This article takes an in depth look at air filters.
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Air filters play a vital role in eliminating airborne contaminants such as particles, pollutants, and microorganisms, all of which can pose significant health hazards and damage the environment. Within industrial settings, these filters are crucial for preserving the quality of products and materials, safeguarding important equipment, and minimizing product wastage.
Cleanrooms utilize a combination of high-efficiency particulate air (HEPA) filters, which are nearly 100 times more effective than standard furnace filters, along with strategies for managing airflow to keep particle levels within permissible limits. Exhaust and stack gases are filtered and purified before being discharged into the environment. Air filters find applications in residential furnaces, corporate offices, laboratories, medical facilities, advanced microelectronics manufacturing, chip and hard-drive fabrication, and the aerospace sector.
Air filters adeptly capture a variety of contaminants present in the air, including dust, dirt, smoke, aerosols, odors, viruses, molds, bacteria, and toxic gases. These pollutants can exacerbate respiratory issues, skin conditions, allergies, and various other health problems.
Air filters function by drawing in contaminated air, directing it through the filter medium that contains minute openings. Particles larger than these openings are captured and held by the filter, effectively reducing their concentration in the air stream. Over time, the filter's efficiency improves as the accumulation of particles on the filter’s upstream side facilitates the capture of additional contaminants. To ensure optimal operation and prevent airflow obstruction due to clogging or blinding, it is essential to replace or clean the filter medium regularly.
Filter media are the core component of air filters, responsible for capturing and removing unwanted airborne particles from circulation within residential, commercial, and industrial environments. These essential filtration materials play a crucial role in improving indoor air quality, reducing airborne contaminants, and safeguarding HVAC systems. Typically, filter media consist of synthetic fibers, meshes, or various engineered substances designed with fine or microscopic perforations that enable effective filtration. The selected media are held within a frame or cartridge that is integrated into HVAC units, air purifiers, commercial ventilation, cleanrooms, or other air filtration systems.
Air filter media are generally classified into two broad categories: pleated air filters and non-pleated air filters, each with unique filtration performance characteristics and applications.
Pleated air filters feature characteristic folds or pleats that dramatically increase the surface area available for filtration. This expanded surface area enhances dust holding capacity and filtration efficiency while prolonging the filter's service life. Pleated filters are housed in a rigid frame or filter cartridge, maintaining their pleated structure for optimal performance in residential, commercial, and industrial HVAC systems.
Thanks to their increased media area, pleated air filters are highly effective at capturing small and microscopic airborne contaminants. They deliver high MERV (Minimum Efficiency Reporting Value) ratings—often ranging from MERV 8 to MERV 13 or higher—making them ideal for trapping fine particles such as dust mites, pet dander, smoke particles, pollen, mold spores, and other common allergens. As a result, pleated filters are preferred for environments where indoor air quality improvement and allergen control are critical, such as medical facilities, schools, offices, and homes with sensitive individuals.
Pleated filters are typically constructed from durable materials like synthetic fibers (polyester or polypropylene), electrostatically charged media, woven cotton, or reinforced paper. However, the dense structure of these materials can restrict airflow if the filter is not properly sized, causing HVAC systems or air filtration equipment to work harder, which may lead to increased energy consumption over time. While pleated air filters generally carry a higher upfront cost than non-pleated options, they offer longer replacement intervals and superior overall performance. HVAC system adjustments or retrofits may be necessary to accommodate pleated filter designs.
Non-pleated air filters, often referred to as flat panel filters or panel air filters, offer less overall filtration surface area but excel at handling higher air pressures and flow rates. Constructed from materials like woven fiberglass, expanded metal mesh, or electrostatic cotton, non-pleated filters provide a cost-effective solution for basic dust and debris removal in HVAC systems and air handlers.
These inexpensive air filters primarily target larger particulate matter such as dust, lint, fibers, hair, and insects, making them less effective for environments where high-efficiency filtration or allergen removal is required. As a result, non-pleated filters typically have lower MERV ratings (MERV 1-4), a shorter service life, and require more frequent cleaning or replacement to maintain peak airflow and prevent unnecessary strain on air handling equipment. For individuals with respiratory problems, asthma, or allergies, a non-pleated filter may not provide adequate particulate removal and enhanced indoor air quality.
The following materials are commonly used in the construction of air filter media. Manufacturers may engineer or combine these substances to maximize air filtration effectiveness while minimizing airflow resistance and pressure drop.
Paper is a widely used yet less durable, cost-effective filter media material. Formed by compressing interwoven wood pulp fibers, paper filters possess fine pores designed to trap small airborne particulates while allowing airflow. Pleating the paper increases both its filtering efficiency and structural strength, making it suitable for applications such as automotive engine intake air filters, furnace filters, and indoor air purifiers. Paper air filters strike a balance between affordability and performance, but they must be replaced regularly to ensure optimal indoor air quality and HVAC protection.
Foam air filters are engineered from open-cell polyurethane, polyether, or polyester foams, often formulated for specific filtration efficiency and durability. Their performance is measured by Pores Per Inch (PPI)—the density of open pores per inch—which impacts both airflow resistance and filtration capability. A higher PPI rating indicates finer filtration but lower airflow, while a lower PPI allows for increased airflow but reduced efficiency at capturing small particles.
Foam air filters excel in dirt holding capacity and depth filtration, making them ideal as pre-filters or for use in automotive intake systems and small engine applications. Foam filters are adept at capturing large airborne debris, dust, and contaminants. Oiling the foam further enhances particle retention, and most foam air filters are washable, reusable, and therefore an environmentally friendly option. Their resilience and ease of cleaning make them cost-effective for repeated use, especially in dusty or outdoor environments.
Activated carbon air filters, also known as charcoal filters, are specialized for gas-phase filtration and odor removal in air purification systems. Utilizing a highly porous form of activated carbon, these filters adsorb and neutralize harmful gases, volatile organic compounds (VOCs), smoke, sulfur dioxide, benzene, cooking fumes, pet odors, and other chemical contaminants. Activated carbon is produced from materials like coconut shells or coal, which are processed to increase their surface area, enhancing the filter's ability to capture gaseous pollutants.
These filters are a vital stage in air purifiers, commercial kitchens (within range hoods), bathroom exhausts, medical facilities, cleanrooms, and microwave ventilation systems, often used in combination with HEPA filters or aluminum mesh pre-filters. While not intended for particle filtration, carbon air filters are essential for improving overall air quality in environments sensitive to odors, fumes, and chemical pollutants.
Aluminum air filters are constructed from multiple layers of expanded or woven aluminum mesh, arranged in alternate patterns to maximize particle capture and retention. Designed for robustness, they are typically installed as pre-filters in multi-stage commercial and industrial air filtration systems to intercept larger debris, grease, and coolant mists before finer filters (such as HEPA or ULPA) handle microscopic particles.
Their corrosion-resistant, washable, and reusable design makes aluminum mesh filters ideal for repeated duty in high-temperature, high-moisture, or corrosive environments, including HVAC systems, commercial range hoods, welding facilities, and automotive applications. Alternatives to aluminum mesh include stainless steel and galvanized steel, which offer further options for specialized air pollution control needs.
Fiberglass air filters, often known as spun glass filters, are constructed using layered fiberglass fibers that provide a basic level of particle filtration. These low-cost filters offer minimal airflow resistance and are effective at capturing larger airborne particles like lint, dust, pollen, and hair. However, their ability to trap smaller, microscopic contaminants is limited, making them suitable for general use in residential HVAC applications where elevated filtration is not required.
Due to their lower MERV ratings (typically MERV 1-4), fiberglass filters require frequent replacement and regular maintenance. While they offer improved airflow and are inexpensive, environments with allergy or asthma sufferers, or those requiring stringent air quality standards, would benefit from upgrading to pleated, HEPA, or electrostatic filter options for enhanced protection against allergens and fine particulate matter.
Plastic air filters are fabricated from high-performance synthetic fibers such as high-density polyethylene (HDPE), ultra-high-molecular-weight polyethylene (UHMW-PE), polypropylene, polyester, and PTFE. These robust filters offer excellent chemical resistance, durability, and long service life. Many plastic filter designs incorporate electrostatically charged fibers for enhanced particle capture, making them suitable for applications where consistent filtration performance and easy maintenance are required.
Common in industrial, laboratory, and cleanroom environments, as well as moisture-prone or corrosive settings, plastic mesh filters combine effective particulate removal with the advantage of being washable and reusable. Their versatility makes them an attractive choice for sustainable air filtration solutions.
Air filter performance is typically assessed using the following ratings:
The Minimum Efficiency Reporting Value (MERV) is a scale used to assess how effectively air filters capture particles ranging from 0.3 to 10 microns in diameter. This rating helps determine the filter’s ability to maintain good air quality and allows for performance comparisons between different air filters. The MERV rating system was developed by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE).
A higher MERV rating indicates a greater ability of the air filter to trap smaller particles. As the rating increases, the filter can capture finer particles. However, filters with higher MERV ratings often have a denser filter medium, which can reduce airflow, lead to higher energy consumption, and put additional strain on the HVAC system and its components.
The MERV rating scale is detailed in the table below:
| 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 |
Filters with MERV ratings exceeding 16 are categorized as HEPA or ULPA filters.
Arrestance is a measure used for air filters with lower MERV ratings to gauge their efficiency. It reflects the filter's capability to capture synthetic dust from the air. A higher arrestance value indicates that the filter is more effective at removing larger particles, such as dust, hair, lint, and dirt.
Dust holding capacity measures the average amount of dust an air filter can accumulate when progressively loaded with synthetic dust under controlled testing conditions.
Arrestance and dust holding capacity are metrics used for air filters with MERV ratings between 1 and 4. These parameters are outlined in the ANSI/ASHRAE Standard 52.2.
Dust spot efficiency measures an air filter's effectiveness at removing airborne dust from the air stream. This is assessed through a staining test that calculates efficiency based on the flow rates and opacity of the air before and after filtration. This metric is specified in the 2009 ANSI/ASHRAE Standard 52.1, which is an earlier version of the ANSI/ASHRAE Standard 52.2.
The Most Penetrating Particle Size (MPPS) refers to the particle size that is most likely to bypass the air filter and pass through it with the greatest ease.
The following are the various types of air filter equipment:
Activated carbon air filters excel at eliminating gaseous pollutants, fumes, vapors, and odors from the air. They function by adsorbing these gaseous molecules onto the surface of the activated carbon. To achieve effective removal, the molecules must have sufficient time to interact with the carbon matrix.
As standalone filters, they are not capable of capturing solid particles, so they are often used alongside HEPA or electrostatic filters to enhance overall air quality. The optimal replacement frequency for activated carbon filters is not well-defined, as they do not exhibit visible 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 are designed to capture particulates and air pollutants, such as soot, ash dust, smoke, and toxic gases, from flue gases emitted by industrial processes like boilers, burners, and furnaces before these gases are released into the environment. The filters consist of multiple layers of pleated materials, including polymers, fabrics, or glass fibers, which are arranged in a frame and contained within a vessel. The captured particles accumulate in a hopper located at the bottom of the vessel. Baghouse filters are known for their high airflow capacity.
Cabin air filters are specialized HVAC filters used in vehicles. Typically constructed from multi-layered paper filter media, these filters ensure that the air entering the passenger compartment is clean and free from contaminants. They also help protect the vehicle’s air conditioning system by filtering particulates from the air before it reaches the air conditioner. Cabin air filters generally have different replacement intervals compared to engine air filters.
Car exhaust filters are installed directly in the exhaust pipes of vehicles to capture fine, harmful particles from engine emissions, thereby reducing atmospheric pollution. However, these filters are not designed to capture toxic gases such as carbon monoxide and nitrogen dioxide.
A Diesel Particulate Filter (DPF) is a specialized exhaust filter designed for diesel engines. It features filter media made from ceramic material arranged in a honeycomb structure, which captures soot, ash, and other particulates.
DPFs are equipped with an oxidative catalytic converter that transforms the carbon in the collected soot into carbon dioxide through passive or active regeneration processes. These regeneration methods can occur automatically without driver intervention, making DPFs self-cleaning to some extent. Nonetheless, regular maintenance is still required to ensure optimal performance.
Engine air filters are typically rectangular and pleated, designed to trap particulates from the air before it enters the engine. By preventing solid particles from entering the engine, these filters help avoid wear and damage to internal components, reduce fuel consumption, and maintain engine efficiency.
Engine air filters generally need to be replaced every 15,000 to 30,000 miles, depending on the vehicle type and driving conditions. A buildup of debris in the filter can restrict airflow, impair acceleration, and lead to increased emissions of harmful 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, also known as grease filters, are used in kitchen hoods to capture grease, oil, smoke, and odors generated during cooking. Typically constructed from metal filter media, these filters are installed above stoves, grills, and fryers to maintain air quality in the kitchen.
HEPA stands for High-Efficiency Particulate Air. To qualify as a true HEPA filter, as specified by MIL-STD-282, the filter must capture at least 99.97% of airborne particles that are 0.3 microns in diameter. According to European and ISO standards, specifically ISO 29463 and EN 1822, the required efficiency is 99.95%. Filters with efficiencies ranging from 85% to 99.95% are classified as EPA filters.
HEPA filters typically use media composed of multiple layers of borosilicate glass fibers or polypropylene fibers arranged in a random, web-like pattern to enhance particulate capture. These filters employ a combination of mechanisms to trap particulates, including:
HEPA filters are crucial for protecting user health as they efficiently capture microorganisms, allergens, odors, irritants, and smoke that could potentially cause illness. They are extensively utilized in settings such as hospitals, clinics, cleanrooms, and chemical manufacturing 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.
Given that HVAC filters rely on proper airflow for effective operation, it is advisable to use furnace filters with a MERV rating below 13 for residential settings to ensure optimal performance and prevent airflow restrictions.
In an ionizer air filter, air first passes through a pre-filter before reaching the ionizer, which charges airborne particles electrically. These charged particles are then attracted to and captured by plates with an opposite charge. This type of filter does not use traditional filter media. Ionizer air filters are often called electrostatic air filters.
Ionizer air filters typically have a MERV rating between 4 and 5. They are cost-effective and have a long lifespan but require regular maintenance to ensure proper airflow. However, they may not be ideal for individuals with respiratory issues, as the electric field they generate can produce ozone, which may exacerbate asthma and other lung conditions.
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 use intense short-wave ultraviolet light to destroy the DNA of bacteria, viruses, molds, and other pathogens, effectively neutralizing them. However, when used alone, they cannot capture solid particulates or gaseous pollutants. To enhance overall air quality, UV filters are often combined with HEPA filters, similar to how activated carbon filters are used.
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 typically have a MERV rating between 6 and 8. They are a safe choice as they do not produce ozone. These filters are cost-effective, durable, and offer a long service life.
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