Air purifier

    An air purifier is a device which removes contaminants from air. Air purifiers are commonly marketed as being particularly beneficial to allergy sufferers and asthmatics and at reducing second-hand tobacco smoke.

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Use of purifiers

Dust, pollen, pet dander, mold spores, and 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.

Purifying techniques

Several 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 become very ineffective for particles smaller than 0.01 micrometer. HEPA purifiers which filter all the air going into 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 that of 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 using a high-powered HEPA machine at a fraction of the initial capital expenditure. Unfortunately, most furnace filters are slid in place without an airtight seal, which allows air to pass around the filters. This problem is worse for the higher-efficiency MERV filters because of the increase in air resistance. Higher-efficiency MERV filters are usually denser and increase air resistance in the central system, requiring a greater air pressure drop and consequently increasing 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[1]; 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 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 produce 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. The only safe use of ozone generators is in unoccupied rooms, utilizing "shock treatment" commercial ozone generators that produce over 3000 mg of ozone per hour. Restoration contractors use these types of ozone generators to remove smoke odors after fire damage, musty smells after flooding, mold (including toxic molds), and the stench caused by decaying flesh which cannot be removed by bleach or anything else except for ozone. However, it is not healthy to breathe ozone gas, and one should use extreme caution when buying a room air purifier that also produces ozone.

Consumer concerns

When selecting air purifiers, consumers are influenced by several factors besides 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 produce low amounts of ozone (<0.05 ppm). The noise level of a purifier can be obtained easily through a customer service department and is usually reported in decibels (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 over long periods 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 cleaner, 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.

In September, 2007, the California Air Resources Board announced a ban of in-home ozone purifiers. This law, which will take effect in 2009, will require testing and certification of all types of air purifiers for ozone emissions.

See also

• Air filter
• Air ioniser
• Electrostatic precipitator
• Ozone generator
• HEPA