Note: All data gathered from the official NAFA website Please check this source for additional sources utilized when gathering data for these articles.

Purolator FAQs

MERV or Minimum Efficiency Reporting Value, or MERV for short, is a filter rating system devised by the American Society of Heating, Refrigeration and Air conditioning Engineers (ASHRAE) to standardize and simplify filter efficiency ratings for the public.

The higher the MERV rating, the higher the efficiency of the air filter. Simply stated, a MERV 12 filter will remove smaller particles from the air than a MERV 8 filter.

For the consumer, this means that you now have the ability to effectively compare one brand to another. Without any value-added additions, any MERV 8 filter will perform about the same as any other MERV 8 filter. The MERV rating only applies to efficiency.

Merv 1-4 Rated filters will collect most particles of 10 microns or larger. Typical applications of these filters are minimum residential filtration, Light commercials, and minimum equipment production.

Merv 5-8 rated filters are used to trap particles in the 3-10 micron range. Some uses are in industrial and commercial building, high-end residential units, and paint booth/spray and finishing areas.

Merv 9-12 rated filters are used specifically for particles in the 1-3 micron range. High-end residences, upgraded industrial workplaces, and commercial boiling frequently use these.

Merv 13-16 rated Filters remove particles in the 0.3-1 micron range and are used in hospitals, healthcare, and high-end commercial buildings. They are also useful in telecommunication manufacturing facilities.

If allergies or asthma are your concern, we suggest you choose a minimum of a MERV 8 filter.

Your home filter is also called a "furnace filter". The purpose of your furnace filter is to keep the coils and heat exchanges on the heating and air conditioning system clean. You will want to keep the filter clean to extend the life of your HVAC unit.

The most important thing to remember about home air filters is to change them regularly. To be safe and keep the air in your home clean we recommend changing 1" filters every month, 2" filters every 1-2 months, and 4-5" filters every 3-6 Months.

Fiberglass air filters most commonly called "throwaway filters", these are the fiberglass weave or "hog hair" filters that are designed to meet the minimum requirements of protecting your air handler (furnace or Air Conditioner) and its components. NAFA (National Air Filtration Association) no longer recommends using anything under a MERV 7 filter for home HVAC systems.

A pleated furnace filter is for those wanting to step up from a basic furnace filter. Pleated filters offer better protection against dust and other airborne particulate.

When it is loaded with dust, of course.

But, unfortunately, there is more to the story than this simple statement.

First, a short lesson on media air filters:

All filters, whether they're commercial or residential, battle the same three forces of nature

  1. Resistance to flow
  2. Amount of dust they will hold
  3. Efficiency or ability to remove particles from the air

Residential heating and air conditioning equipment present several challenges for the homeowner from the standpoint of air filtration.

The first limiting factor involves filter depth. Most older homes have an air handler with a 1-inch slot for an air filter inside the unit. This filter is changed by removing the door to the unit and removing and reinstalling the filter.

Newer homes have a filter grille, located at the return air opening. This grille is hinged and can be opened to expose a track for a 1-inch filter. In short, regardless of where the filter is located in a residential unit, there is usually only 1-inch in depth allocated for the air filter.

The second limiting factor is the blower horsepower of residential units. The standard unit has a 1/4 or 1/3 horsepower blower that allows for a *limited amount of resistance to the flow of air.

Anything in the system is considered resistance, but the typical things in a residential system are ductwork (a friction factor to the flow of air), air conditioning coils, grilles, and registers, sometimes fire dampers and filter(s).

Resistance in an HVAC system is measured in inches of water - pressure forcing water to rise in an enclosed tube. Residential units can normally have about 0.5 inches of added pressure in the system and the typical unit is allocated only about 0.1 inches (w.g.) for filters.

And, media filters increase in the resistance to flow as they load with dust. This increase in resistance leads to a decrease in velocity of airflow in your unit.

Because of the 1-inch restriction combined with a limited allocation for pressure, homeowners are limited as to their choices of filters for their home without retrofitting the system.

The choices in the 1-inch variety are normally:

  • Standard Fiberglass Throwaway Filter - these filters are designed to remove only the larger particles from the air and, in industry-designed testing, do not do very well. These filters do have a very low resistance to the flow of air and, for this reason, they're most often sold for home units.
  • Pleated Filters - these filters achieve more filter surface area by folding the media into a 1-inch frame and can use a higher efficiency media without adversely affecting the resistance to flow.
  • Others - anything that does not fall into the above two categories (metals, plastics, cellulose, electronics, etc.).

In today's marketplace, higher efficiency filters are available at most retail home stores and homeowners need to be aware that some of these filters (usually in the MERV 11 and higher category) may create low airflow problems in their system.
Note: The higher the MERV number, the higher the efficiency.

There have been significant improvements on 1" furnace type filters over the last ten years.

The fiberglass furnace filter was originally designed to keep the house ventilation system clean from large particles and debris. Since most of the house furnace blowers are not designed to operate in high static pressure environment, the fiberglass furnace filter can share only a limited amount of pressure drop.

The 1" fiberglass furnace filter with low-pressure drop and low cost naturally became the most common choice.

Two things have changed. The first is a better filter design by adopting a pleated structure to reduce media velocity. Lower media velocity typically leads to higher filter efficiency and lower pressure drop.

Secondly, the variety of specialty media (e.g., tribo-charged media, split fibers, large effective fiber diameter (EFD) melt-blown electret, etc.), has significantly higher initial efficiency with relatively low-pressure drops.

The enhanced performance of some pleated type furnace filters has elevated the traditional role of 1" fiberglass furnace filters from protecting the residential ventilation system to improving the indoor air quality (IAQ) in a residential environment. Pleated type furnace filters, when selected and used properly, can potentially reduce the indoor air pollutants significantly and the advantages are multifold.

For example, for allergy sufferers, houses installed with specialty pleated type media furnace filters can potentially alleviate the symptoms of sneezing, watering eyes, itching throat, postnasal drip, coughing etc. during the pollen season.

The reduction of indoor air particle concentration by pleated type furnace filters can also slow down the settling of dust and respirable particles inside the house.

Field Tests Setup

Seven different new furnaces were evaluated in this study. Identification of the seven filters can be seen in Table 1.

Tests were performed in an actual residential home located in Florida with total square footage of 2900 ft2. The test house is a high ceiling with two stories in half of it. A heat pump with a slot for a 20"x20"x1" furnace filter is the ventilation system used in the house. Air filters used were acquired from local retail stores.

Two TSI PortaCount Plus used as condensate nuclei counters (CNC) and one TSI 3755 (two channels: 0.5-5 and >5 microns) optical particle counter (OPC) was used as the primary monitoring instruments in this study.

One CNC (#1) was placed on a ground floor dining table close to one corner of the house.

One CNC (#2) was placed in a second-floor bedroom whose location was in the exact opposite end of a diagonal of the house relative to the first CNC.

The two-channel optical particle counter was placed in the first-floor master bedroom. It forms a triangle (with the other two CNCs) that covers three corners of the house. Each monitoring instrument was connected to a computer for data acquisition.

CNC recorded the particle concentration from 0.02-1 mm and the sampling time was set at 15 seconds for each sampling period. The optical particle counter recorded the results of particle concentration in the range of 0.5-5 mm (respirable particle size range) and the sampling time was set at 20 seconds for each sampling period. Combination of those two types of instrument covers the particle size range from 0.02 to 5 mm.

Since the main objective of the study is to see how effective is each furnace filter working as a whole-house room air cleaner. The decay of particle concentration vs. time inside the house is the primary focus of each test.

Table 1. Furnace Filters Used in Field Tests

Description Media Type Property
MERV11 Split Fiber (Fat Fibers) Electret
MERV12 Melt-Blown Electret
MERV8 Cotton and PET Mixed Non-Charged
MERV10 Tribo-Charged (Fat Fibers) Electret
MERV12 Tribo-Charged Composite Electret
Glass Fiber Fiberglass Throwaway-type Non-Charged
CFP* MERV8 Composite Filter Pads* Non-Charged

Field Test Procedures

Several sliding doors and windows were open to let outside ambient particles enter the house through natural ventilation.
Particle concentrations were monitored throughout the house to make sure the particle concentrations were stable before each experiment started.
All the sliding doors and windows were then closed once the particle concentration was stable inside the house.
All three counters were then reset to start to record the data of each test.
The blower of the ventilation system was not turned on for another 10 minutes to establish the initial baseline (particle concentration).
Each test lasted 3-8 hours depending on the performance of each furnace type filter.
No activities or human movements occurred during each test.

A background test was also performed. All the doors and windows were closed. The blower was off and there was no activity or any movement in the house. CNC#1 and OPC recorded the particle concentration over a period of time.

Results and Discussion

The data collected in the first 10 minutes of each test were averaged and used as the initial concentration. The numbers collected by each instrument after the blower was turned on were divided by the initial concentration and represented as the percentage of the original concentration.

Fig. 1 illustrates the decay curves of the CNC #2 placed on the second floor. The CNC #2 was not available when the test was performed for the WEB furnace filter. The results are very close to those collected by CNC#1. The time that requires removing 50%, 75% and 87.5% of the initial particle concentration for each tested filter is shown in Table 3.

Air filtration supplies the means to obtain the level of particulate cleanliness required by any definition of "air conditioning." It extends from the simple task of preventing lint and other debris from plugging heating/cooling coils to removing particles as small as 0.1 microns which could cause a short circuit on a microchip.

In addition to the reasons given above, air filters are used for a wide variety of purposes, some of which include:

  • Protecting the general well-being of the occupants of a space
  • Protecting the decor of occupied spaces by removing the staining portion of airborne dust
  • Reducing maintenance of building interiors by reducing the frequency of washing such items as Venetian blinds and fluorescent bulbs
  • Protecting the contents of occupied spaces including paintings, tapestries, and other items of historic or cultural value
  • Elimination of fire hazards by removing lint and other materials which might accumulate in ductwork
  • Extension of shelf life of perishable dairy products by removing airborne mold during processing operations
  • Removing airborne bacteria from operating room air to help prevent postoperative infection

The Problems

The statistics are a bit unnerving;

53 million school children and 6 million teachers, administrators and others walking into 120,000 school buildings every day – at least 50% of these schools have been diagnosed with indoor air quality problems.

The Department of Energy says, "Our nations K-12 schools are challenged to serve a growing student population and rising community expectations with aging buildings, constrained operating budgets, and ever-increasing energy bills." Each year, taxpayers spend $6 Billion on energy for these schools – about 25 percent more than necessary. That $1.5 Billion could be redirected to hire 30,000 new teachers or purchase 40 million new textbooks annually.

Add to this energy bill another alarming statistic:

The American Lung Association estimates show 6.3 million school-aged kids miss about 10 million days of school with asthma and, as result, asthma is the leading cause of school absenteeism. The Center for Disease Control and Prevention estimates approximately 14 Million school days per year are lost because of asthma exacerbated by poor indoor air quality in schools.

Particulates in the Air

Because schools represent a much denser population percentage than a typical commercial office building, the bio-burden becomes even greater.

Viable and non-viable particulates brought in on people’s clothing and through open doors and windows add to the activity level of most young people which increases the shedding of skin cells and other particulates causing school air to be some of the dirtiest air in any environment.

Many schools utilize low efficiency (MERV 1-4) filters that remove minimal levels of all particulate matter.

For any parent who has taken their child to school first thing in the morning and picked them up in the afternoon, the difference in the smell of the school at the end of the day is astonishing. For those in the school, they have become accustomed to the odor and do not realize their air is full of particulates and odors.

Aerodynamic Diameter (micron) Likely Region of Deposit
> 9.0 Filtered by nose
6.0 to 9.0 Pharynx
4.6 to 6.0 Trachea / Primary Bronchi
3.3 to 4.6 Secondary Bronchi
2.15 to 3.3 Terminal Bronchi
0.41 to 2.15 Alveoli

With these tremendous problems comes tremendous opportunities for collaboration of schools with NAFA Certified Air Filter Specialists (CAFS).

NAFA members across the world have stepped forward to help local schools provide better air filtration and cleaner environments for their students. Here are just two examples:

Case Study #1

Norpsec Filter, Ltd.

Sarnia, ON

President – Bob Jackson, CAFS

Norspec Filtration Ltd. in Canada worked with the Thames Valley District School Board beginning in 2000. TVDSB began to realize that their “low bid” contract for air filters was not working when parents, teachers and custodial staff began complaining. They revised their air filter requirements with the note that they were looking for solutions to their air quality problems.

Norspec made a presentation to TVDSB outlining an "Air Filter Management Program" that included replacement of all low MERV # filters with MERV 8 pleated filters along with MERV 8 synthetic ring and link panels.

Next, Norspec assisted with the development of a change-out schedule that involved a 3-month survey of all 195 school locations to verify size, quantity, and existing status of the air handling system.

Finally, they worked with the school district to assemble a “Filter Committee” with representatives from Norspec, along with school officials and personnel from purchasing, maintenance and health & safety that met on a quarterly basis to assess proposed solutions along with addressing any filter issues brought to the committee.

Each school had its own filter change schedule and filter order sheet with specific times and dates for ordering and changing.

The program was monitored by the Filter Committee. This monitoring revealed that the individuals involved in changing air filters knew little about air filtration.

With more than 400 people involved, Norspec held 5 training sessions – one in each region of the district.

Over the intervening years, this training has become a yearly event to accommodate new personnel and reacquaint existing employees with filtration concepts.

The Filter Committee continues to meet regularly to discuss issues, troubleshoot problems and look for better ways to improve overall air quality.

As a result of this partnership between TVDSB and Norspec, the school has realized cost savings from reduced change-outs in many schools, along with the reduction of storage and damage.

With the improvement in air quality at the schools the Board has reported significant cost savings in other areas such as housekeeping and equipment maintenance.

In 2004, Norspec Filter nominated Thames Valley District Schools for the NAFA Clean Air Award which they subsequently received.

This case study shows the value that NAFA-member companies can bring to facilities with knowledge and training along with higher efficiency filters to help provide clean air in the schools.

Case Study #2

Air Industries, Inc.

North Andover, MA

Stephen W. Nicholas, CAFS, NCT

The Keefe Technical School is a 30-year-old facility with approximately 300,000 sq. ft. of space. They provide classes and training for (13) different vocational/technical careers including automotive, woodworking, plumbing, electrical and various other trades. They also have a gymnasium, swimming pool and offer several cooking classes as well.

The school recently had the Heating, Ventilation, and Air-Conditioning, (HVAC) ductwork and coils cleaned. They were now looking for ways to keep their HVAC system components hygienically clean to improve and maintain acceptable Indoor Air Quality for the students, faculty, and staff.

The Plant Engineer, Ken Whidden arranged for instruction, training, and testing for custodial and maintenance staff including the HVAC Supervisor Tim Rivers with the latest technology required to maintain the school’s HVAC air filtration systems.

The training programs provided HVAC Air Filtration Choices for Today, The U.S EPA’s Tools for Schools Program as well as Indoor Air Quality. The staff also participated in and successfully completed training and testing of the National Air Filtration Association, (NAFA) Certified Technician, (NCT) program.

Original Equipment

The original equipment manufacturer (OEM) HVAC air filters were a 20-25% (MERV 6) 5 cartridge type filter. These filters remove 35-49.9% of particles in the 3-10 micron size range.

The pressure differential gages used were the inclined tube manometer without any gage oil to accurately read air filter pressure drop. The initial (clean) filter static pressure operating @ 400-450 feet per minute, (FPM) is .15” in water gage (w.g.).

The gasket material on the filter holding frames and air handler doors was deteriorated and in many instances missing altogether.

To replace each filter the technician would spend approximately 4-5 minutes to remove and replace the new (clean) filter cartridge.

Filter Upgrade

The School wanted to upgrade the filtration efficiency to meet or exceed the filter efficiency required by ASHRAE Standard 62.1 “Ventilation for Acceptable Indoor Air Quality” under section 5.9 Particulate Matter (MERV 6). They also wanted to spend less time installing the filters that would allow more time to address other maintenance duties.
The other objective was to keep the HVAC system components hygienically clean and to reduce coil and duct cleaning as well. The school also wanted to improve the overall Indoor Air Quality, (IAQ) with higher efficiency air filters.


Several air filter product types were evaluated for:

  1. Efficiency/MERV
  2. Documentation/Test Reports
  3. Construction Quality
  4. Initial Cost vs. Life Cycle Cost/Operating Cost
  5. Labor/Installation

The products selected for the upgrade were a 4" deep high capacity extended surface pleated (MERV 11) air filter effectively removing 65-79.9% of 1-3 micron size particles.

This efficiency level addresses the US EPA PM 2.5 Standard. Particulates of 2.5 microns may potentially cause lung infection and possible disease.

These 4” high capacity pleated filters have approximately the same amount of media (26.1 sq. ft.) as the original (MERV 6) 8" deep cartridge filters (29 sq. ft.).

The initial clean filter static pressure @ 400-450 FPM is .21” w.g which is a negligible .06” w.g. differential.

The 4” filters were installed in the existing filter holding frames with new filter latches.

Closed cell neoprene gasket material was installed on the filter holding frames and doors of the air handling equipment.

The time to remove and install the 4” filters took approximately 15-20 seconds each compared with an estimated 4-5 minutes it took for the original 8” cartridge type.

Magnehelic® gages were properly installed on all air handling units. This allowed the technicians to effectively measure monitor and manage the air filter change-outs by air flow pressure drop.

Having the HVAC technicians and custodial staff successfully complete the NAFA Certified Technician program provided the means for the school to have qualified trained technicians with the skills necessary to maintain the HVAC air filtration system providing cleaner supply air to the students, faculty and staff.


The upgraded filter efficiency and long life cycle of the 4" (MERV 11) pleated filters vs. the (MERV 6) 8" cartridge type filters saved on labor and associated disposal costs.

The higher efficiency filters will also keep the HVAC ductwork clean while operating the heating and cooling coils at peak energy efficiency.

The overall IAQ was also improved with the higher efficiency pleated filters.

Products selected by Ken Whidden and Tim Rivers of the Engineering/Maintenance Department of the Keefe Technical School can be implemented by other school departments and educational facilities that are looking to improve overall IAQ, equipment efficiency and system performance.

Building owners and facility managers will also save on valuable energy consumption scheduling air filter change-outs on pressure drop while providing a safe, clean and comfortable Indoor Air Environment for all the students and occupants in our school systems today.

1. NAFA (NCT) Program based on the NAFA Installation Operation and Maintenance of Air Filtration Systems.

2. ANSI/ASHRAE Std. 52.1-1992 Gravimetric and Dust Spot Procedures for Testing Air-Cleaning Devices Used in General Ventilation for Removing Particulate Matter Cleaner Air and Lower Costs?

YES, The National Air Filtration Association is dedicated to providing training and certification to those involved in providing clean air to building inhabitants.

Most of the time, the lowest initial cost air filter is not the lowest overall cost air filter when energy, storage, change schedules and disposal costs are included.

NAFA member companies have the skills and information along with technology tools to help school personnel determine the correct filter for the application, the appropriate change schedule, and the training and certification for air filter technicians that combine to give value and cost savings in almost every application.

This is a summary of research completed on the fungi growing on insulation within air-handling units (AHUs) in an office building and levels of airborne fungi within the AHUs measured before the use of germicidal UV lights and again after 4 months of operation.

Fungal contamination in air-handling units is a problem in many buildings with central heating, ventilation, and air conditioning systems and is a potential source of contamination for occupied spaces.

Control of fungi in indoor environments has traditionally focused on source control or air cleaning as methods of removal.

UV irradiation, used to disinfect indoor environments in hospitals and other healthcare facilities has various effects on fungi.

This investigation was undertaken to determine the effectiveness of germicidal UV radiation on reducing fungal contamination within AHUs.

The test facility was a 286,000 square foot building in Tulsa, Oklahoma and was originally constructed in the 1920’s and completely remodeled in 1976.

Each of the floors of the 4-story facility is equipped with four primary AHUs and two perimeter units.

When the study was undertaken in 1996, acoustical insulation within many of the AHUs exhibited abundant mold growth, as did the drain pans.

Preliminary air and insulation samples were collected to develop the sampling protocol.

Two floors were selected for investigation; no UV lamps had been installed in these units. The floors that were designated were the study floor and the control floor.

In May 1997, air samples and insulation samples were collected from the eight AHUs.

UV lamps were installed on both floors – each AHU being retrofitted with 10 lamps, installed downstream of the coils.

Output of the lamps was 158 microwatts per square centimeter at 1 meter or 10 microwatts per square centimeter for every 2.54 centimeters of tube length.

UV lamps on the control floor were not operated and on the study floor were operated 24/7 throughout the summer and early fall months – while the AHUs were in air conditioning mode.

Sampling was done using paired-stage Anderson (N-6) samplers with malt extract agar for viable fungi and paired Burkard personal samplers for total spores.

Two-minute Anderson and 5-minute Burkard samples were collected approximately 40 centimeters downstream of the cooling coils.

Pieces of the insulation, approximately 60 square centimeters, were cut from the ductwork directly opposite the cooling coils.

Dominant fungi found within the AHUs for both air and insulation included Penicillium corylophyllum, Aspergillus versicolor and a strain of an unidentified Cladosporium species.

In May, before the UV lights were initiated, mean concentrations of the total fungi isolated from the insulation on the two floors were similar in type and quantity (see table 1), while the total concentration of viable fungi in the AHUs on the study floor and control floor in the fall were significantly different.

While this study indicated that concentrations of fungi were significantly lower when UV lamps were in use, the study did not show what stages of fungal growth were most susceptible, nor did it show whether there was a reduction in spore viability.

Also, the study was not able to show if all of the fungi obtained from the AHUs were susceptible to UV light. Asthana and Tuveson (2) showed that germicidal effects were highly selective for certain species.

In summary, this study indicates that germicidal UV irradiation may be an effective approach for reducing fungal contamination with AHUs.

The use of germicidal UV lamps in AHUs resulted in significantly lower levels of fungal contamination in insulation lining of the study floor as opposed to the control floor (see Table 1).

Also, there were significantly lower levels of viable and total airborne fungi in the study floor units than in the control floor units when samples were taken during the periods (see Tables 2 & 3).

Table 1.

Mean concentrations of fungi isolated from insulation samples in AHUs before and after installation of germicidal UV lamps

Fungal taxon isolated

Concn (103 CFU/cm2)

Study floor

Control floor







0.65 (0.65)

5.81 (5.81)

23.81 (23.68)

Aspergillus versicolor

64.87 (38.56)c

0.96 (0.56)d

87.58 (32.95)

1,765.46 (1,702.1)d

Cladosporium (unknown)

135.28 (50.38)

8.42 (5.22)d

22.68 (10.19)

95.31 (37.74)d

Cladosporium cladosporioides

0.26 (0.26)

5.04 (5.04)

0.65 (0.39)

228.59 (226.92)

Cladosporium (other)


0.13 (0.13)


1.72 (1.60)



0.05 (0.05)




4.65 (3.84)

13.95 (13.95)

83.96 (83.10)

109.66 (72.09)


8.16 (4.35)

1.05 (0.63)

9.27 (8.11)

16.0 (15.59)


0.01 (0.01)




Nonsporulating colonies

0.04 (0.04)


1.94 (1.94)



213.27 (82.53)

30.51 (24.85) d

211.89 (10.80)

2,240.55 (1,622.4) d

a UV lamps were used only on the study floor
b May concentrations were measured before the UV lamps were turned on
c Mean (standard error).
d The concentrations on the control floor and the study floor were significantly different after the use of germicidal UV lamps (P < 0.05).

Table 2.

Mean concentrations of viable airborne fungi during disturbance sampling within AHUs before and after installation of germicidal UV lamps.

Fungal taxon isolated

Concn (102 CFU/m3)

Study floora

Control floor






0.11 (0.10)c


0.16 (0.10)

0.10 (0.10)


0.02 (0.01)

0.01 (0.01)

0.02 (0.01)



3.08 (2.58)

0.91 (0.48)d

1.89 (0.27)

7.46 (3.37)d


15.64 (8.83)

1.28 (0.5)d

14.75 (9.25)

11.87 (1.99) d





0.04 (0.04)




0.01 (0.01)



0.07 (0.03)


0.02 (0.02)



2.18 (0.28)

0.68 (0.28) d

5.39 (2.36)

220.05 (63.06) d


0.11 (0.11)





0.10 (0.03)

0.05 (0.02)

0.06 (0.03)



0.33 (0.09)

0.06 (0.02)

0.25 (0.03)



21.65 (11.27)

2.98 (1.06) d

22.55 (11.1)

239.52 (58.55) d

a UV lamps were used only on the study floor
b May concentrations were measured before the UV lamps were turned on
c Mean (standard error).
d Concentrations on the control floor and the study floor were significantly different after the use of germicidal UV lamps (P < 0.05).

Table 3.

Concentrations of total airborne fungal spores during disturbance sampling within AHUs before and after installation of germicidal UV lamps

Fungal taxon isolated

Concn (103 spores/m3)

Study floora

Control floor






0.04 (0.03) c





29.54 (8.75)

5.43 (3.35) d

19.00 (14.16) d

68.42 (30.91) d


27.49 (20.92)

6.69 (2.09) d

5.63 (2.55)

186.56 (52.51) d


0.01 (0.01)

0.01 (0.01)

0.03 (0.01)



0.12 (0.06)

0.04 (0.02)

0.05 (0.03)

0.06 (0.04)


0.03 (0.01)


0.01 (0.01)

0.04 (0.02)


0.70 (0.25)

0.24 (0.06)

0.47 (0.2)

1.46 (1.09)


57.92 (25.09)

12.41 (4.47) d

25.19 (16.73)

255.54 (82.27) d

a UV lamps were used only on the study floor
b May concentrations were measured before the UV lamps were turned on
c Mean (standard error).
d Concentrations on the control floor and the study floor were significantly different after the use of germicidal UV lamps (P < 0.05).

(Ed. Note: The largest number of cleanrooms is divided between the semi-conductor industry, driven by increased yield (or decreased cull rates), and the pharmaceutical/ medical device industry driven by health-related concerns of not causing death or illness to the public along with FDA regulatory concerns. In this article, Mr. Brande presents his views on what the absolute minimum testing criteria should be for a cleanroom to be "certified.")

There are five tests and/or calculations that must be performed in order to prove four objectives that qualify a cleanroom as functional:

One Demonstrate that the controlled area meets the desired classification (Class ISO 5; Class ISO 6; Class ISO 7 or Class ISO 8).

Required Test

Room classifications according to ISO 14644-1, or the now defunct Federal Standard 209E, to establish that the desired room class has been met.

Two Demonstrate that no particulate will enter the controlled area by way of the supply air mechanical system.

Required Test

Integrity testing of the HEPA filtered supply (or exhaust) air with an oil aerosol (ambient challenge not accepted here) to show no bypass of airflow and potentially detrimental particulate.

Three Demonstrate that no particulate will enter (positive) or exit (negative) the controlled area due to construction (wall and/or ceiling utility penetrations, mouse holes and threshold gaps).

Required Test

Differential pressures will indicate that both direction and magnitude of static pressures is sufficient to control migration of particulate from one controlled area to another (or even an uncontrolled area).

Required Test

Four Demonstrate that, should there be an episode of particulate generation within the controlled area, the design of the room will handle the particulate in a “controlled” and timely manner.

Required Test

Controlled area volumes (sometimes recorded in the form of velocities and converted) and subsequently the room air exchange rates are used to determine if there is sufficient airflow into a controlled area to neutralize (dilute) any short term potential source of particulate in an area and also maintain the required static pressures.


Required Test

Finally, airflow visualization (a picture is worth a thousand words) for the definitive answer as to whether a controlled area can truly control particulate.

“If I could perform only one test in a Class ISO 5 (Class 100) environment, this would be my choice. In my opinion, more pertinent information can be derived from this test than all of the other standard tests listed in either IEST-RP-CC006.3 or ISO 14644 Part 3.”

Note: In areas Class ISO 6 (Class 1000) and higher, this test takes on another form called room recovery and is not quite so ‘visual’. Bear in mind, that both tests are considered destructive in terms of trying to maintain sterility.


The information provided here is intended to assist those responsible for making technical decisions to improve air filtration in commercial buildings.

These would include offices, retail facilities, schools, churches, transportation terminals, and public arena's such as sports coliseums, and malls.

The focus here will be on air filter selection concerning particulate contaminants.

Building owners, operators, managers, designers, service contractors and maintenance personnel need reliable and accurate information regarding air filtration and air cleaning options.

The decision to enhance and upgrade air filtration in a specific building should be based on the building, occupants, it's engineering, and architectural, feasibility and cost.

The information learned will allow one to make a more knowledgeable and informed decision about selecting, installing and upgrading air filtration system.

Effective air filtration can also help improve overall Indoor Air Quality, (IAQ) and worker health and productivity.


Cost is always an issue affected by implementing a filtration upgrade to the HVAC system. Total system costs should be evaluated by the decision makers regarding the enhanced filtration upgrade. Life cycle cost analysis should also be conducted. They should include the following:

  1. Initial cost of the materials to include shipping, warehousing and "shrinkage"
  2. Operating cost, the energy consumption allocated directly to the air filters
  3. Replacement cost which is the labor cost to replace filters when they have reached the end of their service life
  4. Disposal cost

Higher efficiency filters typically have a higher initial cost than commonly used low to medium efficiency products that are specified in most HVAC systems.

Usually, HVAC systems are equipped with filters designed to keep equipment components such as coils, compressors, fans, and ductwork clean. Higher efficiency filters may have a higher resistance to airflow called pressure drop, and fans may have to be changed to handle this increased pressure drop.

Although these systems improvements will normally come at a higher initial cost, the benefits achieved by this change can offset many of the operating costs just by delivering cleaner air throughout the building and keeping the system components operating at peak energy efficiency.

Operating Conditions

Building pressure must also be considered for an effective HVAC filter system upgrade. The building envelope should be as airtight as possible but, as with most construction, this is a very difficult parameter to achieve.

Some outside building walls leak (infiltration) and significant amounts of unfiltered air can enter the building envelope. Field studies have shown that, unless specific measures have been taken to reduce infiltration, as much air can enter the building through infiltration (unfiltered) as through the HVAC mechanical (filtered) system. Therefore, one cannot expect the HVAC filtration system alone to improve overall IAQ.

Instead, one must consider air filtration in combination with other steps, such as building envelope tightness, and building pressurization to, as much as possible, insure that the air entering the building only comes in through the outside air HVAC air intake.

The building envelope should be maintained under a slight positive pressure to inhibit infiltration as recommended by the Department of Health and Human Services (NIOSH) in their publication No. 2002-139 "Guidance for Protecting Building Environments from Airborne Chemical, Biological, or Radiological Attacks".

Particulate Air Filtration

Contaminants of concern should carefully be evaluated to determine the level of filtration efficiency required for the contaminant size. The size of contaminants is measured in micrometers (microns).

Once a comprehensive list of contaminants of concern has been identified one will be able to use the ANSI/ASHRAE Standard 52.2-1999 to select the proper filter with the appropriate Minimum Efficiency Reporting Value, (MERV). A MERV 6 filter, for example, is the minimum required to comply with ANSI/ASHRAE Ventilation Standard 62.1-2004 located in Section 5.9 Particulate Matter (PM).

Filter selection should be based on ANSI/ASHRAE Standard 52.2-1999 "Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size."

This procedure calls for efficiency measurements to be taken on twelve (12) particle size ranges using potassium chloride, (KCI) as the challenge aerosol.

Six efficiency measurements for each of the (12) particle size ranges is taken which gives (72) total efficiency measurements.

The (12) particle size ranges are grouped into (3) wider ranges. They are as follows:

  • E1 - 0.3 - 1.0 microns
  • E2 - 1.0 - 3.0 microns
  • E3 - 3.0 - 10 microns

The lowest efficiency value (minimum efficiency reporting value – MERV) of the 6 measurements taken is recorded. The 52.2 test prescribes that the procedure is to be conducted at one of 7 airflow rates. The tested filters run from 118 feet per minute, (fpm) up to 748 fpm. The MERV allows you to be able to select the proper number to capture and remove the contaminant of concern.

Standard 52.2 provides the industry accepted procedure for measuring filter efficiency by particle size. The need for a more precise measurement of a filter’s ability to remove specific particle sizes has become a concern over Indoor Air Quality, (IAQ) as well as the protection, products, processes and most importantly people.

It is very important that the filters selected for the specific application are provided with an ASHRAE 52.2 Test Report documenting the filter efficiency.

It is also critical that the filters selected have the test data showing the airflow rate of the filter being tested should be of the same velocity rating of the HVAC system using one of the 7 flow rates used in Standard 52.2.

Emerging Technologies

Increasing the HVAC air filtration efficiencies typically results in higher pressure drops. Today there are several air filtration products that are manufactured that provide higher efficiencies with little or negligible increase in pressure drop.

The mini-pleat V-cell filters incorporate up to 4 times the media in the same 24x24x12 filter pack. This is accomplished by manufacturing 1 inch min-pleat panels in a V-style filter pack.

The principle here is the very same as a V-bank filter housing. It allows more filter surface area, thus reducing air flow resistance.

In cases where there is only one filter track the highest MERV number should be considered, providing the airflow pressure drop is not increased beyond the point of system design capabilities.

This V-cell mini-pleat filter incorporates about four times the area of a regular filter, greatly reducing static pressure and lasting about twice as long.


In addition to proper air filter selection, several issues must be considered before installing or upgrading filtration systems.

Air filter bypass is a common problem found in many HVAC filtration systems. Filter bypass occurs when air moves around the filter rather than moving through the filter. This will result in a decrease of collection efficiency and defeating the intended purpose of the filtration system. By simply improving filter efficiency without properly addressing filter bypass, the system will provide very little if any added benefit.

If the system hardware/frames or housing leaks or if the filters are poorly fitted then subsequently filtration efficiency and performance will drop off significantly. The filters must be installed with the proper filter holding clips. Gasket material should be used on the vertical side between filters, on frames, tracks, and definitely on the doors of the unit to insure an airtight seal.

Simply put, in order to have the filtration system perform effectively they must be forced to pass through the filters. Air filter gages must also be installed in order to measure a pressure drop across the filter bank. If the system cannot be measured with an air filter gauge then it cannot be monitored, and if not properly monitored, the filtration system performance cannot be effectively managed.

One More Thing

Everyone assumes that technicians understand the proper way to install and maintain air filters. Experience shows a different story. The author has personally observed incorrect filters, improperly installed and/or missing, with gaps and worn or missing filter holding clips and gasketing.

A 10 millimeter gap (less than ¼ inch) between filters can lower a filter’s MERV rating by at least two levels, thereby taking a high efficiency filter and moving it to a medium efficiency filter. Only adequately trained personnel should perform filter maintenance.

The National Air Filtration Association (NAFA) has designed an accredited program for HVAC field technicians called a NAFA Certified Technician (NCT). This program is comprehensive in its approach with a complete text and tutorial study program followed by a national exam. This certification program has been designed for North American Technician Excellence (NATE) CEU’s, and more information can be found at a local NAFA-member air filtration company or on the NAFA web site at


Consider using periodic quantitative evaluation to determine the total system efficiency.

Building operators should perform various field inspections to ensure filter seals and gaskets are installed properly and gauges are reading pressure drops accurately. This will allow you to properly apply the 3 M’s Measure, Monitor and Manage their HVAC air filtration systems.

HVAC systems should be (locked out/tagged out) while conducting maintenance to avoid and prevent contaminants from being entrained into the moving air stream.

Follow OSHA Standards 29 Code of Federal Regulations, (CFR) 1910.132 and 1910.134 regarding appropriate personal protective equipment, i.e. (gloves, respirators, glasses) etc. when performing filter change-outs.

Maintenance plans and schedule of operations should also be put in place to make sure that the filtration system works as intended.

Life cycle cost analysis will also ensure that the filtration system will satisfy the building needs while providing adequate protection to the building occupants in the office workplace today.

NAFA certified field technicians will assure personnel are trained in the proper installation, application, and maintenance of the system.

Because NAFA supports the position stated by the Environmental Protection Agency that, "...Ozone can be harmful to health," NAFA opposes the use of ozone-producing equipment used as air cleaners or air purifiers in occupied spaces.

Therefore, NAFA adopts the position that ozone air cleaner manufacturers or ozone air cleaner distributors not be allowed into NAFA membership, unless or until they cease the marketing of these types of products as air cleaners in occupied spaces.

This does not preclude membership to those who sell these devices for use in unoccupied spaces.

Contact your local NAFA member company, and ask for a NAFA Certified Air Filtration Specialists (CAFS) to survey your systems and assist in selecting the proper filters for your needs and applications.

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