Air Filters: What Is It? How Does It Work? Types, Uses

15 Nov.,2022


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Air Filters

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This article takes an in depth look at air filters.

Read further to learn more about topics such as:

  • What is an air filter?
  • Types of air filter media
  • Performance of air filters
  • Types of air filters
  • And much more...

Chapter 1: Overview of Air Filters

Air filters are devices used to remove airborne particles, pollutants, and micro-organisms that are health hazards and can damage the ecosystem. In manufacturing, air filters preserve the quality of products and materials and protect critical equipment from damage and product loss.

Cleanrooms use a combination of high efficiency particulate air (HEPA) filters, which are approximately 100 times more efficient than a typical furnace filter, with methods for controlling air flow to keep particulate counts within acceptable parameters. Exhaust and stack gasses are filtered and cleaned before releasing them into the atmosphere. Air filters are used in home furnaces, offices, laboratories, clinics, hi-tech microelectronics manufacturing, chip and hard-drive production and the aerospace industry.

Most commercial air filters are designed to effectively remove solid particles such as dust, dirt, smoke, aerosols, odors, fumes, viruses, molds, bacteria, and particles and pollutants from the ambient air. These contaminants cause and aggravate respiratory illnesses, skin disorders, and allergies. Filters that work on aerosols, odors, and fumes tend to absorb them like a sponge and are made with activated charcoal or carbon. Normal air filters cannot capture gas phase contaminants and work by straining/sifting particles thru a media fiberglass.

How Do Air Filters Work?

Air filters work by drawing in unclean surrounding air and passing it through a filter medium with fine narrow or progressively smaller openings. The unwanted particles that are larger than the openings are trapped inside or by the progressively denser filter media and are separated from the passing air.

Air leaves the filter with reduced particulate content. The efficiency of most commercial particulate filters increases, goes up. or improves as the filter loads up, reducing the opening in the filter media. A filter's efficiency, on day one, can be 50% of what it is when replaced. As the particles in air filters increase, their filtration efficiency, as particles form a layer in the upstream side of the filter media; aids in screening other particles.

Filter medium is replaced regularly or cleaned to prevent a filter from clogging or blinding and restricting airflow. While a filters efficiency increases as it loads up, so does its resistance to the airflow, which can have a negative effect on cooling and heating costs.

Chapter 2: Types of Air Filter Media

Filter media are the filtering component of air filters and are responsible for capturing unwanted particles and preventing the particles from circulating in the air. They consist of a single piece of progressively denser synthetic fiber or mesh with tiny or miniscule perforations, which is contained and carried in a frame and installed in air filter equipment. The filter media is sealed and secured to a frame to eliminate unfiltered air from bypassing. The structure of the frame makes it easier to install filters in HVAC equipment or duct work.

Air filter media is classified as pleated or non-pleated:

Pleated Air Filters

Pleated air filters have folds or pleats that increase the filtration surface area. They are placed in a frame or a cartridge that maintains the pleated form of the filter. In most cases, as a filter's efficiency goes up, so does its restriction to air flow. To combat and counter this effect, more surface area is needed.

Pleating the filter media has three positive results.

  1. The increased surface area spreads the air over a larger area resulting in lower air velocities and lower pressure drop or lower resistance.
  2. In most commercial filters, lower air velocities can result in slightly higher efficiencies.
  3. The increased surface area allows the filter to capture and hold more particles, particulate contaminations, or dirt extending the filter life. A common 20x20x1 furnace filter has 2.8 sq. ft. of surface area while a pleated filter might have 5-8 sq. ft.

Pleated air filters are effective at capturing small particles and have high filtration efficiency to greatly improve indoor air quality as they can capture pollutants such as odor, bacteria, pollen grains, molds, and other allergens.

Residential pleated air filters are effective at capturing particles down to 3.0 to 10 microns (µ) (3.0 microns = 1/8,400 of an inch). Under the right conditions, the smallest particle that can be seen with the human eye is in the 60 to 100 µ range. Residential pleated air filters have high filtration efficiency and greatly improve indoor air quality.

Pliable materials such as polyester, synthetic materials, and cotton are used to manufacture pleated air filters. Due to their material density, they have some restricted airflow, which can cause the motor of an HVAC unit or air filter equipment to work harder to push the air out of the filter media. The reduced air flow causes cooling or heating to take longer.

Pleated air filters are more expensive than non-pleated ones but easily fit into a standard HVAC unit. A one or two inch pleated filter easily slides into the same track or holding frame as the same size throwaway filter. Larger pleated filters, such as four and six inch ones, require adjustments and modifications to the HVAC system.

Non-Pleated Air Filter

Non-pleated air filters have less filtration area or filter surface area. They are typically made of blown fiberglass or a less common synthetic material that can generate an electrostatic charge that attracts particles like a magnet.

Fiberglass filters are less expensive than pleated air filters and can have a shorter service life, which requires that they be replaced frequently or cleaned. They are only effective in capturing large particles such as dust, debris, and insects. Their use is not advisable for patients with respiratory illnesses or allergies.

The materials used in constructing air filter media are enumerated below. These materials are engineered or reinforced with another material to improve the filter's efficiency and reduce its resistance to airflow.

Paper Air Filter

Paper is the most inexpensive and least durable filter media. They are used to separate small particles by allowing air to pass through their fine pores. Paper filters are constructed by compacting waste and recycled cotton or spun plastics that are pleated to increase their mechanical strength, filtration efficiency, air flow resistance, and static pressure. They are used as automobile air cleaners, in industrial filtration systems, and paint spray booths.

The filter media used in more efficient pleated filters (MERV 11-20) are made from statically charged synthetic media and varying diameters of fiberglass.

Foam Air Filter

Foam air filters are made of porous foams made from polyurethane, polyether, polyester, or a combination of those materials. They are characterized by their Pores Per Inch (PPI), which is the number of open pores per linear inch. Foam filters with higher PPI have lower resistance to airflow than those with lower PPI of the same thickness and have lower efficiency due to their larger open pores and less filter material for trapping particles.

Air is purified as it passes through a foam filter. A benefit of foam air filters is their high dirt loading or holding capacity, which makes them ideal exhaust filters and filters in automobile intake systems. They capture and retain large particles such as dust, dirt, and debris.

Filtration efficiency and dirt retention are further improved in foam air filters by applying filter oil. They are easy to clean, washable, and can be recycled.The combination of higher air flow with lower static pressure, higher dust and dirt loading capacity, and being washable makes them ideal for the automotive market.

Carbon Air Filter

Carbon air filters remove toxic gasses such as VOCs, sulfur dioxide, and benzene as well as fumes, and odors (i.e., from smoking, painting, automobile exhaust) that are present in the air by adsorbing them into the surface of the activated carbon’s molecules. Carbon air filters are commonly used in air purifiers, range hoods in conjunction with aluminum screens, bathroom fans, and microwaves.

Virgin activated carbon is made using the same techniques used to make charcoal, but under higher temperatures and pressures. Used active carbon works like a sponge, which can be generated using pressure and heat but not as hot as it was when it was originally produced.

Carbon is derived from things like wood and coconut charcoal by physical pressure and temperature or chemical treatment to increase its porosity. Chemicals are added to improve its performance on specific gasses and fumes. Much like a sponge, the greater surface area of carbon translates into more single pass efficiency and longer filter life.

Unlike particulate air filters that are replaced because of static pressure, carbon filters are replaced once there is an odor or gas fume break thru.

Aluminum Air Filter

Aluminum air filters are made of multiple alternating layers of aluminum screens or mesh to maximize their filtering and particle retention capabilities. They are supported in a rigid carrier frame made of aluminum. The main use of aluminum air filters is as pre-filters in multi-stage air filtration systems to keep large particles from reaching the main filter. They can capture aerosols, coolant mists, and grease, kitchen and industrial.

A characteristic of aluminum air filters is their high strength and durability, and their ability to be washed or cleaned and reused several times. Their performance can be enhanced with coating and spraying their screens with oils approved for ventilation systems. Aluminum filters are used in a variety of environments containing airborne aerosols due to their corrosion and thermal resistance. Their applications include HVAC units, range hoods, automobiles, and more.

Aluminum is the most widely used metal mesh air filter material. Alternative metals include stainless steel and galvanized steel.

Fiberglass Air Filter

Fiberglass air filters provide better airflow in HVAC units and are suitable for capturing large particles. Microscopic particles are able to still pass through the fiberglass medium, making fiberglass air filter capabilities suboptimal. Fiberglass air filters are not recommended for spaces where occupants have respiratory problems and allergies as well as environments with high concentrations of smaller particles that can affect the performance of an HVAC system.

Inexpensive fiberglass air filters have to be replaced frequently but are less expensive than pleated air filters. They require constant maintenance since they clog. Fiberglass filters allow particles that can coat and restrict air flow thru A/C coiling requiring longer run times and higher cooling costs.

Plastic Air Filter

Plastic air filters inherently have high strength, durability, and chemical resistance and are made from woven HDPE, UHMW-PE, polypropylene, polyester, and PTFE fibers. Some plastic filters have electrostatic properties that can increase their efficiency.

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Chapter 3: Efficiency of Air Filters

The performance of air filters is reported by the following ratings:

Minimum Efficiency Reporting Value (MERV)

The Minimum Efficiency Reporting Value (MERV) is a scale that evaluates the minimum efficiency of air filters in capturing particles between 0.3 µ and 10 µ. Since the efficiency of most filters goes up as the filter loads with dirt, this test determines how well the filter works from day one.

Previous tests gave the average efficiency based on the particle sizes that could be skewed and not a true representation of the filter’s performance. The MERV test provides the user with information so they have a better idea of the air quality they can expect and a more accurate evaluation of the performance between the different ratings. MERVE is used to compare the performance of air filters. The test method to determine the MERV rating was developed by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE).

The higher the MERV rating, the more effective the air filter is at filtering smaller particles. As the rating increases, the air filter becomes more capable of capturing even smaller particles. However, those air filters have a denser filter media, which creates a higher initial static pressure and could result in a lower airflow depending on your HVAC design.

Higher static pressure can result in higher operating costs, which can be eliminated by replacing the filter more often while its static pressure is at its lowest. Along with the health and production benefits, a higher efficiency filter would also keep your Heating and A/C coils cleaner.

The table below presents the MERV rating scale:

MERV Rating Air Filter will trap Air Partiles size .3 to 1.0 microns Air Filter will trap Air Partiles size 1.0 to 3.0 microns Air Filter will trap Air Partiles size 3 to 10 microns Filter Type
Removes These Particles MERV 1 < 20% < 20% < 20% Fiberglass & Aluminum Mesh MERV 2 < 20% < 20% < 20% ~ MERV 3 < 20% < 20% < 20% Pollen, Dust Mites, Spray Paint, MERV 4 < 20% < 20% < 20% Carpet Fibres MERV 5 < 20% < 20% 20% - 34% Cheap Disposable Filters MERV 6 < 20% < 20% 35% - 49% ~ MERV 7 < 20% < 20% 50% - 69% Mold Spores, Cooking Dusts, MERV 8 < 20% < 20% 70% - 85% Hair Spray, Furniture Polish MERV 9 < 20% Less than 50% 85% or Better Better Home Box Filters MERV 10 < 20% 50% to 64% 85% or Better ~ MERV 11 < 20% 65% to 79% 85% or Better Lead Dust, Flour, Auto MERV 12 < 20% 80% to 90% 85% or Better Fumes, Walking Fumes MERV 13 Less than 75% 90% or Better 90% or Better Superior Commercial Filters MERV 14 75% to 84% 90% or Better 90% or Betterr ~ MERV 15 85% - 94% 95% or Better 90% or Better Bacteria, Smoke, Sneezes MERV 16 95% or Better 95% or Better 90% or Better *MERV 17 = HEPA 13 99.97% 99% or Better 99% or Better HEPA & ULPA *MERV 18 = HEPA 14 99.997% 99% or Better 99% or Better ~ *MERV 19 = HEPA 15 99.9997% 99% or Better 99% or Better Viruses, Carbon Dust, < 0.3 p *MERV 20 = HEPA 16 99.999997% 99% or Better 99% or Better *ASHRAE does not recognize Merv 17-20

Air filters with MERV ratings above 16 are classified as HEPA and ULPA filters.


Arrestance is a rating given to air filters with low MERV ratings to indicate their efficiency. It is defined by the ability of an air filter to remove synthetic dust in the air. Higher arrestance means the air filter can remove large particles, such as dust, hair, lint, and dirt more effectively.

Dust Holding Capacity

Dust holding capacity is the average weight of dust an air filter can hold before reaching a specified pressure. This test uses a manufactured synthetic dust with a known average particle size.

Both arrestance and dust holding capacity are used for air filters with MERV ratings of 1 to 4; they are documented in the ANSI/ASHRAE Standard 52.2.

Dust Spot Efficiency

Dust spot efficiency refers to the ability of air filters to remove atmospheric dust from test air. It is determined by a staining test used to calculate the efficiency based on upstream and downstream airflow rate and opacity. This measure is documented in the 2009 ANSI/ASHRAE Standard 52.1, an older version of the ANSI/ASHRAE 52.2.

Most Penetrating Particle Size

The Most Penetrating Particle Size (MPPS) is the size of the particle that can most easily pass through the air filter.

Chapter 4: Types of Air Filters

The types of air filter equipment are as follows:

HEPA Filters

HEPA stands for High-Efficiency Particulate Air filter. According to MIL-STD-282, to be considered an authentic HEPA filter, the filter must capture at least 99.97% of particulates in the air measuring 0.3 µ in diameter and larger. A 0.3 µ particle is considered to be the most difficult particle size to capture. In European and ISO standards, which are ISO29463 and EN 1822, the required efficiency is 99.95%. Air filters with efficiency between 85% and 99.95% are considered to be EPA filters.

The filter media for HEPA filters are commonly made from several layers of borosilicate glass fibers or polypropylene fibers randomly arranged like a web to maximize particulate arrestance. HEPA filters rely on the combination of these mechanisms to capture the particulates:

  • Impaction Impaction happens at the surface of the filter media to sieve the particles larger than the mesh openings of the filter media. These particles crash into the fibers and are directly captured. This mechanism usually works for particles larger than 1 micron.
  • Interception Particles that are not sieved at the surface of the filter medium move to a depth of the filter media. However, these particles collide between the fibers and are removed from the airflow.
  • Diffusion Very small particles collide with the gas molecules; this causes them to move erratically within the filter medium and eventually strike and stick to the fibers. This phenomenon is called Brownian motion. For particles with sizes near the MPPS, interception and diffusion are the two most important filtration mechanisms.

HEPA filters protect the health of the users as they can effectively capture disease-causing microorganisms, allergens, odors, irritants, and smoke. They are widely used in hospitals, clinics, cleanroom environments, and chemical production facilities.

ULPA Filters

Ultra-low particulate air (ULPA) filters can remove even smaller particles in the air, down to 0.12 µ in diameter with at least 99.9995% efficiency. ULPA and HEPA filters have the same working mechanism; however, ULPA filters have lower filter medium porosity. Hence, their greater filter media density reduces airflow; this results in higher energy consumption than HEPA filters. Moreover, they have a shorter service life and are more expensive.

Ionizer Air Filters

In an ionizer air filter, the unclean air first passes through a pre-filter, then through an ionizer to give the airborne particles an electrical charge. The electrically charged particles are then attracted to and retained on the oppositely charged plates. The filtering mechanism does not involve filter media. Ionizer air filters are commonly referred to as electrostatic air filters.

Ionizer air filters have a MERV rating ranging from 4-5. They are cost-effective, have a long service life, and must be maintained regularly to maximize airflow. However, they are not the best choice for users with respiratory problems. The electric field created can produce ozone molecules that can worsen asthma and lung diseases.

Washable Electrostatic Air Filters

Washable electrostatic air filters rely on an electrostatic filter media, which is typically made from woven polypropylene or polyester. Airborne particles encounter friction as it passes through the filter media. They eventually become charged and are attracted to the inner layers of the filter media. The filter media holds the particles by static electricity until it is washed to remove those particles. Its electrostatic property remains constant over time and is not affected by washing.

Washable electrostatic air filters have a MERV rating ranging from 6-8. They are safe since they do not produce ozone. They are inexpensive, durable, and have a long service life.

Activated Carbon Air Filters

Activated carbon air filters are effective at removing gaseous pollutants, fumes, vapors, and odors present in the air. The gaseous molecules in the air are absorbed and retained in the outer surface of the carbon atom. However, to effectively remove those molecules, they must be allowed to settle in the activated carbon matrix for a certain amount of time.

When used as a stand-alone filter, they cannot capture solid particulates. Hence, they are typically used in conjunction with HEPA and electrostatic air filters to improve air quality. The required replacement frequency of the activated carbon filter media is unknown since it shows no signs of saturation.

Ultraviolet Air Filters

Ultraviolet (UV) air filters use a special light that is installed inside the air flow stream before or after an air flow unit to ensure that the air circulation passes through the filtration system. As air passes through the filter, a focused UV light shines into the air flow sterilizing the air and removing microbes, viruses, bacteria, and mold spores.

A combination of regular filters and UV filters cleans physical particles and viruses. UV filters use electromagnetic waves that do not bond with air molecules. They are very low maintenance and only need to be replaced once a year. UV filters are highly efficient and clean constantly without producing any noise. Unlike traditional filters, UV filters do not accumulate dirt, dust, or particles that block other filters.

UV air filters are unable to remove solid particulates and gaseous pollutants when used as a stand-alone filter. When they are combined with HEPA filters, the combination produces significant improvement in air quality. Some forms of UV filters are rated for removing the COVID virus.

HVAC Filters

HVAC filters prevent particulates (e.g., dust, dirt, debris) and other contaminants in the air from entering the internal components of the HVAC system. These solid particles can damage and deteriorate the efficiency of the HVAC system. HVAC filters also improve the circulating air quality inside a room or facility. AC filters and furnace filters are types of HVAC filters and basically have the same design and construction.

Since the operation of HVAC filters is based on airflow, it is recommended you use furnace filters with a MERV rating lower than 13 if they will be installed in residential spaces.

Engine Air Filters

Engine air filters are rectangular pleated filters that remove particulates from the air before it flows to the engine of a vehicle. Accumulation of solid particles inside the engine can cause wear and damage to the engine's internal components, increased fuel consumption, and deterioration of its efficiency.

Engine air filters should be replaced between 15,000 and 30,000 miles depending on the type of vehicle and driving conditions. Accumulation of solids in the filter restricts the airflow, limits acceleration, and causes the vehicle to emit toxic gases.

Cabin Air Filters

Cabin air filters are HVAC filters designed for vehicles. They are usually made from multi-layered paper filter media. They provide clean air to the passengers and protect the air conditioning system of the vehicle. They filter particulates in the air drawn from the passenger compartment before it enters the vehicle's air conditioner. Cabin filters have different replacement frequencies than engine air filters.

Car Exhaust Filters

Car exhaust filters are directly installed in the exhaust pipes of vehicles. They capture fine, harmful particles from engine emissions to prevent them from polluting the atmosphere. However, they are unable to capture toxic gases like carbon monoxide and nitrogen dioxide.

Diesel Particulate Filters

A Diesel particulate filter (DPF) is a special type of car exhaust filter designed for diesel-powered engines. Their filter media are made from a ceramic material formed into a honeycomb structure. The filter media collects soot, ash, and other particulates.

DPFs have an oxidative catalytic converter that converts the carbon content of the accumulated soot into carbon dioxide by passive or active regeneration. Passive and active regeneration can take place automatically without the initiation of the driver. This makes DPFs a self-cleaning filter; however, regular maintenance is still necessary.

Exhaust Hood Filters

Exhaust hood filters, or grease filters, are installed in kitchen hoods in order to remove grease, oil, smoke, and odors in the air produced during cooking. They are usually made from metal filter media. They are installed on top of stoves, grills, and fryers.

Baghouse Filters

Baghouse filters remove dust particulate and air pollutants from dirty air streams created by manufacturing and processing equipment. Any industry where nuisance dust is created needs a dust collection system. Typical industries are wood processing, food manufacturing, and metalworking.

A baghouse filtering system contains filter bags, filter cartridges, or pleated filter elements. Filter Cartridges are made of polyester and paper media treated with coatings like PTFE and Nanofiber, while filter bags are made from polyester felts and treated cotton materials.

The disposable filters of a baghouse system capture dust particles from the air that passes through them. Clean air exits the baghouse either venting outside or is recirculated through the environment. Excess dust released from the filters falls into a hopper below the baghouse housing. Baghouse filters have a large airflow capacity and are designed for industrial applications and use.

Exhaust Filters

Exhaust filters are installed in ventilation systems to filter the air from a closed space before releasing it to the environment.

Air Compressor Filter

Air compressor filters, or airline filters, are installed in condensed airlines, which are used to remove water, solid particulates, oil, and other contaminants in a multi-stage filtration process. They prevent these contaminants and protect the internal components of the air compressor unit, ensuring the unit is in top condition.


  • Air filters are devices used to remove airborne particles and pollutants that are hazardous to health and the ecosystem. These devices utilize a filter media, which captures those particles on its upstream side.
  • Pleated air filters have folds or pleats, which increase their surface area. Non-pleated air filters provide maximum airflow and are suitable for higher airflow pressures.
  • Air filter media can be made from paper, foam, carbon, aluminum, steel, fiberglass, or plastic.
  • The performance of air filters is reported by the MERV rating, arrestance, dust holding capacity, dust spot efficiency, and MPPS. The MERV rating is commonly used; it refers to the air filter's efficacy in removing particles with sizes between 0.3 and 10 microns.
  • HEPA filters capture at least 99.97% of particulates in the air measuring 0.3 microns in diameter. In ISO and European standards, the efficiency must be at least 99.95%.
  • Impaction, interception, and diffusion are the filtration mechanisms that occur in HEPA filters.
  • ULPA filters remove particles of up to 0.12 microns in diameter for at least 99.99% efficiency. Their filter media density is higher than HEPA filters; hence, airflow is more restricted.
  • Ionizer air filters and washable electrostatic air filters attract particulates by static electricity.
  • Activated carbon filters and UV air filters remove toxic gases and microorganisms in the air respectively.
  • HVAC air filters protect the critical components of the HVAC unit and purify the air for the users. The types of HVAC air filters are AC filters and furnace filters.
  • Air filters used in vehicles are the engine air filters, cabin air filters, car exhaust filters, and diesel particulate filters (DPF).
  • Baghouse filters are pollution control equipment that removes particulates and pollutants in flue gases from process equipment before releasing them into the atmosphere.
  • Exhaust hood filters and exhaust filters are used in commercial and residential ventilation systems.

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