conditioning (HVAC) system filtration. Utilising UVGI in conjunction with HEPA filtration creates a specific, controlled dosage of UV light, which has been proven to kill virtually all known organisms, showing the effectiveness of UVGI given the following conditions:

- Room air mixing is designed in a way to utilise top flow discharge and bottom flow return.

- Dwell time or dosage time of the air passing through the UV light path is sufficient to have a killing effect on micro-organisms.

- The ambient moisture level, which could protect the microbes, is destroyed.

- Keeping the UV lights clean and free from dust.

The control of tuberculosis (TB) is a major protocol in hospital and patient care facilities worldwide. Bacteria called Mycobacterium tuberculosis cause the disease. The disease primarily affects the lungs but can attack any organ in the body. TB is transmitted through the air from one person to another. An infected person sneezing or coughing generates the aerosols or droplets; inhalation of these bacteria may cause infection. The ease in which this disease can be transmitted has been the inherent problem for infection control for millennia. However, not everyone who is infected will develop full-blown symptoms of the disease and become sick. The two specific conditions of TB are latent infection and active TB disease. Both conditions are treatable if diagnosed in time. The effects of UVGI on these Mycobacteria are well documented and are proven to be effective. The amount of UV dosage required to kill TB is 10,000 microwatts per centimetre squared.

The first UV system employed successfully to disinfect water contaminated by pathogens was implemented in Marseilles, France, in 1909. This first experiment was multiplied numerous times and was adapted from water to surface disinfection methods for hospital medical equipment with measured success. The introduction to airborne applications could not be measured or studied as easily with the same proof of efficacy.

The first laboratory studies on UVGI disinfection of air in the 1920s showed such promise that the elimination of airborne disease seemed possible. In 1936 Hart used UVGI to sterilise air in a surgical operating room. In 1937, the first application of UVGI to a school ventilation system reduced the incidence of measles dramatically, with subsequent applications enjoying similar success.2 Further experiments by both Riley and O'Grady resulted in destroying TB bacteria from hospital ward exhaust air.3 These early applications in the use of UVGI were way ahead of their time in both theory and design. The basics of medical disinfection and sterilisation were primarily topical with heat, alcohol and other various chemicals. The first inventions that utilised sterilisation with UVGI were developed in the 1930s but gained no immediate or implied attraction by the medical community.

Not until the 1950s was there any sort of plausible attempt to re-introduce UVGI into the mainstream of physical application. In the 1970s with the onset of energy shortages, architects and engineers began to build structures with little or no leakage of outside air, thereby closing these buildings to little outside air, creating energy conservation but causing a problem that only became known later in the 1980s as sick-building syndrome. The introduction of Legionella bacteria or the result, legionnaires'Õ disease became well-known worldwide. The treatment of these and many other moulds, spores and bacteria were to increase filtration, introduce chlorine and sodium-based chemicals into the airstreams and bring in more outside air to dilute indoor air. These methods worked but some of the cures were possibly as lethal as the disease itself. In this new mindset, a chemically sensitive society is turning slowly back to the thought of utilising UVGI in disinfecting air in critical hospital applications.

There are mainly three types of UVGI applications being utilised in medical disinfection currently:

- Upper room UVGI, which simply introduces a UV device onto an upper wall surface and allows the room convection currents to pass the airborne pathogens in front of the light field to achieve a killing effect.

- Portable, forced air re-circulation equipment utilising both UVGI and HEPA filtration to trap and kill pathogens by introducing an artificial air current to achieve room air mixing.

- Application by addition to existing HVAC filtration systems thereby enhancing the already functioning medical facilityÕs filtration system.

Upper-air UVGI systems have uncontrolled airflow patterns and their performance depends solely on local air currents. The performance of upper air systems does not lend itself to prediction by any empirical methods and limited data is available currently.

Stand-alone re-circulation units are designed for use inside the room. It utilises both barrier filtration as well as UVGI to disinfect the air. It supplements the existing HVAC systems and thereby adds to the room air mixing effect, enhancing air changes per hour for design purposes. The limiting factor is that the device will be seen in its placement in the space to ensure proper airflow patterns and its application to disinfect the air only. In introducing the disinfection appliance at the source of concern, the patterns of control are kept within the room.

In-duct HVAC systems utilise the same forced air method. Such systems are employing surface disinfection primarily to keep the cooling coils and the air filters free from fungal growth. This method is validated by laboratory testing of the filtration systems and has been proven to be effective. However, this design is too limited to the HVAC unit only and relies on the fact that the existing air circulation systems are adequate, designed properly and well maintained. It does not approach the great runs of ductwork to and from the contaminated spaces or the assumption that the room air mixing is both adequate and balanced properly, and the HVAC equipment is sized and maintained properly. This would possibly create a false sense of security utilising this method alone.

There are many different schools of thought on the application of these and other methods of room air disinfection. No single system is completely effective in all applications that may arise or any condition that pre-exists. The CDC uses their protocol with a great deal of conservative verve. The one specific notion that can be surmised from all of the information that exists on the uses of UVGI is that, like general lighting and a typical hospital HVAC system, every application has to be approached in a unique and specific manner. No single system is all encompassing and completely effective. UVGI can not be accepted categorically as an end-all solution to eliminating airborne pathogens forever. These devices all have their place in the medical and commercial sphere, and the proper design and application of one or more of these methods coupled with existing proven practices will move the industry towards a better understanding of the uses and finally the acceptance of UVGI technology.