Air filtration has always played an important role in health care facilities. However, the role of air filtration has been elevated as the manner in which health care is provided continues to evolve. Concerns over hospital-acquired infections and bioterrorism have propelled filtration solutions into the forefront as a primary tool for both infection control and engineering staff members. The main applications where air filtration solutions are employed are infection control, odor control, protection of mechanical equipment, facility cleanliness, and bioterrorism applications.

Infection Control

Infection control in health care facilities deals largely with protecting building occupants from patients with communicable infectious diseases. Infections, which may result from activities and procedures taking place within the facility, have also become a cause for great concern. The principal measures used to control airborne infections are the containment of spaces with potentially contaminated air, and removal of airborne contaminants from those contained spaces either by exhaust or filtration.

The three areas within a facility with the greatest concern for communicable disease control are rooms with known infectious cases (e.g., airborne infectious isolation rooms or AIIR), special treatment or procedures rooms (e.g., bronchoscopy, sputum induction, etc.), and areas that are most likely to contain undiagnosed infectious patients (e.g., emergency department waiting areas and treatment rooms). The likelihood of airborne contaminants leaving these rooms is reduced by keeping the rooms under negative pressure, relative to surrounding areas. Air is exhausted from these rooms either directly to the outside or through HEPA filters.

HEPA filters are used when exhausting air to the outside is not practical, when the exhaust is located near a potential air intake, or when the air is recirculated within the space in order to increase ach while reducing the total exhaust requirements. They have been chosen for these applications due to their high removal efficiency on micrcontaminants as well as the predictable and repeatable nature of their performance. HEPA filters have a minimum initial efficiency of 99.97% for removing particles 0.3 microns in size. Each HEPA filter is individually tested at the factory in order to confirm their conformance to this standard. They may also be field-tested in order to confirm their ongoing adherence to efficiency requirements.

HEPA filters are tested for their efficiency at the 0.3 micron particle size because it is theorized that this is the most penetrating particle size for this type of filter. In other words, HEPA filters become more efficient when removing particles both larger and smaller than 0.3 microns. This is a critical point as these filters are being used to remove mold and bacteria, typically 1 to 5 microns in size when airborne, as well as viral particles which are submicron in size.

Using HEPA filters may provide several advantages over direct exhaust in certain infection control applications. When exhausting from a potentially infectious area, the ductwork should be a dedicated (not shared) run that has all penetrations and seams sealed. The duct run needs to be labeled as containing potentially contaminated air.

Labeling the ductwork helps prevent unnecessary exposure to maintenance personnel who may unknowingly cut into the ductwork for the purpose of testing airflow or repairing equipment. Using a HEPA filter at the point of exhaust in the room allows you to use non-sealed ductwork (after the HEPA), which may be on a shared exhaust run. The ductwork located after the HEPA filter does not need to be labeled as potentially contaminated. Also, in areas where exhaust to the outside is not feasible, HEPA filters may be installed in order to remove contaminants prior to recirculation of air within the room or to other areas within the facility.

Protective Environments

Unlike applications which provide a contained environment around the infectious patient, protective environments provide a sanitary setting for immune compromised patients by guarding them from contaminants which may be generated in other areas of the facility. The immune systems of cancer and transplant patients are intentionally suppressed in order to prevent their bodies from fighting off treatments or rejecting transplants. Other patients have weakened immune systems as a result of disease (e.g., AIDS/HIV). These immune compromised individuals make up an increasingly large percentage of the patient population in today's health care facilities.

Immune compromised patients have been shown to be susceptible to infections resulting from exposure to airborne molds whose source may be renovation work being performed within the facility. These molds would not normally provide a threat to individuals with a healthy immune system. Protective environments are designed to reduce a patient's exposure to these normally innocuous contaminants, allowing their bodies to focus on healing rather than fighting off infection.

Protective environments are provided with HEPA filtered supply air and are typically kept under positive pressure to surrounding areas. This greatly reduces the risk of airborne contaminants entering the room either through the supply air or through the room door. HEPA filters are also often used to recirculate the air within these rooms in order to provide additional ach of clean air.

A unique challenge occurs when there is an immune compromised patient who also has a communicable infectious disease such as tuberculosis (TB). This patient needs to be in a protective environment for his own health but also needs to be isolated to protect others from his communicable disease. This can be achieved by having within the facility a positive pressure, protective environment room with an anteroom that is under negative pressure relative to the corridor and protective environment.

Renovation Considerations

As mentioned above, renovations within a health care facility have been shown to be a source of airborne pathogens such as aspergillis mold which may be harmful to immune compromised patients. In order to prevent these contaminants from spreading throughout the facility, new source capture and control recommendations have been made. In 2003, the Centers for Disease Control (CDC) published Guidelines for Environmental Infection Control in Health-Care Facilities. These guidelines recommend that both short- and long-term renovation projects be sealed off and kept under negative pressure created by HEPA filtration exhaust equipment. In addition to the immediate infection control benefits, this practice will also help to reduce cleaning requirements created by the renovation work.

HEPA Maintenance

When HEPA filters are used in infection control applications it is imperative to have a meticulous maintenance program in place. It is critical that HEPA filters be installed in equipment which seals the filter in place in order to prevent contaminated air from bypassing the filter. HEPA filters used for infectious disease control should be tested on site when they are first installed and every six months thereafter to confirm that they are operating at their design efficiency.

A HEPA filter, when loaded with particulates, will decrease the amount of air flowing through a system. The appropriate final pressure should be determined for these filters at the time of system installation in order to maintain proper airflow throughout the life of the equipment. Therefore, HEPA filters should be monitored on a regular basis with manometers or other pressure indicating devices to determine the remaining filter life.

It should be noted that there may be a significant difference in the manufacturers changeout recommendations between HEPA filters used in large air-handling systems and point of use HEPA filters. HEPA filters used in air handlers are typically changed out at twice their initial static pressure (2.0 in. wg), while point of use HEPA filters may be changed when their pressure drop increases by as little as 0.4 in. wc. Air handlers using HEPA filters have larger blowers which can be designed to deliver the appropriate amount of air even at the filter's final pressure drop. Point-of-use units have smaller blowers, which due to noise considerations and other restrictions, are not capable of overcoming high resistance to the airflow without greatly diminishing the airflow.

According to the CDC, special care should be taken to not jar or drop the filter element during or after removal and that appropriate respirators should be used during the filter changeout process. It is not necessary to red bag HEPA filters in most applications, but local and regional regulations should be consulted prior to making a final determination.

HEPA filters are a costly budget item. In order to extend the life of a HEPA filter and reduce ongoing replacement costs, it is strongly recommended to provide a roughing prefilter prior to the HEPA. According to the CDC TB guidelines for health care facilities, a low-efficiency prefilter may extend the life of a HEPA filter by 25%, while adding higher efficiency intermediate filters such as a MERV 14 (ASHRAE 95% dust spot test) filter can extend the life of the HEPA filter by as much as 900%.

Odor Control

There are several areas within a health care facility where odors or gaseous contaminants are common. Some of these contaminants may only be nuisance or comfort related, while others may represent a threat to personal health. An additional problem with gas phase contaminants is the variable tolerance levels of different individuals. A contaminant that is perceived as a nuisance odor to one person may make another person ill.

Combustion Fumes

Two applications with similar effects and solutions are air intakes that are located near heliports and loading docks. Each is affected by intermittent odors that are caused by combustion engines. There are multiple contaminants in each application, complicating the solution. Combustion contaminants, especially diesel fumes, have a high concentration of small particulates. Most of these particles are less than 5 microns in size, which make them respirable. This is exacerbated by the fact that there are hundreds of different gases contained within combustion exhaust, many of which are adsorbed onto the surface of the particulates.

Systems designed to remove combustion fumes from air intakes are typically provided in multiple stages in order to address each contaminant type. The first two stages consist of a pre- or roughing filter and a high-efficiency filter. The prefilter removes larger particles and extends the life of the high efficiency filter. The high-efficiency filter should be at least a MERV 14 (95% by ASHRAE 52.1) rated filter. This should ensure that 90% or more of all particles greater than 1.0 micron in size will be removed from the airstream.

Finally, when high concentrations of particulate matter are present in gas phase filtration applications, it is important to incorporate particulate filtration upstream of the gas phase filters. This will help prevent the blocking of filter pores, which must remain accessible in order for contaminant adsorption to occur.

The third and fourth stages may consist of various gas phase filters, with the first stage typically being a carbon filter that is designed to remove heavy weight molecules such as hydrocarbons. The last stage is usually either an activated carbon or activated alumina filter that is designed to remove lightweight molecules such as aldehydes and oxides of nitrogen. Additional stages may be added to convert gases that are not readily removed by adsorption to a form which may be eliminated by the above mentioned filters.

Laboratories and Special Procedure Rooms

Laboratories and special procedure rooms that are known to contain gaseous contaminants are typically designed to be under negative pressure in order to prevent these gases from spreading throughout the facility. Examples of these areas include cytology labs where xylene and toluene may be part of the process, or morgues, were formalin may be used. These chemicals are both irritants and carcinogenic. Such areas typically employ 100% pass-through ventilation where no air is recirculated within the facility. Conditions do occur, however, where it is possible for these chemicals to be re-entrained into the facility due to the close proximity of exhaust to potential air intakes. In applications like this, gas phase or multi-stage filtration equipment may be installed.

It is important to discuss with the room occupants all of the potential contaminants that may exist in the room. This helps to ensure that appropriate control measures are employed. For instance, in a morgue where formalin is used, we must not only be concerned with gas phase filters to deal with the formalin, but we must also be concerned with the removal of potentially infectious airborne bacteria. There have been documented cases of transmission of m. tuberculosis from viable TB bacteria which may become airborne during the autopsy procedure. In situations where air from a morgue must be filtered at the exhaust point, it is important to utilize a system with both HEPA and gas phase filtration. As with other applications, the filter should be located upstream of the gas phase filter in order to ensure that filter pores are not blocked with particulates.

Gas Phase Filter Maintenance

Proper monitoring of gas phase filters is critical to ensure their optimum performance. In applications with health consequences, it is not acceptable to rely on smelling the breakthrough of contaminants when the filter media is spent or loaded. You must work with your filter supplier to establish the appropriate test method for determining the remaining capacity of the filters to remove contaminants and the frequency that such tests should be performed.

Protecting Mechanical Equipment

This is the most prominent and historical use of filters in health care facilities. For decades filters have been used in order to maintain the cleanliness of heating and cooling coils in HVAC equipment. The original intent was to maintain the heat transfer efficiency of the coil and that purpose remains today. It is, however, enhanced by the realization that coils are an excellent breeding ground for mold. The HVAC coil and the filter which protects it may both become reservoirs for infection causing molds if they are not properly maintained.

Common practices for protecting HVAC coils include locating the filter upstream of the coil and having a filter rating of at least MERV 8. The filters should fit snugly into the holding frames; be of rigid, moisture resistant construction; and be constructed from materials that will not support microbial growth. Care should be taken to ensure that no air bypasses around the filters. Bypass can be reduced by gasketing the filters to seal them in place and by installing filter blanks in spaces were the filter track does not contain filters.

Air Filter Maintenance

Proper monitoring of air filters is crucial to the effectiveness and efficiency of the system containing them. A filter that is changed too infrequently will lead to low airflow conditions and the need to run HVAC equipment for longer periods of time in order to maintain temperature control. A filter that is not changed frequently enough can become a reservoir of microbial contamination. Should this loaded filter start to degrade or collapse over time, the contaminants contained on it will be introduced to the airstream.

On the other hand, changing a filter too soon may lead to subpar removal efficiencies, allowing more contaminants to pass through to the coil. This will lead to lower heat transfer efficiencies and may cause the coil to become a breeding ground for microbial contaminants.

A manometer or other type of pressure gauge should be used to measure the pressure drop across the filter so that the filters may be changed at the correct intervals. Regular visual inspections should also be made to ensure that the filter integrity is not compromised and to determine if there is mold growth present on the filter or coil. Dirty filters should be placed in a sealed bag before transporting them through the facility for disposal.

Facility Cleanliness

Given the dwindling labor resources in today's health care market, providing proper filtration can be a cost effective method to reduce housekeeping's heavy workload and costly budget.

It is commonly accepted that airborne particles that are larger than 5 microns in size tend to settle quickly out of the air onto horizontal surfaces. These particles are readily removed when sweeping or dusting. Airborne particles that are less than 5 microns in size (especially those less than 2 microns in size) tend to settle slowly out of the air and are more likely to come in contact with vertical surfaces. When these particles come into contact with surfaces they tend to adhere to them and as the concentration of particles increase, create visible staining.

All public areas of health care facilities, except for administration, are required to have two banks of filters - a 30% (ASHRAE 52.1) prefilter and 90% final filter. Providing that the final filter is properly installed and maintained and providing that there is little or no bypass around the filter, these filters should adequately control airborne particles that affect a facility's aesthetics. Increasing filter efficiency in the administrative areas may be an important part in maintaining a clean appearance throughout the facility.

Bioterrorism Applications

After September 11 and the subsequent postal anthrax attacks of 2001, bioterrorism threats are being taken seriously by the health care community. Due to their "first line of defense" status, health care facilities have been identified by the federal government as one of the top priorities for protection from bioterrorism.

We are likely to discover that we are in the midst of a biological event when individuals who have become infected begin to show up at hospital emergency rooms. If the bioterrorism agent used is communicable (able to be spread from person to person), such as small pox, it is possible for secondary infections to occur in the emergency room. The primary difference between a bioterrorism event and a naturally occurring infectious disease outbreak is the source of the outbreak. Once an individual with a communicable disease enters the ER, the use of the containment and filtration principles discussed earlier should adequately protect building occupants.

A possible avenue for the direct introduction of infectious bioterrorism agents is the deliberate insertion of these agents into air-handling intakes. Administrative steps should be taken to eliminate or severely reduce access to facility air intakes. Additionally, a facility should perform a bioterrorism risk assessment to determine if the filtration on all accessible air intakes should be upgraded to appropriate levels. Both particulate and gas phase filters should be considered if this avenue is pursued. Great attention should be paid to eliminating any bypass around the filters on all air intakes.

Lastly, the mailroom remains as another potential gateway for bioterrorism agents to be introduced into a health care facility. Where risk assessment warrants, the mailroom may be placed under negative pressure with HEPA filtered exhaust and recirculation. HEPA filtered containment hoods may also be used to isolate or open suspicious packages as needed.


Whether we are trying to remove an infectious agent resulting from bioterrorism or whether we are trying to protect the aesthetics of a facility, air filtration plays a critical role in our strategic responses to these issues. Implementation of these solutions should only be carried out by trained individuals and should be the result of a joint effort by engineering, medical, and infection control staff. ES