Between July and September of 1994 technicians in the clinical microbiology laboratory of Wilford Hall Medical Center at Lackland Air Force Base in Texas found Aspergillus niger in 17 clinical samples submitted for 15 patients at the hospital. This caused a great deal of consternation for two reasons:
1. The incidence of Aspergillus niger had been very low. In fact, there had been only two positive A. niger tests in the preceding twelve months.
2. Exposure to Aspergillus species could lead to very serious consequences for the immunocompromised patients (such as those who had received bone marrow transplants) at the hospital. Such exposure could lead to invasive aspergilliosis which has been shown to be fatal in 85% to 95% of these patients.
The concern was that the hospital might have a serious epidemic on its hands. Except there was something else that was strange. None of the patients were showing any signs of illness and over half of the samples were taken from patients who were in a very low risk group for a positive test including one woman whose urine specimen was taken during a routine obstetrical evaluation.
Fearing the worst, the hospital immediately called in a team of experts. They conducted a series of experiments, but were unable to determine the cause of the high levels of test results for A. niger. As a precaution they asked the laboratory personnel to clean the laboratory biological safety cabinet (BSC). In doing so the lab people noticed that there was a black material on the bottom panel, but they did not take a culture. After six more failed samples, they tested the high efficiency particulate air (HEPA) filters in the BSC and found they needed to be replaced 4.5 years prematurely. Once the HEPA filter in the BSC was replaced the “pseudoepidemic” ended.
Investigation of the events leading up to the outbreak showed just after the BSC was installed there was construction at the hospital to revise the ventilation for the pediatric clinic one floor below the microbiology lab. This included the installation of a new ventilation duct from the pediatric clinic through the lab. To construct the new duct workers jack-hammered a 6- foot by 6-foot hole in the ceiling. No barriers were used to minimize the dust in the laboratory and specimen processing continued as usual during construction. No wonder they had contaminated samples!
As bizarre as this true story sounds, other instances of careless and unsafe construction related incidents continue to happen in other medical facilities. According to the Centers for Disease Control and Prevention (CDC) an estimated 90,000 patients die each year from nosocomial (hospital-acquired) infections. From that number, it is estimated that about 2,000 to 3,000 deaths are associated with infections caused by airborne microbes. It is probable that at least some of these deaths are related to improper construction techniques and controls. Using proper procedures during construction in medical facilities is important for infection control. In this article we would like to review some of these procedures with particular emphasis on controlling contaminants with differential air pressure and filtration.
Air Pressure is Powerful Stuff
Air moves to achieve pressure equilibrium. Consequently, a room that is under positive pressure will have air moving out of it to equalize with the surrounding area. Conversely, a room under negative pressure will retain air in the space and draw air in from the surrounding areas. This phenomenon is very powerful. In fact, most of the winds on earth are created by differential pressure which is why we hear the meteorologists talking about high and low pressure fronts. This differential in air pressure between spaces can be used to control particle movement.
For example, in a hospital there are protective environment (PE) rooms to protect patients from airborne contaminants. These are used for immunocompromised patients such as those who have had bone marrow transplants, are HIV positive, or have open wounds or burns. These rooms are kept with a positive differential pressure in relationship to their surrounding environment. On the other hand, there are also airborne infection isolation rooms (AIIR) that are kept in negative pressure so that whatever is in the room stays in the room and does not contaminate the rest of the indoor environment. A prime example for the use of AII rooms is for the management of patients with tuberculosis.
Differences in pressure are created by mechanically adjusting the supply and exhaust air. A negative pressure room will have greater exhaust air (going out of the space) than supply air. A positive pressure room will have more exhaust air than supply air. According to the American Institute of Architects (AIA) “Guidelines for the Construction of Health Care Facilities” the pressure differential should be about 0.01” water gauge (w.g.) or about 2.5 Pascals (Pa) for AII rooms and a pressure differential of 0.03” w.g. or 8 Pascals for PE rooms. For construction purposes the difference in pressure can be greater but never less than the above recommendations.
To simplify this a little, just remember that if you want positive pressure in a space so that you can minimize the introduction of particles, you bring in air from outside the space. If you want negative pressure so that you can minimize the release of particles from a space, you need to exhaust air outside that space.
Filtering the Air for Particle Control
The proper differential pressure allows us to keep construction particles flowing in the desired direction. The next step is removal of those particles from the air before they can create a problem – either to the health care facility and its patients or to the outside environment. To accomplish this we need to use an array of different filters from a polyester pad or pleated filter used as a pre-filter to a final HEPA (High Efficiency Particulate Air) that is challenge tested with results showing a removal efficiency of 99.97% of the particles 0.3 microns and above.
Health care facilities have special filtration requirements depending upon the purpose of the area served. These requirements are specified by various bodies including the AIA, the CDC, the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) and others. While these requirements go beyond the scope of this article, they are readily available through public sources. Suffice it to say that the filtration requirements are designed to provide the best and safest air for patients and health workers.
Of equal importance is the proper installation of filters and filtration devices. Following sound installation procedures such as gasketing and “in-place” challenge testing is the only way to ensure the filters will perform as required.
The third important component of controlling airborne infection is ventilation to provide enough air exchanges per hour (ACH) for the dilution of pollution. We don’t have enough space here to discuss this component in detail. Recommendations call for 12 ACH for both PE and AII rooms.
Recommendations for Controlling Contaminants In Medical Facilities During Construction Activities
There are three basic types of construction in and around health care facilities that need to be dealt with for particle and contaminant control. The first is construction (or demolition) that is taking place near an existing facility. The second is the construction of a new facility. The third is reconstruction or adding on to an existing facility. Each has different differential pressure and filtration requirements.
First, would be the control of contaminants in a health care facility in close proximity to construction (or demolition). The most obvious problems would be a big increase in construction dust and unearthing and aerosolizing of microorganisms. It is recommended that all pre-filters and all standard filters used in non-critical areas be changed before construction begins to minimize disruption during the construction process. It is very important to check all filters regularly during construction since they obviously will get dirty quicker. Replace as needed.
While this sounds easy, unfortunately, it is where some people involved in the maintenance of health care facilities decide to become “creative.” The first thing they think about doing is to decrease the efficiency of the filtration. After all, the existing filtration is loading up with contaminants way too quickly. Less efficient filters will load slower and will last longer. This has the consequence of final filters loading more quickly (at a much higher replacement cost) and all of the components of the HVAC system including coils and duct work building up with more dirt. This results in higher HVAC system cleaning costs and lower operating efficiencies. So, it is important to resist the temptation to go with less efficient filters. Likewise, it is also important to replace filters when they are loaded. Loaded filters can drop air velocity to the extent that the building can become depressurized. More frequent filter changes is just a cost of doing business in a health care facility in close proximity to a construction site.
The second “creative” approach is to shut down outside air intakes on the HVAC system. The HVAC systems of health care facilities are designed to take in “fresh” air at a ratio of anywhere from 10% to 100% of the return air in the system. This is done for a number of reasons including the dilution of pollution and the replenishment of oxygen in the building. They also do it to create POSITIVE PRESSURE. This positive pressure is the best defense against infiltration of the contaminants from the construction site. When someone closes the dampers and shuts down the outside air intake they might think they are keeping the dirt out of the HVAC system, but they are depressurizing the building and allowing contamination of the structure, the equipment and the patients in the facility. If anything, the dampers on outside air intakes should be kept open at all times during the construction to maintain adequate positive differential pressure.
New construction of a health care facility generates its own set of issues on avoiding contamination. Most health care building construction today involves the completion of the outside building envelope and then the activation of the HVAC system. This is done before any interior finish out. This procedure helps to “dry out” the building. However, it does present some challenges to keep the HVAC system clean. If possible, cutting return openings in the duct work is done very late in the construction process. In this case the system is run using outside air.
Once the return air openings have been cut they should be covered with filtration media or pleated filters. These filters load quickly with construction dust and should be changed frequently. Changing these construction filters on a weekly basis is not unusual.
Depending on the amount of return air used and the speed of the HVAC fan, there can be very high air velocities at these openings. This usually necessitates the installation of a temporary filter rack with a support grid to keep the filter in place and to avoid “bowing” of the filter.
Before turning on the HVAC system, construction filters should be installed in the holding frames or modular track framing systems. According to the National Air Filtration Association “Installation, Operation and Maintenance of Air Filtration Systems Manual” (IOM Manual) these filters should be at least the same type, efficiency and capacity as the filters specified for the operating system. These filters should be replaced when they are “loaded” as indicated by pressure drop measurements or when the HVAC system is ready to be turned over to the building owners.
The NAFA IOM Manual recommends that in any supply system incorporating HEPA filters that they be installed as soon as ductwork is completed and cleaned so that the ductwork will stay in this condition. To prevent damage to the HEPA filter the IOM Manual also recommends covering the face of the filters with temporary covers until the ductwork containing the filter is put into operation.
Proper installation of filters is extremely important in health care facilities. Special attention should be given to ensure that filter banks have the proper spacers, seals and gaskets to provide an airtight fit and avoid filter bypass. The consequences of improper air filter installation can be substantial. Dr. Jeffrey Siegel of the University of Texas at Austin has found that small gaps between filters can dramatically drop filter efficiency. For example, he found that with a 10-millimeter gap between filters the efficiency of a MERV 15 filter would be reduced to that of a MERV 8 filter.
Air follows the path of least resistance. Generally, the higher the MERV, the greater the resistance and the more loss in efficiency to air bypass. Considering the fact that many hospital applications require MERV 14 filters as the final filter, the consequences of improper filter fit can be substantial.
And now for the biggest challenge – what does one do to control contaminants in a health care facility in operation during construction or reconstruction at the facility? For advice on this subject we talked with Andrew J. Streifel, MPH, of the University of Minnesota. He is a hospital environment specialist and has been involved in this field for over 25 years. He is on the AIA guidelines revision committee and has served as a technical expert for the CDC on environmental infection control issues pertaining to air and water.
“Communication is the key to a successful project. Everybody has to be involved. That includes the staff, medical practitioners, contractors, architects and engineers.” said Streifel. In order to facilitate this communication, Streifel was involved in developing the “Infection Control Risk Assessment Matrix of Precautions for Construction & Renovation” for the American Society of Healthcare Engineering (ASHE) of the American Hospital Association (www.ashe.org/ashe/codes/cdc/index.html).
This “decision-tree” document lays out the types of construction (Type A being inspection and non-invasive activities going to Type D which is major demolition and construction), and an identification of the patient risk groups (patients in office areas to patients in AII rooms) into a matrix. The end results are “Classes of Precautions” required for each project type. Each Class has specific precautions required. For example, it states that a negative air machine with a HEPA filter needs to be used in Class III and Class IV construction projects.
Both Class III and Class IV construction projects use the same basic precautions when it comes to differential pressure and filtration. The first step is to seal off the area where the construction is being done. This can either be accomplished with sheetrock, plywood or plastic. The HVAC system should be removed or sealed to prevent contamination of the duct system. Once the sealing has been completed it is necessary to use a negative air machine. These machines have a blower and run the air through a HEPA filter before exhausting it to the outside of the building. The machines create negative differential pressure and remove airborne contaminants from inside the construction area.
The HEPA filters on a negative air machine can load up quickly with large particles unless they are pre-filtered. Polyester pads make an inexpensive but effective pre-filter for most applications.
Using proper air pressure differentials and proper filtration play a major role in controlling contaminants during construction at health care facilities. How well do they work? According to Streifel, who has assisted more than 400 hospitals with environmental control issues pertaining to air and water, they are among the most important considerations: “If you have your pressure, filtration and ventilation rates right, you will be able to keep the air clean. Each of these physical ventilation parameters needs to be measured and monitored regularly. Particle counts and sampling for fungi are useful to reassure you that things are working correctly. But it is the air control functions that prevent infection.”