There are few places where indoor air quality (IAQ) is of greater concern than hospitals and other healthcare facilities. Proper air filtration including filter selection, installation and maintenance plays an important role in this area. This article will focus on the requirements for central ventilation filter selection in healthcare facilities, show how proper installation is essential to obtain good performance and discuss some of the aspects of filter maintenance.
Filter Selection
The American Institute of Architects (AIA) Academy of Architecture for Health with assistance from the U.S. Department of Health and Human Services has established the following “Filter Efficiencies for Healthcare Central Ventilation and Air Conditioning Systems.” These are minimum filter efficiencies in all cases.
The efficiencies are expressed in both MERV numbers as determined by ANSI/ASHRAE Standard 52.2 and in percentages as determined by ANSI/ASHRAE Standard 52.1. Although ANSI/ASHRAE Standard 52.1 has now been officially withdrawn as an ASHRAE Standard, it will be years before this change is fully implemented and recognized throughout the industry and both numbers are included here to avoid confusion.
Table 1 – Filter Efficiencies for General Hospitals
Area Designation |
Minimum Number of Filter Beds |
Filter Bed No. 1 MERV (%) |
Filter Bed No. 2 MERV (%) |
All areas for inpatient care, treatment and diagnosis and those areas providing direct service or clean supplies such as sterile and clean processing, etc. |
2 |
8 (30%) |
14 (90%) |
|
|
|
|
Protective Environment Room |
2 |
8 (30%) |
17 (99.97%) |
|
|||
Laboratories |
1 |
13 (80%) |
|
|
|||
Administrative, bulk storage, soiled holding areas, food preparation areas and laundries |
1 |
8 (30%) |
|
|
|||
Recirculation of air within Individual Isolation Rooms |
1 |
17 (99.97%) |
Table 2 – Filter Efficiencies for Psychiatric Hospitals
Area Designation |
Minimum Number of Filter Beds |
Filter Bed No. 1 MERV (%) |
Filter Bed No. 2 MERV (%) |
All areas for in-patient care, treatment and diagnosis and those areas providing direct service |
2 |
8 (30%) |
14 (90%) |
Administrative, bulk storage, soiled holding, laundries and food preparation areas |
1 |
8 (30%) |
Table 3 – Filter Efficiencies for Nursing Care Facilities
Area Designation |
Minimum No. of Filter Beds |
Filter Bed No. 1 MERV (%) |
Filter Bed No. 2 MERV (%) |
All areas of resident care, treatment and/or diagnosis, and those areas providing direct service or clean supplies |
2 |
7 (30%) |
13 (80%) |
Administrative, bulk storage, soiled holding, laundries and food preparation areas |
1 |
7 (30%) |
Table 4 – Filtration Efficiencies for Outpatient Facilities
Area Designation |
Minimum No. of Filter Beds |
Filter Bed No. 1 MERV (%) |
Filter Bed No. 2 MERV (%) |
All areas for inpatient care, treatment, and/or diagnosis and those areas providing direct service or clean supplies such as sterile and clean processing, etc. |
2 |
8 (30%) |
14 (90%) |
Laboratories |
1 |
13 (80%) |
|
|
|
||
Administrative, bulk storage, soiled holding areas, food preparation areas and laundries |
1 |
8 (30%) |
Again, it is important to note that these Tables call for the minimums required by most states and the Joint Commission. There are some cases where HEPA filtration is recommended such as in orthopedic operating rooms, bone marrow transplant rooms and organ transplant rooms. Airborne Infection Isolation (AII) and Protective Environment (PE) rooms also need HEPA filters with a 99.97% efficiency at 0.3 micrometers. Pharmacies located within healthcare facilities have their own set of filtration requirements that go beyond the scope of this article.
Unless someone is working in a healthcare environment or in some other area where high efficiency filters are required, these filters are somewhat confusing. Most people have seen pleated filters of various efficiencies and the old stand-by “throw-away” panel filters but a MERV (Minimum Efficiency Reporting Value) 14 – 24”X24”X12” rigid cell-type filter or a MERV 14 V-cell filter is a “new animal.” It might be helpful to explain a little more about these filters so that they will not be such a mystery.
To achieve their higher efficiencies these filters are often made with denser filtration media. This can increase airflow resistance or pressure drop creating more strain on the HVAC system. Pressure drop can be decreased by increasing the filter media area. There are several mechanisms for doing this including pleating and forming the media into bags or pockets.
To give you an idea of what changing the amount of filter media can do to pressure drop, let’s take the example of a MERV 7 pleated filter in thicknesses of 1”, 2” and 4”. A filter with outside dimensions of 24”X24” in a 1” variety will have about 7 square feet of media area, a 2” thick 24”X24” filter will have about 11.5 square feet of media and a 4” thick 24”X24” filter will have about 22 square feet of media. When tested at 500 feet per minute the 1” filter could have a pressure drop of .31” w.g. (water gauge), the 2” filter a pressure drop of .27” w.g. and the 4” filter a pressure drop of .23” w.g.. All of these filters use the same media. The rule of thumb is – the more filter media, the lower the resistance or pressure drop.
According to the ASHRAE HVAC Design Manual for Hospitals and Clinics rigid filters are preferred for healthcare facilities. Therefore, these high efficiency filters usually are Rigid Box Filters, Rigid Cell Filters or Mini-pleat Filters.
Rigid Box Filters
Rigid box filters are available in depths of 4”, 6” and 12”. They are made with pleated high efficiency media (synthetic or fiberglass) bonded to an expanded metal grid. The pleats are separated and supported by “fingers” made from plastic, cardboard or metal. The boxes are generally made from metal or plastic. They either come as a square box or have a “header” on the front of the filter that enables them to be installed in a 1” filter track/frame. Pleats in a rigid box filter are about an inch apart. A typical 24”X24”X12” rigid box filter will have about 60 square feet of media. Pressure drop for a MERV 14 rigid box filter at 500 fpm is approximately .50” w.g..
Rigid Cell Filters
Rigid cell filters are made with microglass media separated by corrugated aluminum. They are available in either 6” or 12” depths and are sealed into a box constructed from metal or wood. Rigid cell filters come with a single or double “header” or are made in a straight box configuration. A typical 24”X24”X12” MERV 14 rigid cell filter has a media area of about 110 square feet and a pressure drop of about .60” w.g. at 500 fpm.
Mini-pleat Filters
Mini-pleat filters are made from microglass or synthetic media with very tight pleat spacing. While a standard pleated filter might have one pleat per inch, a mini-pleat filter typically has 4-6 pleats per inch. Therefore, it has 4-6 times more media surface area for the same depth filter. Mini-pleat filters come in thicknesses of 2” or 4.” They are also made into 1” panels. These 1” thick panels are then configured into a 12” deep frame in a “V” shape to give the maximum surface area. There are anywhere from 2 to 4 “V’s” in a 24”X24”X12” filter. Several manufacturers state that their 24”X24”X12” V-cell mini-pleat filter incorporates up to 190 square feet of media surface area and a pressure drop of about .35-.40” w.g. at 500 fpm.
Which One?
There are a number of factors that influence the choice of the best high efficiency filter for a particular application including pressure drop, energy usage, projected filter life, ease of installation, speed of the air flow in the HVAC unit and more. When all of these factors are considered it just may be that the more expensive filter in initial cost will turn out to be the least expensive filter to operate in the long run. Filter test results vary significantly and it is always a good idea to insist that all filters being considered are supported by testing of an independent third-party laboratory. The best way to assess what filter to use is to work with someone with extensive knowledge of filtration such as an individual with a Certified Air Filter Specialist (CAFS) designation from the National Air Filtration Association (NAFA).
Recent advances in filter media technology have enabled filter manufacturers to build filters achieving a MERV 13 while using the same frames and pleat configurations as standard pleated filters. These new filters have relatively low resistance and high efficiency. While they are significantly more expensive than a standard pleated filter, they are less expensive than the more traditional filters mentioned above. It is anticipated that they will be widely used in hospitals and healthcare facilities in the future.
Proper Filter Installation
A healthcare facility can have the best filters, but still have poor results unless the filters are installed properly. Filter holding frames should be durable and should be sized properly to provide an airtight fit within the enclosing ductwork. Because high efficiency filters have higher resistance, they are more susceptible to the problems of air bypass. Small gaps between filters or between filters and the surrounding ductwork and unit doors and panels can create serious efficiency loss. For example, in a study conducted by Dr. Jeffrey Siegel at the University of Texas in Austin, it was found that just a 10 millimeter gap between filters could reduce filter efficiency from a MERV 15 to a MERV 8.
All joints between filter frame banks and the enclosing ductwork should be caulked or gasketed to provide a positive seal against air leakage. Gaskets should also be used in the space between the last filter in the row and the filter bank door and between high efficiency filters. Gasket material is made from neoprene or some other compressible rubber-like material that will provide an airtight seal. It usually has an adhesive on one side protected by a peel off backing.
Are Pre-filters Necessary?
Several of the filtration guidelines above do not call for pre-filtration of high efficiency filters. Generally, whenever there are filters of MERV 12 or above,it is a good idea to use a pre-filter to capture the larger particles. This extends the life of the high efficiency filters and preserves their integrity for capturing smaller particles. When you consider that the high efficiency MERV 12 and above filters cost anywhere from 10 times to 40 times the cost of a pleated filter, it is easy to see the savings involved. Changing pleated filters on a quarterly basis is more economical than replacing an $80 filter six months prematurely because it had been used to remove dirt, dust and debris.
High Efficiency Final Filters Should be Downstream from the Coils
It is important to understand that in all healthcare filtration when there are two filter beds, the first one is installed before the blower and coils. The second one is always downstream of the coils. This set-up ensures that the first filter is used to protect the equipment from dirt and other contaminants in the air stream. The second filter bank captures anything that makes it through the first filter or comes off of the HVAC equipment itself. For example, if the coils have mold on them, the high efficiency filter would capture the mold spores before they could enter the healthcare facility.
Use Filter Pressure Gauges
A filter pressure measuring device should be installed across each filter bed having an efficiency of 75% (MERV 12) or more. These devices measure the pressure change in the HVAC unit upstream of the filter to downstream of the filter. Pressure gauges can give the user important information about the filters in a system. Initial pressure drop significantly below what it is expected from manufacturer’s specifications usually indicates the filters have not been installed properly and/or are not adequately sealed.
A common question regarding high efficiency filters is: “How long will they last?” The only true answer is: “It depends.” Some will operate effectively for a year or more. Others will need to be replaced in six months. Many factors determine filter life including exposure to contaminants in the HVAC system, system run time and filter composition. Pressure drop change is an indicator of when a high efficiency filter needs to be changed. As the filter loads with dirt the pressure drop increases. A general rule of thumb is that high efficiency filters need to be changed when their pressure drop is double the initial pressure drop. For example, a MERV 14 filter with an initial pressure drop of 0.32” wg should be changed when the filter pressure measuring device shows a reading of 0.64” wg.
Although pressure drop is a good starting point, there are other considerations for determining filter life. For example, recent research has shown that perceived indoor air quality is decreased when reactions occur between what is on filters and things that are in the air stream such as ozone. In addition for management reasons many building owners and managers find that changing filters at set intervals such as monthly, quarterly, semi- yearly or yearly is the most reliable means of ensuring filters are maintained properly.
Filtration for Individual Room Units
Many healthcare facilities have separate fan-coil HVAC units for individual patient rooms. This allows for each room thermostat to be set at the desired temperature for the occupants. The AIA recommendations allow for these units to have either permanent (cleanable) or replaceable filters of at least a MERV 3. It would seem that an upgrade to MERV 7 or MERV 8 pleated filters would be a natural way to increase patient satisfaction.
Is it a HEPA filter?
There are few things that cause more confusion with HVAC contractors and building engineers than the identification of a high efficiency filter. Most people automatically, but incorrectly, assume that all thick filters are HEPA (High Efficiency Particulate Arresting) filters. In the vast majority of cases, they are not. By checking filter labels, one can avoid costly mistakes.
A Good Idea
When working with healthcare facilities, it is important to understand that there are some areas where the untrained should not tread. Many hospitals and healthcare facilities use HEPA filters to capture potentially dangerous contaminants. They should be installed in what is referred to as “Bag In/Bag Out” (BIBO) housings. These HEPA filters should never be exposed outside of the filter housing. Therefore, the housing is designed in such a way that the filter can be installed and removed without ever being opened to the surrounding environment.
Special training on the appropriate methods to handle these filters can be obtained from the National Air Filtration Association (NAFA). Certification in “Bag In/Bag Out” filter handling is a part of the NAFA Certified Technician – Level II program. Without this training, there is a simple rule. Do not touch these filters – that is unless you want to expose yourself, patients, workers and the environment to some hazardous or dangerous contaminant.
Keep it simple
Healthcare filtration is really pretty straightforward, but it is serious business. There are good guidelines to follow on selecting the right filters for each area of a healthcare facility. In our experience, errors in the level of filtration selected are rare.
Most problems seem to occur are in the areas of installation and maintenance. Special care needs to taken to ensure that filters in healthcare facilities are installed properly – without air bypass. Filters need to be changed at regular intervals. A recent to trend to “defer” filter replacement for budgetary reasons is false economy. This can lead to increased operating costs as well as create potential health and safety problems for the staff and patients of healthcare facilities.
This is an area where the “KISS” formula works well. “Keep it simple, Sir.” Select the right filters. Then install and change them properly.