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How Air Cleaner Efficiency Is Measured

Air cleaner efficiency is measured by how thoroughly an air cleaner or purifier captures particles, and the amount of air it can process in a given period of time. In other words, an air purifier is only as good as its ability to catch particles of all sizes and process enough air to make a difference.air cleaner efficiency

To this end, the finer the sieve-like action of a mechanical filter, the smaller the particles it can block. However, the finer the sieve, the more quickly the filter clogs up with dust. Though most mechanical filters actually catch more dust as they become more saturated, airflow grows increasingly stymied, reducing the overall air cleaner efficiency. The best mechanical filters have a very fine sieve and a large surface area to maximize both dust collection and airflow, resulting in high air cleaner efficiency.

Warning: Ionization air cleaners that emit ozone have fallen under serious scrutiny because of the hazards associated with ozone’s effects on the body. California, in fact, has banned the sale of all air cleaners that emit ozone. For more about this, see the California Air Resources Board site.

Filters that use electrical attraction to grab particles (electrostatic precipatator) are different. Most don’t slow air as they “fill up,” but their charge is diminished by the accumulation of particles. They’re most effective when they’re clean and when the air passes through them at a relatively slow rate.

air cleaner efficiency

Clockwise from top left: HEPA filter, electret filter, electrostatic precipitator, negative ion generator

Dust particles are measured by the micron (one micron equals 1/25,400 of an inch). The human eye can see lint, pollen, and dust particles larger than 10 microns. Most air cleaners eliminate a large percentage of visible dust that passes through them, but to be really effective, an air cleaner should eliminate most microscopic particles, too.

The standard of air cleaner efficiency at eliminating tiny particles was long ago set by the HEPA filter, which is tested at its ability to eliminate particles that average .3 micron in size. This is the particle size most likely to be inhaled and deposited in the lungs and the most difficult for mechanical filters to catch. More recently, the ULPA filter was developed and is capable of removing particles as small as .1 micron.

The effectiveness of a filter is tested and given an ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) rating. The first rating, Initial Staining Dirt Efficiency, is a critical measurement of how well the filter removes microscopic staining particles such as tobacco tar, allergens, and grease. The second rating, Average Arrestance Test, refers to the percentage of ordinary dust, hair, lint, and other large particulate matter removed. When comparing ratings, it’s important to know which of these two is being listed.

The Association of Home Appliance Manufacturers (AHAM) certifies some portable air cleaners with a Clean Air Delivery Rate (CADR). This measures how many cubic feet per minute (CFM) of air are cleared of a certain size particle. Three particle sizes are rated: smoke (minute), dust (medium), and pollen (large). The higher the CADR rating, the less time the unit needs to remove a given quantity of certain particles from a specified room size. AHAM-certified models are marked with CADR data.

Improve Your Home’s Air Quality Now

How to improve air quality with air cleaners & purifiers. Find expert information on how air cleaners work, buying the right air cleaner, and other methods of improving your home’s air.

Did you know that every cubic foot of air in the average home is loaded with about 3 million dust particles?

air cleaners & purifiers

[/media-credit] Portable air cleaner removes dust and pollen using a true HEPA filter.

Though most of these particles are so small they’re not visible, they can nonetheless be a health hazard, particularly for people who suffer from allergies or bronchial problems. That’s where air cleaners come in.

How Air Cleaners Work

Air cleaners (sometimes called air filters or air purifiers) can help reduce the problem by filtering out the microscopic particles caused by smoking, fireplaces, pets, molds, pollen, and other sources of indoor pollution. A variety of technologies are employed by these devices to clean the air. Some draw room air through finely woven filters that catch particles before returning air to the room. Others, such as electrostatic precipitating air cleaners, use electrical charges to attract particles in the air.

For more about various types and how they work, please see How Air Cleaners Work.

Air Cleaners Buying Guide

You can buy air cleaners that filter the air in a single room or models that connect to the home’s central air conditioning or heating system to handle cleaning the air throughout the entire house. When buying an air cleaner, you’ll want to focus on effectiveness, the unit’s size and capacity, price, and more. For help with these decisions, see the Air Cleaners & Purifiers Buying Guide.[GARD align=”right”]

How Efficiency Is Measured

An air cleaner is only as good as its ability to extract contaminants, dust, and particles from the air. But how can you tell the abilities of various air cleaners do do this? Fortunately, air cleaners are given standardized ratings. For more about this, see How Air Cleaner Efficiency Is Measured.

Tips for Indoor Air Quality

Buying an air cleaner isn’t the only way to improve your home’s indoor air quality. You can also reduce the number of pollutants expelled into the air to make it cleaner. For tips about what you can do, see How to Improve Interior Air Quality.

Improve Your Home's Air Quality Now

How to improve air quality with air cleaners & purifiers. Find expert information on how air cleaners work, buying the right air cleaner, and other methods of improving your home’s air.

Did you know that every cubic foot of air in the average home is loaded with about 3 million dust particles?

air cleaners & purifiers

[/media-credit] Portable air cleaner removes dust and pollen using a true HEPA filter.

Though most of these particles are so small they’re not visible, they can nonetheless be a health hazard, particularly for people who suffer from allergies or bronchial problems. That’s where air cleaners come in.

How Air Cleaners Work

Air cleaners (sometimes called air filters or air purifiers) can help reduce the problem by filtering out the microscopic particles caused by smoking, fireplaces, pets, molds, pollen, and other sources of indoor pollution. A variety of technologies are employed by these devices to clean the air. Some draw room air through finely woven filters that catch particles before returning air to the room. Others, such as electrostatic precipitating air cleaners, use electrical charges to attract particles in the air.

For more about various types and how they work, please see How Air Cleaners Work.

Air Cleaners Buying Guide

You can buy air cleaners that filter the air in a single room or models that connect to the home’s central air conditioning or heating system to handle cleaning the air throughout the entire house. When buying an air cleaner, you’ll want to focus on effectiveness, the unit’s size and capacity, price, and more. For help with these decisions, see the Air Cleaners & Purifiers Buying Guide.[GARD align=”right”]

How Efficiency Is Measured

An air cleaner is only as good as its ability to extract contaminants, dust, and particles from the air. But how can you tell the abilities of various air cleaners do do this? Fortunately, air cleaners are given standardized ratings. For more about this, see How Air Cleaner Efficiency Is Measured.

Tips for Indoor Air Quality

Buying an air cleaner isn’t the only way to improve your home’s indoor air quality. You can also reduce the number of pollutants expelled into the air to make it cleaner. For tips about what you can do, see How to Improve Interior Air Quality.

How Air Cleaners & Purifiers Work

Dust can be removed from air using any of several technologies. Most involve cycling the air through a filter that traps or attracts dust particles to withdraw them from the air. In actuality, most tabletop and room air cleaners send air through a gauntlet of filters: a foam pre-filter to screen out large particles, the primary filter or electrical-attraction device to catch smaller particles, a carbon filter to remove odors and some gases, and a post-filter for any remaining particles.

The spectrum of air filters and cleaners is differentiated in a number of ways. Here’s a closer look at the main types:

Four different types of air cleaner (clockwise from top left): HEPA filter, electret filter, electrostatic precipitator, negative ion generator

HEPA, which stands for High Efficiency Particulate Accumulation, filters are highly effective. These filters were originally designed for removing radioactive particles from air. Today, HEPA filters are in certain portable and room-sized residential home air filters. Made from finely woven glass particles formed into a pleated filter that has a very large surface area, a HEPA filter—by definition—removes 99.97% of all .3-micron particles from the air.

Unfortunately, cost is a disadvantage of HEPA filters—they generally must be replaced once every year to 18 months, depending upon the extent of use, and they typically cost from $70 to $125. And, because the filters are so dense, they require a strong blower to move air through, racking up higher energy costs. The larger the surface area of a HEPA, the better, so find out its square footage before buying one.

ULPA, which stands for Ultra Low Penetrating Air, filters were developed for use in hospitals and manufacturing “clean rooms” such as those employed in the semiconductor industry. Now available in self- contained home air cleaners, the filter is an intercut web of micro-fibers that traps 99.99% of .1 micron particles.

“HEPA-type” filters, made for both self-contained units and forced-air systems, have not met the rigorous standards of a genuine HEPA, but some, like the pleated types, are almost as effective. Most range from 25% to 95% effective at microscopic particle removal. Before buying one, be sure you know how good it is at blocking respirable-size particles. With continuous use, HEPA-type filters must be replaced every year at a cost of $30 to $45.

Though ordinary 99-cent furnace filters catch about 75% of bulk dirt, they capture very little— often less than 3%—of minute pollutants. Slightly more effective for a forced-air system are treated filters. These disposable products, which typically sell for under $10, are coated with dirt-trapping glycol or mineral oil. Though they stop about the same percentage of bulk dirt, they block up to about 10% of the smaller, microscopic particles.

An electret, or electrostatic air filter, uses synthetic fibers that create a static charge to attract particles. Costing from $15 to $100, electrostatic filters come plain or pleated, disposable or reusable. You just hose down reusable models to clean them, but they still have to be replaced periodically. Disposable models should be replaced quarterly. Depending on the make, an electrostatic filter blocks between 10% and 35% of microscopic particles. Some are sold as kits; the frame is permanent and the filter pad is replaced every couple of years.

A negative-ion generator, or ionizer purifier, uses tiny charged wires or needles to create ions (gas molecules with a negative charge) that latch onto airborne particles. Some types have a fan in the unit that speeds and improves the cleaning and distribution of air. These are noisier but more effective than other types that don’t have a fan. All produce trace amounts of ozone and oxidants. With this type of air cleaner, airborne particles tend to stick to walls, floors, tables, and other surfaces near the unit.

An electrostatic precipitating (“electronic”) air cleaner draws particles in by fan and charges them with a series of high-voltage wires. A precipitating cell (a series of plates) that carries the opposite electrical charge attracts the particles as they pass by. You can buy portable versions or whole-house models that connect to the cold-air return plenum on the furnace. They are quite effective, removing about 95% of bulk dirt and 85% of microscopic particles. Whole-house electronic air cleaners, sold through heating contractors, cost $300–$900 installed, depending on the complexity of the installation.

Ozone generators produce significant amounts of ozone—the main ingredient in air pollution. They have come under heavy scrutiny for the serious health risks they present. In 2007, the state of California banned their sale. Beware of buying or using a room air purifier that produces ozone.

How Air Cleaners & Purifiers Work

Dust can be removed from air using any of several technologies. Most involve cycling the air through a filter that traps or attracts dust particles to withdraw them from the air. In actuality, most tabletop and room air cleaners send air through a gauntlet of filters: a foam pre-filter to screen out large particles, the primary filter or electrical-attraction device to catch smaller particles, a carbon filter to remove odors and some gases, and a post-filter for any remaining particles.

The spectrum of air filters and cleaners is differentiated in a number of ways. Here’s a closer look at the main types:

Four different types of air cleaner (clockwise from top left): HEPA filter, electret filter, electrostatic precipitator, negative ion generator

HEPA, which stands for High Efficiency Particulate Accumulation, filters are highly effective. These filters were originally designed for removing radioactive particles from air. Today, HEPA filters are in certain portable and room-sized residential home air filters. Made from finely woven glass particles formed into a pleated filter that has a very large surface area, a HEPA filter—by definition—removes 99.97% of all .3-micron particles from the air.

Unfortunately, cost is a disadvantage of HEPA filters—they generally must be replaced once every year to 18 months, depending upon the extent of use, and they typically cost from $70 to $125. And, because the filters are so dense, they require a strong blower to move air through, racking up higher energy costs. The larger the surface area of a HEPA, the better, so find out its square footage before buying one.

ULPA, which stands for Ultra Low Penetrating Air, filters were developed for use in hospitals and manufacturing “clean rooms” such as those employed in the semiconductor industry. Now available in self- contained home air cleaners, the filter is an intercut web of micro-fibers that traps 99.99% of .1 micron particles.

“HEPA-type” filters, made for both self-contained units and forced-air systems, have not met the rigorous standards of a genuine HEPA, but some, like the pleated types, are almost as effective. Most range from 25% to 95% effective at microscopic particle removal. Before buying one, be sure you know how good it is at blocking respirable-size particles. With continuous use, HEPA-type filters must be replaced every year at a cost of $30 to $45.

Though ordinary 99-cent furnace filters catch about 75% of bulk dirt, they capture very little— often less than 3%—of minute pollutants. Slightly more effective for a forced-air system are treated filters. These disposable products, which typically sell for under $10, are coated with dirt-trapping glycol or mineral oil. Though they stop about the same percentage of bulk dirt, they block up to about 10% of the smaller, microscopic particles.

An electret, or electrostatic air filter, uses synthetic fibers that create a static charge to attract particles. Costing from $15 to $100, electrostatic filters come plain or pleated, disposable or reusable. You just hose down reusable models to clean them, but they still have to be replaced periodically. Disposable models should be replaced quarterly. Depending on the make, an electrostatic filter blocks between 10% and 35% of microscopic particles. Some are sold as kits; the frame is permanent and the filter pad is replaced every couple of years.

A negative-ion generator, or ionizer purifier, uses tiny charged wires or needles to create ions (gas molecules with a negative charge) that latch onto airborne particles. Some types have a fan in the unit that speeds and improves the cleaning and distribution of air. These are noisier but more effective than other types that don’t have a fan. All produce trace amounts of ozone and oxidants. With this type of air cleaner, airborne particles tend to stick to walls, floors, tables, and other surfaces near the unit.

An electrostatic precipitating (“electronic”) air cleaner draws particles in by fan and charges them with a series of high-voltage wires. A precipitating cell (a series of plates) that carries the opposite electrical charge attracts the particles as they pass by. You can buy portable versions or whole-house models that connect to the cold-air return plenum on the furnace. They are quite effective, removing about 95% of bulk dirt and 85% of microscopic particles. Whole-house electronic air cleaners, sold through heating contractors, cost $300–$900 installed, depending on the complexity of the installation.

Ozone generators produce significant amounts of ozone—the main ingredient in air pollution. They have come under heavy scrutiny for the serious health risks they present. In 2007, the state of California banned their sale. Beware of buying or using a room air purifier that produces ozone.

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