Maureen Spencer, RN, BSN, Med, CIC, Corporate Director of Infection Prevention, Universal Health Services
Helen Boehm Johnson, MD, Pediatrician
Loretta Litz Fauerbach, MS, CIC, Infection Preventionist
Denise Graham, Infection Prevention and Control Consultant
AJIC’s September 2016 supplement focused on an area of increasing concern among infection prevention and control professionals: the degree to which hospital air can be contaminated with pathogens and the potential for airborne transmission of those pathogens. As Sattar et al stated, “Air, a universal environmental equalizer…has profound health implications in all indoor environments” (Sattar).
There is a bounty of evidence documenting the presence of nosocomial organisms, beyond traditional respiratory pathogens, in hospital air (Muszlay, Durmaz, Shiomori, Kumari, Munoz-Price, Gao, Shimose). Numerous bacteria, fungi, and viruses can survive for extended periods of time in the relatively harsh airborne environment and have been captured by both active and passive sampling methods in patient wards, operating rooms, and outpatient waiting rooms (Edmiston, D’Arcy, Shiomori, Lee, Marchand). In many cases, these organisms have shared clonality with the patient or staff isolates (Edmiston, Shiomori, Shimose).
Perhaps more concerning is the fact that aerobiology research has indicated that measured air bioburdens may underestimate true air bioburdens because some vegetative bacteria can enter a non-replicative starvation state in the air which renders them non-culturable by standard microbiological techniques despite the fact the organisms are still viable (Beggs). Multiple outbreaks of virulent pathogens, including methicillin-resistant Staphylococcus aureus (MRSA) and Acinetobacter spp. in a variety of healthcare settings ranging from trauma units to orthopedic wards intensive care units have been linked to airborne transmission (Munoz-Price, Wagenvoort, Bernards, Allen).
Vulnerable patient populations such as those in otolaryngology units where patients lack the normal host defense mechanisms in the upper respiratory tract or burn units where patients lack an adequate skin barrier are particularly at risk for airborne transmission.
With this increasing evidence, the question for the infection preventionist becomes how to treat contaminated air? An answer to that question is ultraviolet germicidal irradiation (UVGI), a relatively recent air disinfection strategy utilizing an “old” technology for air disinfection.
Ultraviolet Germicidal Irradiation: Early Roots
There is a considerable research establishing UVGI’s ability to eliminate pathogens in the air effectively. (2, 3, 5, 6, 7) The CDC states that ultraviolet irradiation of air is an effective means of “reducing the transmission of airborne bacterial and viral infections in hospitals.” (1) The germicidal effect is achieved as UV light induces the formation of thymine dimers in nucleic acids, causing breaks in the DNA of microorganisms preventing them from growing and replicating. Ultraviolet Germicidal Irradiation (UVGI) has a surprisingly long history of air disinfection, dating back as early as the late 1800s. (4) Early uses included disinfection of OR air, barriers in infant wards, and upper room systems to control measles and TB outbreaks. However, as Nicholas Reed discusses in his article, “The History of Ultraviolet Germicidal Irradiation,” the use of UVGI lost momentum in the early to mid-1900s for many reasons. They included the development of antibiotics for treatment of tuberculosis (TB), improved vaccination programs, anxiety over about the risks of UV exposure, and concerns about the high maintenance associated with U V systems. (4) The resurgence of TB and, in particular, multidrug-resistant TB cases in the 1980s prompted researchers to begin to take a second look at UVGI for air disinfection. (4)
UVGI for Air Disinfection: Traditional Options
Historically, there have been two primary types of UV air treatment systems: so-called duct and upper room systems. Duct systems involve placement of UV lamps inside the ducts of HVAC systems. While highly effective at disinfecting cooling coils, drip pans, and other surfaces within the HVAC systems, they can only provide minimal airstream disinfection because of the rapid rate of air movement (400 to 1,000 cubic feet per minute) and therefore minimal UV exposure time. In their review of available literature, Memarzadeh et al found no evidence to support a decrease in HAI rates from the use of UV duct systems. (8) Upper room systems typically involved either suspension of upward-facing irradiation units or parallel systems that deflect the UV light outward. An advantage of these systems is the fact that they are directly treating occupied spaces, the sources of contamination. Unlike HEPA filters, which are used in conjunction with HVAC systems and require the air to pass through the filter in the supply or return ductwork to achieve their effect, upper room UV systems can neutralize airborne microorganisms within the room before they either reach the air filtration system or are allowed to settle. Significant barriers to the implementation of many of these UV systems designed to treat the air, however, have been the structural or engineering changes required to accommodate them.4 Additionally, the air disinfection efficacy of the suspended upper room systems is passive and is dependent on adequate air circulation and mixing necessary to bring the air to the light.2 The inverse square law establishes that the distance from the UV source and the intensity of radiation (and thus the level of germicidal effect) are inversely related such that the shorter the distance from the UV source, the greater the germicidal irradiation. Therefore, if the air does not reach a closer proximity to the units, the germicidal effect is significantly diminished. (9)
Active UVGI for Air Disinfection: A New Approach
Recent advances in UV technology, however, have resulted in the development of active UV upper air/upper room treatment systems that assume the footprint of a standard 2’ x 4’ ceiling panel or light fixture. These systems utilize fans to actively draw air at a rate of 50 ft3 per minute (cfm) through filters that remove larger particles such as dust and then pull the air into a shielded UV chamber where it is irradiated. The emitted light, occurring at a wavelength of 253.7 nanometers (nm), is amplified by mirrored UV chambers that reflect and thereby intensify the light, a proven means of amplifying the irradiation.11 Safety baffles prevent the light from escaping, thus eliminating UV exposure risk to patients and healthcare providers alike. Additionally, the chambers are only three inches deep, allowing them to capitalize on the inverse square law by minimizing the distance between the light and its target. Once the air is irradiated, it is then pushed out of the chambers by fans at a 30-degree angle to promote circulation throughout the room and to prevent the continuous re-treatment of the same air. It is notable that these units do not affect the existing HVAC operation in a room, nor do they affect the pressurization, whether positive or negative, but rather treats and recirculates the air within a room. To further understand this, it is helpful to think about the aerodynamics involved. Globally air moves from an area of higher pressure to lower pressure. In the hospital/building ventilation system, the fan speed and air pressure are greater than that of the UV air treatment system. The system operates at 50 cfm utilizing a pressure differential into the UV-C chamber. The fan blades on the system scoop air and are on the downside of the air stream. Once it passes the fans and UV-C lamp, the air is then on the pressure side of the fan. The system operates similarly to other medical equipment in the room and because of these low fan pressures it does not interfere with the higher fan speeds, air volume and ambient static room pressures from the main HVAC units according to the manufacturer. The room calculation for pressure and room duct pressure is TP=SP+VP. Where TP is total pressure, VP is average velocity pressure and SP is static pressure. (10)
Aerobiology modeling of these new systems has demonstrated an average single pass removal rate of 97 percent for 44 nosocomial pathogens with a known or suspected airborne component in their transmission cycle, including bacteria, viruses, and fungi.11 Perhaps more significantly, for some of the most virulent pathogens, including MRSA, VRE, and C.difficile, the removal rate (reflecting both filtration and UV disinfection) was 100 percent modeled for those pathogens that pass through the chamber. (11)
Air Disinfection is Only One Piece of the Puzzle
Despite the growing evidence supporting the fact that many nosocomial pathogens, including multidrug-resistant organisms, are present in hospital air, it is important to remember that treating hospital air is only one front in the battle against healthcare-associated infections. A comprehensive approach to environmental disinfection is clearly necessary as there is a constant interplay between all aspects of the environment. Airborne pathogens can settle onto surfaces much as disturbances of those surfaces can resuspend pathogens into the air. As Ijaz et al state, “continual redistribution of microbes indoors occurs at the air-surface-air nexus” (Ijaz). Evidence suggests facilities have been making strides with addressing surface contamination, but it might just be time to reconsider addressing the air.
1 Centers for Disease Control and Prevention. 2007 Guideline for Isolation Precautions: Preventing Transmission of Infectious Agents in Healthcare Settings.
2 Fernstrom A, Goldblatt M. Aerobiology and its role in the transmission of disease. J Pathog. 2013. Vol. 2013. doi: 10.1371/journal.pone.0034867
3 Van Griethuysen AJ, Spies-van Rooijen NH, et al. Surveillance of wound infections and a new theatre: unexpected lack of improvement. J Hosp. Infect. 1996; 34(2): 99-106.
4 Reed, NG. The History of UVGI for Air Disinfection. Public Health Rep. 2010; 125(1): 15-27.
5 The American Society for Heating, Refrigeration, and Air-Conditioning Engineers. ASHRAE position document on Filtration and Air Cleaning. 2015.
6 Riley RL, Wells WF, Mills CC, et al. Air hygiene in tuberculosis: quantitative studies of infectivity and control in a pilot ward. Am. Rev Tuberc 1957; 75:420–31.
7 Perkins JE, Bahlke AM, Silverman HF. Effect of ultra-violet irradiation of classrooms on spread of measles in large rural central schools. American Journal Public Health Nations Health 1947; 37:529–37.
8 Memarzadeh F, Olmsted RN, Bartley JM. Applications of ultraviolet germicidal irradiation disinfection in health care facilities: effective adjunct, but not stand-alone technology. American Journal of Infect Control. 2010; 38(5): S13-24.
9 Kowalski WJ, Bahnfleth WP, Mistrick RG. A specular model for UVGI air disinfection systems. International Ultraviolet Association News. 2005; 7(1): 19-26.
10 The Occupational Environmental: Its Evaluation, Control and Management. Second Edition, Edited by Salvatore R. DiNardi, American Industrial Hygiene Association, 2003, p873-8781
11 Kowalski, W., American Green Technology, Inc. Report on the Performance of the VidaShield System. 2011.