Glossary of Filtration Terms

Glossary of Filtration Terms

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Actual dimension

The exact size of a furnace filter is its actual dimension. The actual dimensions typically are smaller than the nominal dimensions. Most filters have the nominal measurements printed on their outer frames. The numbers may read 16 x 25 x 1; the actual dimension for this size filter would then be 15 1/2 x 24 3/4 x 3/4. These measurements are always calculated in inches.

Aftermarket filter

Aftermarket air conditioning filters, furnace filters or refrigerator water filters are products made by a third party instead of the original manufacturer. Typically, aftermarket filters are less expensive than factory original (OEM) filters. These filters are designed to replace the factory originals and share many of the same characteristics, offering the consumer a quality replacement at a discounted price.

Air cleaner

An air cleaner is a filter or device that reduces and/or removes dust and other particulates from air. Residential air cleaners may vary from devices installed into the home’s HVAC system to a portable room air cleaner that can sit atop a table or counter. A portable air cleaner does not provide whole-house filtration. Contaminants that air cleaners can remove include dust, pollen and other allergens.

Air duct

Typically constructed out of sheet metal, air ducts carry air to all the rooms in a home and are part of a home’s HVAC system. Round air ducts offer the most efficiency based on the volume of air this style of duct can move. Round air ducts also create the least resistance to airflow. Air ducts should be inspected regularly as cracks and holes can affect airflow efficiency.

Air filter

A device designed to trap particles, an air filter keeps contaminants out of circulating indoor air. Air filters can be installed in a central HVAC system or in portable air cleaners. They range in size and can be custom fit for most heating and cooling units. Filters may be flat panels, pleated, box cartridges and even washable. They have a broad range of filtration capabilities from capturing particles as large as 10 microns to particles as small as 0.3 microns.

Air pollution

Air pollution is any contaminants found in the air, both indoors and outdoors. Indoor air pollution can often be higher and more severe than outdoor air pollution. Outdoor air pollution may include industrial and automobile exhaust. Indoor air pollution includes fumes from cooking, heating appliances, tobacco smoke, vapors from paints, cleaning materials, pesticides, fabric treatments and radon.

Air purifier

An air purifier is a device that’s designed to remove contaminants from the air. Air purifiers vary in size and are produced by several manufacturers. They may remove allergens such as dust, mold spores, pollen, smoke and pet dander as well as airborne bacteria and odors. Some may have ionic purifiers while others contain HEPA filters.

Air quality

Air quality refers to the cleanliness of the air we breathe. The EPA lists the six air pollutants monitored by the National Ambient Air Quality Standards (NAAQS): carbon monoxide, nitrogen dioxide, lead, ozone/smog, particulate matter and sulfur dioxide. The 1990 Clean Air act also helps protect citizens from bad air quality and is followed by all levels of government from national to state and local.


An allergy is the immune system’s overreaction to a specific substance or allergen. Allergies can be triggered by something that is eaten, touched, injected or breathed into lungs. An allergy can cause physical symptoms such as itchy and watery eyes, sneezing, runny nose, stuffy nose, coughing, scratchy or itchy throat and skin irritations like rashes and hives. Serious allergic reactions may result in problems breathing, lowered blood pressure and even death if not treated immediately.


The United States Environmental Protection Agency (EPA) defines an allergen as “A substance capable of causing an allergic reaction because of an individual’s sensitivity to that substance.” Residential homes can harbor a multitude of allergens including dust mites, pollen, mold spores and pet dander. Allergens can cause sinus and eye irritations as well as affect people with respiratory conditions, including asthma.

American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)

Established in 1894, this professional group works for the advancement of the arts and sciences of heating, refrigeration, air conditioning and ventilation through research, education, publications, establishing industry standards/testing and promoting a sustainable world. Headquartered in Atlanta, Georgia, ASHRAE has members worldwide. ASHRAE sets the standards for air filters with through their creation of MERV (Minimum Efficiency Reporting Value) ratings.


A health threat, asbestos belongs to a mineral group distinguished by their long, skinny fibers that can float throughout the air and become inhaled, causing lung diseases. Asbestos fibers are microscopic, but can pose a serious health risk to anyone exposed to high concentrations of the fibers, exposed frequently or for an extended duration. Serious health conditions resulting from asbestos exposure include lung cancer, mesothelioma and multiple pleural conditions.


A lung disease, asthma affects 20 million Americans. It is a chronic disease, in which the airways to the lungs narrow or become blocked, causing difficulty breathing. Asthma can be grouped into two groups, allergic and non-allergic asthma. Allergic asthma is caused by allergic reactions, while non-allergic asthma is caused by conditions such as stress, exercise, cold or dry air, smoke, viruses and other irritants.

Asthma triggers

Non-allergic asthma is triggered by a variety of conditions ranging from stress and exertion/exercise to smoke and hyperventilation. Triggers for allergic asthma include multiple allergens such as pet dander, pollen, mold spores and dust mites. According to the Asthma and Allergy Foundation of America, immunoglobulin E (IgE) is produced by your body when an allergen is detected. When your body makes too much IgE, breathing can become difficult. Limiting exposure to triggers may help control asthma attacks.


An agricultural pesticide, atrazine is typically applied to crops such as corn, sugar cane and sorghum after they have been planted in order to reduce the occurrence of weeds. In some areas of the United States, it is used as weed control on residential lawns. Atrazine is regulated and monitored by the EPA.

Biological contaminants

The EPA lists biological contaminants as living organisms that, when inhaled, can cause respiratory issues, allergic reactions and infections or disease. These organisms can include bacteria, fungi and viruses as well as toxins from birds and mammals. A biological contaminant is also known as a microbial.

Carbon monoxide

Carbon monoxide (CO) is a colorless, odorless and tasteless gas. When inhaled, CO can cause serious health problems. In your home, CO can come from the fuel combustion of heating units such as furnaces or boilers. Concentrations of CO in a home also can result from the exhaust of vehicles in an enclosed, attached garage. Moderate CO exposure can cause dizziness, impaired vision and reduced brain function. High concentrations can cause death.

Centers for Disease Control and Prevention (CDC)

A governmental agency, the CDC works throughout the United States and the world monitoring health conditions, conducting research, working to prevent disease/injuries and preparing for potential new health threats. They are advocates for health and safety, setting standards for public health policies. The CDC was established in 1946 and is headquartered in Atlanta, Georgia.


A member of the halogen group, chlorine (CI) in its gaseous state is yellow-green and heavier than air. Chlorine often is used as a disinfectant in municipal water supplies and in swimming pool water to control bacteria. Chlorinated drinking water has been linked to certain cancers and as an asthma trigger.

Combustion pollutant

Gases or particulates released from the burning of fuels are considered combustion pollutants. These fuels may include but are not limited to coal, kerosene, natural gas, oil or wood. Combustion pollutants also can come from burning tobacco, carbon monoxide, nitrogen dioxide and sulfur dioxide, reports Utah State University. Health issues can result from exposure including headaches, trouble sleeping, dizziness, nausea, eye irritation and respiratory difficulties.


A contaminant is a substance that does not belong in a specific environment, potentially causing ill effects to the beings or matter in that environment. Contaminants can range from allergens such as pollen, dust mites and pet dander found in air to lead, chlorine and turbidity often present in unfiltered tap water.

Custom filter

When replacing a HVAC unit filter, you may discover the dimensions printed on the filter frame do not match any of the listed standard sizes. If the filter dimensions are non-standard, a custom filter can be the solution. Custom filters can be created to fit heating and cooling units when standard sizes will not.


Cysts are forms of protozoa, which can be the cause of parasitic diseases in humans and animals. Oval-shaped cysts such as Giardia are excreted by an infected host through feces. The cysts are resilient, able to survive for months in a cold-water environment. When humans or animals consume water containing cysts, illness can occur. Symptoms of infection can include diarrhea, stomach distress and vomiting.

Dust mites

Dust mites are microscopic bugs that feed on dead human skin cells. Dust mites can be found in every room of a home in carpeting, upholstered furniture, mattresses, pillows, bedding, clothing and stuffed toys, and are allergens to many people. According to the EPA, dust mites can trigger asthma in some people and actually cause an asthma attack in children who have never had prior asthma symptoms.

Electrostatic air filter

An electrostatic air filter uses electricity to reduce and remove particles from circulating air. Generally, this type of filter is washable or reusable. Its construction contains mold- and bacteria-resistant materials. Their initial cost may be more than a disposable filter but being reusable, cost savings accrue over time. Like disposable filters, electrostatic filters vary by size and MERV ratings.

Environmental Protection Agency (EPA)

Established in 1970, the EPA’s mission is “to protect human health and the environment.” The EPA writes regulations to implement environmental laws passed by Congress, sets standards for environmental issues at the state and local levels and works with companies and manufacturers to ensure all standards are followed. The EPA also provides continuing education and information regarding environmental issues affecting private

Fiberglass panel filter

Fiberglass or flat panel air filters offer an inexpensive air filtration choice for people without asthma or allergies. Fiberglass panel filters remove light to medium particles and typically are rated MERV 1 through 4. Their outer construction is sturdy paperboard attached to a fiberglass media panel used to trap dust, dirt and other particulates. It can be used to reduce the buildup of dirt and dust on HVAC unit coils as well.

Flow rate

The amount of fluid that moves in a specific time is the flow rate and is measured in gallons per minute, or gpm. Flow rates vary between faucets, showerheads, toilets and refrigerator water dispensers. A toilet may have a flow rate of 5 gpm while a showerhead may measure at 2.5 gpm. Refrigerator water systems and their filters have flow rates as well, which vary by manufacturer.

Furnace filter

Furnace filters are devices used to capture dust, dirt, allergens and other particulates before they travel through the HVAC unit and into the home via the ventilation ductwork. Furnace filters vary in type and include flat panel/fiberglass filters, pleated filters, box-style cartridges, HEPA filters and reusable/washable filters. Some furnace filters simply slide into a slot alongside the furnace while others are located in the blower compartment, requiring the owner to remove a panel to change the filter.


A microscopic parasite, Giardia has a protective outer shell known as a cyst. It can survive for a long time in cold water, often infecting people and animals with Giardia intestinalis after they have ingested contaminated water or food. Water is the most common way Giardia is spread. It can cause diarrhea, stomach upset, cramps, gas, dehydration and vomiting. Filtering your home’s water can reduce your risk of Giardia intestinalis.

Heating, Ventilation and Air-Conditioning System (HVAC)

HVAC refers to the unit or overall system that provides a home, business or building’s heating, ventilation and/or air conditioning. It also refers to the various systems, devices and technology used to regulate heating and cooling comfort in an indoor space. Some HVAC units may be a furnace, an air conditioner or both.

High Efficiency Particulate Arrestance (HEPA)

Designed during World War II by the U.S. Atomic Energy Commission, the HEPA filter’s original purpose was to keep radioactive particles from leaking out of laboratories. HEPA filters for HVAC filtration purposes can trap 99.97 percent of particles 0.3 microns or larger. They often are recommended for homes with people suffering from serious allergies or asthma.

HVAC filter

HVAC is the acronym for heating, ventilation, and air-conditioning (AC). HVAC filters are devices that fit into a heating and/or cooling unit or system and work to trap air contaminants such as dust, dirt and allergens. HVAC filters vary in design, size and filtration capabilities. Styles include flat panel fiberglass, pleated and washable. Other HVAC filters may be box cartridges or HEPA style filters.

Humidifier water panels

When the air in a home drops below 30 percent, health conditions such as dry, itchy skin and symptoms of sinusitis can occur. Low humidity also can cause static electricity shocks and even wood shrinkage in furniture. A humidifier can help maintain a balanced humidity level. The water panel within the humidifier is the component that once saturated with water, distributes the water as vaporized moisture.


The amount of water vapor in the air at a given time is humidity. Too much humidity in a home – 70 percent or higher – can lead to mold development, corrosion and other moisture related problems. Too little humidity – less than 30 percent – can contribute to sinus issues, skin irritations and an increase in static electricity.

Indoor air pollution

Indoor air containing contaminants can be considered polluted. Contaminants causing indoor air pollution include tobacco smoke, radon, carbon monoxide, asbestos, mold, pollen, pesticides and even household cleaners. Indoor air pollution can cause health problems such as headaches, dizziness, respiratory diseases and some cancers. Proper ventilation and air filtration can help reduce indoor air pollution. Indoor air pollution is one of the top five environmental health risks, according to the EPA.

Indoor air pollutant

An indoor air pollutant is a substance (gaseous or particulate matter) that contributes to indoor air pollution. The EPA lists particles, dust, fibers, mists, bio aerosols, and gases/vapors as air pollutants. When inhaled, indoor air pollutants can cause respiratory illnesses, headaches and dizziness. Long-term exposure to some pollutants, like asbestos, can cause certain cancers.

Indoor air quality (IAQ)

The condition of the air inside a building is its indoor air quality. When indoor air pollutants are present, the indoor air quality diminishes. Proper ventilation and air filtration can help improve indoor air quality by reducing/removing harmful contaminants found in commercial, public and residential buildings.


A metal found in the crust of the earth, lead is present in many things. It has multiple uses ranging from products such as pipes to batteries and protective shields for patients undergoing X-rays. It has been used in paint, ceramics, gasoline, pipe solder and other products. However, it has been determined through scientific research that exposure to lead can cause serious health conditions including reproductive issues, cardiovascular disease and neurological problems.


Part of a group of medicines called scabicides and pediculicides, lindane is typically used to treat scabies and lice. According to the U.S. National Library of Medicine, lindane can have serious side effects including headache, drowsiness, dizziness, uncontrollable shaking, seizures and more. Lindane, when prescribed, is available in lotion or shampoo form.


A microbial is a microscopic living organism. When inhaled, it can cause a variety of health issues including respiratory illnesses, allergic reactions and infectious diseases. Microbial material includes bacteria, viruses and other biological contaminants from birds and mammals.

Minimum Efficiency Reporting Value (MERV)

A filter’s MERV rating is a number given by ASHRAE that identifies the filter’s abilities. MERV ratings are an industry standard, with numbers ranging from 1 to 16. The larger the number the more particles and the smaller the particles the filter can trap and hold. A MERV rating of 11 traps more particulates than a filter with a MERV 4. MERV ratings do not apply to the filter’s ability to trap gasses.


A type of fungi that grows in moist environments, mold produces tiny spores to reproduce. When airborne, these spores can cause health issues including respiratory issues and allergic reactions, and can aggravate asthma. The EPA reports that the complete elimination of mold spores from a household is almost impossible; however, they can be reduced by controlling moisture and filtering indoor air.

NSF International

The National Sanitation Foundation International, established in 1944, is a not-for-profit organization dedicated to evaluating and certifying the quality for products ranging from food equipment to bottled water, faucets and refrigerator water filters. NSF International is the worldwide leading provider of “public health and safety risk management solutions.” They are headquartered in Ann Arbor, Michigan.

Nitrogen dioxide

A nonflammable gas comprised of both nitrogen and oxygen, nitrogen dioxide is a highly reactive gas. Nitrogen dioxide, according to the EPA, can form from vehicle emissions as well as emissions from off-road equipment and power plants, causing fine particle pollution. Inhalation of nitrogen dioxide can affect breathing and the overall respiratory system.

Nominal dimension

Air filters typically have numbers printed on the side of their frames. The number may read 15 x 20 x 1 or 20 x 25 x 1. These numbers are the nominal dimension (size) of the air filter. The nominal measurement is not the exact measurement of the filter. When measuring the filter, the exact or actual dimension is rounded up to the nearest 1-inch. This provides the nominal measurement. Use the nominal measurement to search for the correct size filter replacement.

Odor-control filter

Sometimes filtering particles from a home’s circulating air is not enough. An odor-control filter takes the filtration process one step further by using a medium such as activated carbon to reduce and remove odors from indoor air. Odors such as tobacco smoke, cooking aromas, vehicle exhaust and even certain pet odors may be reduced with an odor-control filter.

Organic compounds

Chemicals containing carbon atoms are organic compounds. The exceptions are CO and CO2 as well as graphite and diamond. Organic compounds are found in most fuels, cleaning products and many items around the house. Several of these can contain VOCs or volatile organic compounds that emit harmful vapors at room temperature. Breathing in VOCs can cause headaches, dizziness and other health issues.

Original Equipment Manufacturer (OEM) Filter

Any filter labeled OEM has been produced by the original factory brand manufacturer. It has been sanctioned officially by the original company that manufactured the HVAC unit. Sometimes OEM filters may cost a bit more than an aftermarket filter. However, more OEM filters receive standards testing for quality and often come with a manufacturer’s guarantee.


Microscopic components make up particulates. The particulates often found in homes can be a mixture of solid and liquid particles such as fumes and smoke. Particulate matter or PM also can be biological in composition such as bacteria, dust mites, mold spores, bacteria, pollen and pet dander. Particulates range in size. The finest or smallest particles can pose the most health concerns as these are inhaled more easily.

Pet dander

The pet population in the United States is more than 100 million. But pets can trigger allergies and even asthma attacks in many people. Pet fur is not the allergen, but pet dander is – pet dander is the tiny bits of dead skin your pet sheds. Dander can collect on an animal’s fur and when touched can transfer to the person and cause an allergic reaction. Pet dander also can collect on fabric, furniture, walls and carpet and circulate throughout your home’s air.

Pleated filter

The pleated filter is designed to create less airflow resistance, allowing the filter to capture more particulates than a fiberglass filter. This can help keep a home’s indoor air quality at a healthier level. Pleated filters range in MERV ratings, typically from a MERV 4 to the highest level of MERV 16 for certain pleated box-style filters.


An allergen, pollen is the very small, dry particles from plants like weeds, grass, flowers and even trees. When inhaled, pollen sticks to the mucous membranes inside the nose, causing an allergic or histamine reaction. Pollen can be carried indoors on clothing, hair and on the fur of pets. Pollen allergies symptoms include itchy and watery eyes, sneezing, stuffy nose and other respiratory problems.


Any substance that contaminates air, water or soil is considered a pollutant. Pollutants such as dust and mold spores can affect indoor air quality aggravating allergies and causing respiratory issues. Lead and chlorine are pollutants that can be found in tap water. If ingested regularly, they can make drinking water taste or smell bad, and even pose potential health problems.

Pounds per square inch (PSI)

A unit of pressure, PSI measures the amount of pressure on an area that is one square inch. The water pressure for a home is measured in PSI. This pressure provides the energy the water needs to flow through the pipes and plumbing system. Refrigerators with icemakers or water dispensers have a PSI measurement as well. When replacing a refrigerator water filter, the filter’s PSI measurement should match the refrigerator’s measurement.


A radioactive gas, radon is colorless and odorless, and forms from the breakdown of uranium. Uranium may be present in soil, rocks, ground water and natural gas. It also can be found in building materials made from minerals. Radon produces radioactive particles that attach to dust particles, reports the EPA. It can be inhaled and deposited into the lungs, resulting in respiratory issues.

Reusable filter

A reusable or washable air filter may also be referred to as an electrostatic air filter. These filters may use static electricity to remove/reduce dust, dirt and allergens from a home’s circulating air. As the air moving through the reusable filter generates an electrostatic charge, the particles/contaminants become attracted to the filter’s charged fibers. Reusable filters need to be washed regularly in order to continue to work properly.

Refrigerator water filter

Refrigerators with water dispensing systems contain a water filter to clean the tap water before it is consumed. Refrigerator water filters vary in size, price and filtration capabilities. They are designed to reduce impurities found in unfiltered tap water. Refrigerator water filters may reduce and remove contaminants such as turbidity, lead, cysts, chlorine and other harmful chemicals.

Secondhand smoke

Also called “environmental tobacco smoke”, secondhand smoke is defined by the American Cancer Society (ACS) as either side stream or mainstream smoke. Side stream smoke is the smoke emitted from the lit end of any smoking material including cigarettes, cigars and pipes. Mainstream smoke is any smoke exhaled by a smoker, states the ACS. The ACS also states that side stream smoke contains higher concentrations of carcinogens than mainstream smoke. Breathing in any secondhand smoke can cause exposure to harmful levels of chemicals.


One of water’s main physical characteristics, turbidity is the cloudiness often found in water caused by microscopic particles. It causes light to fan out instead of allowing it to stream in straight lines. The contaminants that cause turbidity may be from both organic and inorganic matter. Often turbidity is caused by silt and clay particles.


Ventilation is the movement of air. According to the U.S. Department of Energy, “Ventilation refers to the exchange of indoor and outdoor air. Without proper ventilation, an otherwise insulated and airtight house will seal in harmful pollutants, such as carbon monoxide.” It also can be defined as a mechanical system or equipment that keeps fresh air circulating throughout an environment or space. Proper ventilation is needed to maintain healthy indoor air quality.

Volatile organic compounds

Also known as VOCs, these compounds contain carbon and become a vapor or gas at room temperature. Cleaning products, paints, pesticides, new carpeting, kerosene, gasoline, cigarette smoke and more are examples of substances that can emit VOCs. Volatile organic compounds can cause ill health effects – headaches, dizziness, vision problems, cognitive difficulties and even certain cancers.

Water filter

A water filter is a device to reduce/remove contaminants from water. It may be a cartridge-style unit that fits into a refrigerator, a smaller unit designed to fit into a gallon pitcher or even an individual water bottle. Water filters typically have a hard plastic outer housing that encases a carbon media that traps contaminants.

Water pressure

The strength of water’s flow as it moves through plumbing (or any other water system) is its pressure. The more water that pushes through a system, the more pressure it exerts or the stronger its flow. Water pressure can be measured using PSI or pounds per square inch. PSI measures the pressure on an area of one square inch. Water pressure in residential homes will vary by their proximity to the water source as well as their elevation in relation to the water source.

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