This article takes an in depth look at air filters.
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Air filters are devices used to remove airborne particles, pollutants, and microorganisms hazardous to health and the ecosystem. In industrial facilities, air filters preserve the quality of products and materials and protect critical equipment from damage. Cleanroom environments are heavily dependent on air filters to control particle count. Exhaust and stack gases are filtered and cleaned before releasing them into the atmosphere. Air filters are found in many homes, offices, commercial spaces, laboratories, clinics, and hospitals.
Air filters effectively remove dust, dirt, smoke, aerosols, odors, viruses, molds, bacteria, toxic gases, and other particles and pollutants that could exist in ambient air. These pollutants cause and aggravate respiratory illnesses, skin disorders, allergies, and other illnesses.
Air filters work by drawing unclean surrounding air to pass through their filter medium with fine openings. The unwanted particles larger than those openings are trapped inside the filter medium and are separated from the passing air. The air leaves the filter with significantly reduced particle content. Air filters increase their filtration efficiency as particles form a layer in the upstream side of the filter media; this aids in screening other particles. The filter medium is replaced regularly (or cleaned) to prevent it from clogging or blinding and restricting airflow.
Filter media are the filtering component of air filters. They are responsible for capturing the unwanted particles and for preventing them from circulating in the air. They consist of a single piece of synthetic fiber or mesh with tiny or microscopic perforations; this is contained and carried in a frame and installed in the air filter equipment.
Air filter media can be classified as a pleated or non-pleated filter:
Pleated air filters have folds or pleats that increase the filtration surface area. They are contained in a frame or a cartridge that maintains the pleated form of the filter.
Pleated air filters are effective at capturing small and microscopic particles. They generally have high filtration efficiencies and greatly improve indoor air quality as they can capture microscopic pollutants such as odor, bacteria, pollen grains, molds, and other allergens.
Pliable materials such as paper, polyester, cloth, and cotton are used to manufacture pleated air filters. However, due to their material density, they tend to restrict airflow. This causes the motor of an HVAC unit or air filter equipment to work harder to push the air out of the filter medium; this consequently results in higher energy consumption. Pleated air filters are also more expensive than non-pleated air filters. HVAC units must be modified to enable use of pleated air filters.
Non-pleated air filters have less filtration area. They are suitable for higher air pressures and volumetric flow rates. They are typically made of woven fiberglass or electrostatic material. They are cheaper than pleated air filters, however, they have a shorter service life and require more frequent replacement or cleaning. They are only effective in capturing large particles (e.g., dust, debris, insects); hence, non-pleated air filters are not suitable for users 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 is the most inexpensive but least durable filter media material. Paper air filters allow the separation of small particles by allowing air to pass through its fine pores. They are usually constructed by compacting woven wood pulp fibers together, then they are pleated to augment their mechanical strength and filtration efficiency. They are commonly used in automobile air cleaners, furnace filters, and indoor air purifiers.
Foam air filters are made of engineered 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. They also have lower efficiency due to their larger open pores and have less filter material that strangles the particles. Air is progressively purified as it passes through the depth of the foam filter.
Foam air filters have a high dirt retention 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 by applying filter oil. Moreover, foam air filters are easy to clean and washable and can be recycled for several uses.
Carbon air filters remove toxic gases (e.g., VOCs, sulfur dioxide, benzene), allergens, fumes, and odors (i.e., from smoking, painting) present in 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.
Activated carbon is typically the used form of carbon material. They are derived from charcoal by physical or chemical treatment to increase their porosity and surface area.
Aluminum air filters are made of multiple alternating layers of aluminum screens to maximize their filtering and particle retention capabilities. These filters are supported in a rigid carrier frame. They are used as a pre-filter in multi-stage air filtration systems to keep the large particles from reaching the main filter. They also can capture coolant mists and grease.
Aluminum air filters have high strength and durability and can be washed or cleaned and reused several times. They perform well in a variety of environments 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. The metals alternative to aluminum include stainless steel and galvanized steel.
Fiberglass air filters provide better airflow in HVAC units and are suitable for capturing only large particles. Microscopic particles can still pass through the fiberglass filter medium. Hence, their filtration capabilities are suboptimal, and they are not recommended for spaces where occupants have respiratory problems and allergies, though the air quality is still safe for healthy individuals. They are much cheaper than pleated air filters but are replaced more frequently. Fiberglass air filters also require frequent maintenance since they are prone to clogging.
Plastic air filters inherently have high strength, durability, and chemical resistance. They are commonly made from woven HDPE, UHMW-PE, polypropylene, polyester, and PTFE fibers. Some plastic filters have electrostatic properties.
The performance of air filters is reported by the following ratings:
The Minimum Efficiency Reporting Value (MERV) is a scale that evaluates the efficacy of air filters in capturing particles with diameters between 0.3 and 10 microns. It determines how well the filter provides good air quality, and it 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 preventing smaller particles from passing through the filter medium. As the rating increases, the air filter becomes more capable of capturing even smaller particles. However, those air filters have a filter medium with lower porosity, which creates resistance to airflow and consequently causes low airflow, high energy consumption, and high stress to the components of the air filter equipment and/or HVAC unit.
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 is the average weight of dust an air filter can retain when loaded by synthetic dust incrementally under specified test conditions.
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 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.
The Most Penetrating Particle Size (MPPS) is the size of the particle that can most easily pass through the air filter.
The types of air filter equipment are as follows:
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.
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.
Baghouse filters remove particulates and air pollutants (e.g., soot, ash dust, smoke, toxic gases) in flue gases coming from process equipment such as boilers, burners, and furnaces before releasing them into the atmosphere. The filter media is made from multiple layers of pleated polymer, fabric, or glass fibers that are spaced and held in a frame; they are all housed in a vessel. The particles filtered by the filter media are collected on the hopper at the bottom of the vessel. Baghouse filters have a large airflow capacity.
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 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.
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.
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.
Exhaust filters are installed in ventilation systems to filter the air from a closed space before releasing it to the environment.
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.
HEPA stands for High-Efficiency Particulate Air. 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 microns in diameter. In European and ISO standards, which are ISO29463 and EN 1822 respectively, the required efficiency is 99.95%. Air filters with efficiency between 85% and 99.95% are considered 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:
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.
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.
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.
Ultra-low particulate air (ULPA) filters can remove even smaller particles in the air, down to 0.12 microns in diameter with at least 99.99% 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.
UV air filters kill bacteria, viruses, molds, and other pathogens by destroying their DNA using powerful short-wave ultraviolet light. However, they are incapable of removing solid particulates and gaseous pollutants when used as a stand-alone filter. Like activated carbon filters, they are usually incorporated in HEPA filters to improve air quality.
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.
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