By Kelly M. Pyrek
Pathogenic microorganisms are transmitted in numerous ways in hospitals. One important consideration is the role that the environment plays in pathogen transmission, specifically leading to airborne and waterborne infections.
As Ulrich, et al. (2004) note, "Evidence from many studies leaves no doubt that hospital air quality and ventilation play decisive roles in affecting air concentrations of pathogens such as fungal spores (Aspergillus) and, in this way, have major effects on infection rates. Well-conducted research has linked all of the following to air quality and infection rates: type of air filter, direction of airflow and air pressure, air changes per hour in room, humidity, and ventilation system cleaning and maintenance."
Joseph (2006) explains that airborne pathogens are transmitted in three ways:
- When an environmental reservoir of a pathogen is disturbed, fungal spores may be released into the air and make their way into the hospital environment.
- Microorganisms can also be transmitted directly from person to person in the form of droplets in the air produced by coughing or sneezing within three feet to six feet
- Other infectious diseases such as tuberculosis are transmitted via residuals of droplets that remain indefinitely suspended in the air and can be transported over long distances.
Sources of airborne pathogens are numerous and include:
- Construction and renovation activities
Airborne pathogens such as Aspergillus survive in the air, dust and moisture present in healthcare facilities and are usually released into the air during site construction and renovation.
- Ventilation system contamination and malfunction
Studies have identified the type of air filter, direction of airflow and air pressure, air changes per hour in room, humidity and ventilation-system cleaning and maintenance as factors related to air quality and infection rates. Accumulation of dust and moisture within HVAC systems increases the risk for the spread of environmental fungi and bacteria.
- Vectorborne transmission via pigeon droppings or insect/rodent droppings
Healthcare policies must appropriately address engineering controls for airborne disease, and they must educate and train these workers on the Occupational Safety and Health Administration (OSHA)’s hierarchy of administrative, engineering and personal protective equipment (PPE) for protection in AIIRs, according to Paul Ninomura, PE, chairman of the Project Committee for Standard 170 Ventilation of Healthcare Facilities of the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE).
Some areas of the hospital – such as the operating room (OR) or the emergency department (ED) -- are especially vulnerable to movement by healthcare workers that could stir up infectious microorganisms and particulates. Ninomura and Judene Bartley, MS, MPH, a member of ASHRAE’s Standard 170 Committee, add that when designed in accordance with ASHRAE Standard 170, the OR is a clean area with clean filtered air supplied in a unidirectional airflow from the ceiling to the operating room table. The design utilizes low velocity airflow to minimize turbulence that may occur as surgeons and staff work within the airflow in the operating room. ED waiting rooms have relatively high air exchange rates, and have requirements that all air is exhausted outside, providing a dilution ventilation strategy that controls the propagation of infectious agents from undiagnosed patients with airborne infectious disease.
As Joseph (2006) notes, "The importance of good air quality in controlling and preventing airborne infections in healthcare facilities cannot be overemphasized. Providing clean filtered air and effectively controlling indoor air pollution through ventilation are two key aspects of maintaining good air quality."
Airborne infections can be controlled and prevented by use of the following:
- HEPA filtration
The control of microorganisms and other pollutants and particulates at the source is the most effective way to maintain clean air. Filtration, which is the physical removal of particulates from air, is a key step toward achieving acceptable indoor air quality. HEPA filters are at least 99.97 percent efficient for removing particles 0.3 μm (as a reference, Aspergillus spores are 2.5 to 3.0 μm in diameter), and their efficiency can be increased to 99.99 percent where needed.
After filtration, the second most effective way of controlling the level of pathogens in the air is through ventilation. Ventilation guidelines are defined in terms of air volume per minute per occupant and are based on the assumption that occupants and their activities are responsible for most of the contaminants in the conditioned space. Most ventilation rates for healthcare facilities are expressed as room air changes per hour (ACH). Peak efficiency for particle removal in the air space occurs between 12 ACH and 15 ACH. Ventilation rates vary among different patient-care areas of a healthcare institution and ventilation standards are provided in guidelines from the American Institute of Architects or the American Society of Heating, Cooling and Air-conditioning Engineers (ASHRAE).
According to Ninomura, ASHRAE Standard 170 should be a primary reference along with the FGI Design and Construction of Health Care Facilities when designing a new facility. ASHRAE Standard 170 addresses the removal of airborne pathogens through a combination of filtration, the direct exhaust of contaminated air and dilution via introduction of outdoor air.
One of the most important waterborne pathogens in the hospital environment is Legionellosis, the infection produced by the pathogen Legionella, a bacterium found in water environments. Many hospital water systems are colonized with legionellae, which is introduced into institutional water distribution systems from municipal water systems that do not routinely screen water for the presence of Legionellae.
According to Joseph (2006), waterborne microorganisms proliferate in moist environments and aqueous solutions, especially under warm temperature conditions and presence of a source of nutrition. Waterborne infections spread through direct contact (such as during hydrotherapy), ingestion of contaminated water, indirect contact and inhalation of aerosols dispersed from water sources.
Transmission of Legionella includes inhalation of airborne droplets from bacteria from showers, faucets, room-air humidifiers, cooling towers, evaporative condensers, decorative fountains and medical equipment such as nasogastric tubes and bronchoscopes that are rinsed with contaminated tap water. Immunocompromised patients are most at risk to this bacterium, and the factors that enhance colonization and amplification of legionellae are warm water, water stagnation, and sufficient food for bacteria, including scale, sediment and biofilms.
The Centers for Disease Control and Prevention (CDC), in its Guideline for Environmental Infection Control in Healthcare Facilities, advises following special infection control measures for settings providing care to immunosuppressed patients so that aerosol production is reduced and direct contact with tap water is avoided. Additional measures to prevent exposure of high-risk patients to waterborne pathogen include restricting patients from taking showers if water is contaminated with legionellae; performing supplemental treatment of water for the unit; conducting periodic monitoring/culturing of the unit water supply. Healthcare providers should not use large-volume room air humidifiers that create cold aerosols unless these are subjected to high-level disinfection daily and filled with sterile water.
Engineering controls also are an important way to address the presence of Legionella, and these include ensuring that water temperature and chlorine concentration are correct. As Joseph (2006) affirms, “An important aspect of preventing contamination through the water supply involves designing the water supply system to minimize stagnation and back flow as well as provide temperature control to prevent growth of bacteria.”
Joseph (2006) makes the following recommendations for controlling and eliminating infections transmitted in the hospital environment through air, surface, and water:
- Careful design and maintenance of the HVAC system including incorporation of HEPA filters reduces the threat of airborne diseases.
- Proper precautions during design and construction activities are also critical to preventing the spread of airborne infections.
- Single-bed rooms are strongly recommended from an infection-control perspective—it is easier to isolate infectious pathogens and disinfect single-bed rooms than multi-bed rooms once a patient has been discharged. The threat of infections spread through contact transmission of pathogens is also reduced in single-bed rooms.
- An important aspect of reducing infections spread through surface contact involves providing environmental support for handwashing—visible, conveniently placed sinks, hand sanitizer dispensers, and alcohol handrubs.
- Regular cleaning, maintenance, and testing of water systems and point-of-use fixtures is important for preventing the spread of waterborne infections such as Legionnaires’ disease.
Ninomura and Bartley advise infection preventionists and environmental services professionals to consult ASHRAE Standard 170, Ventilation of Health Care Facilities, which is the source for many of ASHRAE’s best practices for the healthcare environment with respect to minimizing the spread of airborne infectious contaminants. For example:
- The design requirements for the air distribution in an operating room are stipulated including the supply air diffusers, MERV 14 filters/MERV 7 pre-filters, air velocity, etc.
- Use of ducted returns is required for rooms that have pressure relationships with adjacent rooms.
- Relatively high air exchange rates in waiting areas, and with requirement that all air is exhausted outside, provide a dilution ventilation strategy that controls the propagation of infectious agents from undiagnosed patients with airborne infectious disease.