Function

HEPA filters are composed of a mat of randomly arranged fibres. Key metrics affecting function are fibre density and diameter, and filter thickness. The air space between HEPA filter fibres is much greater than 0.3 μm. The common assumption that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through is incorrect. Just as for membrane filters, particles so large that they are as wide as the largest opening or distance between fibres can not pass in between them at all. But HEPA filters are designed to target much smaller pollutants and particles are mainly trapped (they stick to a fibre) by one of the following three mechanisms:

1. Interception, where particles following a line of flow in the airstream come within one radius of a fibre and adhere to it.
2. Impaction, where larger particles are unable to avoid fibres by following the curving contours of the airstream and are forced to embed in one of them directly; this increases with diminishing fibre separation and higher air flow velocity.
3. Diffusion, an enhancing mechanism is a result of the collision with gas molecules by the smallest particles, especially those below 0.1 µm in diameter, which are thereby impeded and delayed in their path through the filter; this behaviour is similar to Brownian motion and raises the probability that a particle will be stopped by either of the two mechanisms above; it becomes dominant at lower air flow velocities.

Diffusion predominates below the 0.1 μm diameter particle size. Impaction and interception predominate above 0.4 μm. In between, near the 0.3 μm MPPS, diffusion and interception predominate.

The initial filter air flow resistance and final filter air flow resistance are typically measured as pressure drop across the filters.

This type of air filter can remove at least 99.97% of airborne particles 0.3 micrometres (µm) in diameter. Particles of this size are the most difficult to filter and thus the most penetrating particle size (MPPS). Particles that are larger or smaller are filtered with even higher efficiency.

History

The original HEPA filter was designed in the 1940s and was used in the Manhattan Project to prevent the spread of airborne radioactive contaminants. It was commercialised in the 1950s, and the original term became a registered trademark and a generic term for highly efficient filters. Over the decades filters have evolved to satisfy the higher and higher demands for air quality in various high technology industries, such as aerospace, pharmaceutical processing, hospitals, healthcare, nuclear fuels, nuclear power, and electronic microcircuitry (computer chips).

Today, a HEPA filter rating is applicable to any highly efficient air filter that can attain the same filter efficiency performance standards as a minimum and is equivalent to the more recent NIOSH N100 rating.

Nuclear industry application

HEPA filters must be correctly installed in a filter housing or frame to achieve proper results. In the Nuclear Fuels and Nuclear Power Generation industries, these housings are sometimes referred to as filter trains. Filter Housings are usually arranged in an array with 24 inch by 24 inch by 11½ inch deep filters (Size # 7, DOE-STD-3020-2005) having a nominal capacity of 1500 cfm (0.7 m³/s) each (see the DOE Nuclear Air Cleaning Handbook).

A good general reference for Nuclear Facility HVAC design is Chapter 26 “Nuclear Facilities” found in the ASHRAE 2003 HVAC Applications Handbook.

Bio-medical applications

HEPA filters are critical in the prevention of the spread of airborne bacterial and viral organisms and, therefore, infection. Typically, medical-use HEPA filtration systems also incorporate high-energy ultra-violet light units to kill off the live bacteria and viruses trapped by the filter media. Some of the best-rated HEPA units have an efficiency rating of 99.995%, which assures a very high level of protection against airborne disease transmission.

Vacuum cleaners

Many vacuum cleaners also use HEPA filters to filter the exhaust air. This is beneficial for asthma and allergies sufferers, because the HEPA filter traps the fine particles (such as pollen and dust mite feces) which trigger allergy and asthma symptoms. However, for a HEPA filter in a vacuum cleaner to be effective, the vacuum cleaner must be designed so that all the air drawn into the machine is expelled through the filter, with none of the air leaking past it. Also, because of the extra density of a HEPA filter, the vacuum cleaner’s motor needs to be more powerful to overcome the extra air resistance of the filter while still providing adequate cleaning power.

Use of Purifiers

Dust, pollen, pet dander, mold spores, dust mite feces can act as allergens, triggering allergies in sensitive people. Smoke particles and volatile organic compounds (VOCs) can pose a risk to health. Air purifiers are used to reduce the concentration of these airborne contaminants and are especially useful for people who suffer from allergies and asthma. They also reduce the need for frequent household cleaning. Air purifiers use a small amount of electrical energy, causing some expense and environmental effect.Clean Air Delivery Rate (CADR)

CADR, or Clean Air Delivery Rating, is a standardized indicator of how well an air purifier functions. The higher the number is, the stronger the power of purification. CADRs are rated by AHAM (Association of Home Appliance Manufacturers), and are based on stringent requirements as produced by the association itself. For a given air purifier, AHAM certifies 3 CADRs for its removal ability of smoke, dust and pollen, respectively. The CADR value is expressed in cubic feet per minute and can be related to the advised floor space. The advised floor space for a purifier is usually calculated under an 8 ft ceiling with a 5 air change per hour setting. For example, a 250 CADR purifier can deliver 250 ft³ (7 m³) of clean air per minute, thus 15,000 ft³ (425 m³) per hour. Dividing 15000 ft³ (425 m³) per hour by a 8 ft ceiling and then by a 5 air change per hour results a recommended 375 ft² floor space. The recommended floor space can also be calculated by multiplying the CADR value for smoke by 1.55 [1] Although floor space is an easier concept to relate to, there is no legal guideline for it and some companies use a lower air change number to calculate their suggested floor space. Consequently, it is safer to compare purifiers based on the CADR values. Few purifier companies do not participate in the CADR testing, instead reporting the fan speeds in cubic feet per minute (CFM). Because a purifier cannot clean more air than it circulates, the CADR value of a purifier is smaller than its fan speed. The performance of an uncertified purifier can be estimated based on its fan speed in CFM, while understanding that the fan speed is a upper limit of the true CADR value.Purifying techniquesSeveral different processes of varying effectiveness can be used to purify air. Different processes may remove different contaminants, so there is advantage in using more than one process in a purifier.

• Filter-based purification traps airborne particles by size exclusion. Air is forced through a filter and particles are physically captured by the filter.

HEPA filters can, by definition, remove at least 99.97% of 0.3 micrometer particles, and are usually more effective for particles which are larger or slightly smaller. They are effective down to 0.01 micrometers in many cases, but becomes very ineffective for particles smaller than 0.01 micrometer. HEPA purifiers which filter all the air going into, say a clean room, must be arranged so that no air bypasses the HEPA filter. In dusty environments, a HEPA filter may follow an easily-cleaned conventional filter (prefilter) which removes coarser impurities so that the HEPA filter does not need to be changed or cleaned frequently.

Filter for HVAC at MERV 13 or above can remove airborne particles of 0.3 micrometers or larger. A medium efficiency MERV 13 has a capture rate of 75% for particles between 0.3 to 1.0 micrometers. Although the capture rate of a MERV filter is lower than a HEPA filter, a central air system can move significantly more air in the same period of time. Using a high grade MERV filter can be more effective than a high-powered HEPA machine at a fraction of the initial capital expenditure. Unfortunately, most furnace filters are slided in place without an airtight seal, which allows air to pass around the filters. This problem is worse for the higher efficiency MERV filters due to the increase air resistance. Higher efficiency MERV filters are usually denser and increase air resistance in the central system, requiring a greater air pressure drop, consequently increases energy costs.

• Activated carbon is a highly porous material that can absorb volatile chemicals on a molecular basis, but does not remove larger particles. It is normally used in conjunction with other filter technology, especially with HEPA. Air cleaners that do not feature at least 15 lb (7 kg) of carbon can not efficiently remove VOCs, chemicals, and strong odors from the air^{[citation needed]}. Other materials also can absorb chemicals, but at higher cost.
• Photocatalytic oxidation (PCO) uses short-wave ultraviolet light (UVC), commonly used for sterilization, to kill 99.9% of bacteria and viruses^{[citation needed]}. Independent research confirms its effectiveness against molds, bacteria, and viruses^{[citation needed]}. UVC in-duct units can be mounted to an existing forced-air HVAC system. PCO is not a filtering technology, as it does not trap or remove particles. It is sometimes coupled with other filtering technologies for air purification. UV sterilization bulbs must be replaced about once a year[2]; manufacturers may require periodic replacement as a condition of warranty.
• Electrostatic precipitators are HVAC air cleaners which remove particles from air effectively if used properly.

Some purifying technologies are less effective at cleaning large volumes of air, and may be more suitable for use in smaller spaces or in specific situations.

• Ionizer purifiers use charged electrical surfaces or needles to generate electrically charged ions. Ions are attach to airborne particles which are then electrostatically attracted to a charged collector plate. This mechanism produces trace amounts of ozone and other oxidants as by-products. Most ionizers produces less than 0.05 ppm of ozone, an industrial safety standard. There are two major subdivisions: the fanless ionizer and fan-based ionizer. Fanless ionizers are noiseless and use little power, but are less efficient at air purification. Fan-based ionizers clean and distribute air much faster.
• Ozone generators produce ozone, and are sometimes sold as whole house air cleaners. Unlike ionizers, ozone generators are designed to produce significant amounts of ozone, a strong oxidant gas which can oxidize many other chemicals. Ozone can be used in unoccupied rooms for odor or smoke removal, but is harmful to health and should not be used in occupied rooms. Unless those ozone generators which produces ozone emission certified to be below toxic level of 0.05ppm are deemed to be safe for use in occupied rooms even for 24 hours of exposure.

Consumer concerns

When selecting air purifiers, consumers are influenced by several factors beside the cleaning ability. These include possible hazardous gaseous by-products, noise level, frequency of filter replacement, electrical consumption, and visual appeal. Ozone production is typical for ionizing purifiers and has received much attention recently. Although high concentration of ozone is dangerous, most ionizers produces low amounts of ozone (<0.05 ppm). Some purifiers also produce hydroxyl radicals and nitrogen oxides, and they can be dangerous to health. The noise level of a purifier can be obtained easily through consumer service department and is usually reported in decibel (dB). The noise levels for most purifiers are low compared to many other home appliances and are not expected to cause hearing loss. However, purifiers are expected to operate long period of time. Therefore, even a moderate level of noise can be disturbing to some people. Frequency of filter replacement and electrical consumption are the major operation costs for any purifier. There are many different types of filters; some can be cleaned by water, by hand or by vacuum-cleaning, while others need to be replaced every few months or years. Some purifiers are certified as energy star and are energy efficient.HEPA technology is often used in portable air purifiers as it removes common airborne allergens. The Department of Energy has rigid requirements manufacturers must pass to meet HEPA requirements. The HEPA specification requires removal of at least 99.97% of 0.3 micrometres airborne pollutants. Products that claim to be “HEPA-type”, “HEPA-like”, or “99% HEPA” do not satisfy these requirements and may not be tested in independent laboratories.Air ionizers and ozone

As with all health-related appliances, there is some controversy surrounding the claims of certain companies, specifically involving ionic air purifiers. Particularly, some ionic air purifiers generate the pollutants ozone (an energetic allotrope of oxygen) and NOx. Either can be toxic in sufficient concentrations. Ironically, people who have asthma and allergy are most prone to the adverse effects. For example, increasing ozone concentration can increase the risk of asthma attacks. Due to the below average performance and potential health risks, Consumer Reports have advised against using ionizers.

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