Air Purifier and Air Cleaner Technologies

The science behind HEPA filtration, ionic air purification, and ozone can be complicated, and is still the subject of lively debate in many air quality and health arenas, but it all essentially boils down to one concept: some air cleaning technologies perform better than others at certain tasks.

While there are a wide variety of air purifier and air cleaner products on the market today, almost all of them employ one or more of three main types of air cleaning technology: HEPA filters, Ionic or Electrostatic technology, or Ozone Generation. We'll take a look at each type of air purification process, and the strengths and weaknesses of each.

HEPA Filters

High-Efficiency Particulate Air (HEPA) filters, once known as high-efficiency particle arrestors, are perhaps the best-known type of air cleaners on the market today, and certainly one of the most effective. HEPA filter air cleaners employ a fan to move air across a pleated fabric filter which is embedded with submicronic glass fibers, which traps particulate matter in the air, holding it in the filter media. HEPA filters were originally developed during World War II to filter dangerous radioactive particles from the exhaust of nuclear reactor facilities, and are now widely used in hospitals, laboratory "clean rooms," and even home air cleaners and allergen reducers.

To qualify as a "true" HEPA filter, the filter must achieve a minimum 99.97% removal efficiency for all airborne particles of 0.3 micron diameter, meaning no more than 3 in 10,000 particles of the 0.3-micron size may pass through the filter. The standard follows US Military Standard MIL-STD-282, and the 0.3 micron size is particularly important because particles of 0.3 microns in diameter are proven to be the most likely particles to enter the lungs where they can cause irritation or infection.

A micron is a metric measure of measurement equaling one millionth of a meter, less than 0.00004 inches! To give you some idea what the 0.3 micron figure represents, a human hair, for example, is typically 70-100 microns thick, while a single grain of pollen may measure from 5 to 100 microns in diameter. Anything smaller than 10 microns in size is invisible to the human eye.

The strength of a HEPA filter is that the special filter media can remove almost all of the particle pollutants in the air, including allergens such as dust, mold spores, and pollen, greatly improving the air quality in a room, and alleviating many symptoms of allergy and asthma sufferers. While not all consumer air cleaners employ "true" certified HEPA filters, even the so-called "HEPA-type" filters can still be very effective at ridding the air in your home of airborne irritants and pollutants. Because HEPA filters are not designed to remove gaseous chemicals or odors from the air, they are often combined with other technologies which remove smoke and odors. Similarly, the HEPA filter does not actually kill disease-causing agents like bacteria and viruses: a HEPA filter may trap most bacteria and prevent them from re-entering the air, some bacteria and almost all viruses are too small to be filtered.

Ionic Air Purifiers

Ionic or electrostatic air purifiers as they're sometimes known, do not use a physical filter; instead they work by ionizing the air, imparting a positive or negative charge to particles in the air as they pass through the unit. These ionized particles are then removed from the air by one of two major methods. An ion generator imparts a charge to pollutant particles in the air, which then are attracted to and deposited on walls or furniture, or stick together and fall out of the air. More advanced ionic air purifiers use a principle called electrostatic precipitation to trap particles within the unit, collecting them on an oppositely-charged metal plate or sleeve, which must then be routinely cleaned. Either type of ionic air cleaner may also include a fan to help the air circulate, though many do not.

While ionic or electrostatic units are theoretically capable of removing even the smallest of airborne particles, they are generally not as efficient as HEPA filters in removing pollutants, and their effectiveness decreases with use as the collection plates become covered in collected particulates. In models without collection plates, the pollutants are simply deposited on the walls and floors of the room, and may be stirred up and re-enter the air.

On the other hand, the benefits of an electronic air cleaner lie in its low energy consumption and quiet operation. HEPA filters, for example, use densely-packed filter fibers requiring a powerful fan to move air through the unit, generally resulting in higher energy consumption and a noise level that many people find uncomfortable, especially in a bedroom setting. In contrast, air ionizers are typically very quiet or nearly silent in operation, and require very little energy to work. Also, since there are no filters to replace, the long-term operating costs of an electronic air purifier can be much lower than their HEPA filter counterparts.

Though we'll cover it in more detail in the following section about Ozone Generators, a word is needed about the health effects of ozone. Ozone is an important component of the earth's upper atmosphere, and the stratospheric "ozone layer" is essential to the health of every living creature on earth. Low-level Ozone, however—ozone at ground level—is a known irritant and can be dangerous to human health, especially for asthma sufferers or other people with respiratory problems of all types. People in the Air Quality regulatory community have long used the phrase "Good up high, bad nearby" to explain the difference between stratospheric ozone and toxic low-level ozone.

All ionic air cleaner devices produce some ozone; it's a natural by-product of the ionizing process. Some manufacturers actually claim this as a benefit, as ozone does in fact neutralize odors in the air, leaving a "fresh" smell. For this reason, ionic air cleaners do have some effect on odors, whereas HEPA filters do not. For asthma sufferers in particular, even the low levels of ozone produced by these units is not worth the tradeoff, but the effects of low-level ozone on otherwise healthy individuals is debatable. There are no definitive standards set for home air purifiers with regards to ozone levels, though the FDA does set standards for "medical devices." Some reviewers, such as Consumer Reports, recommend only using electronic air cleaners which produce less than the FDA standard of 50ppb (parts per billion, or 50 molecules of ozone per billion molecules of air); other agencies, including the American Lung Association, recommend avoiding any amount of ozone.

Ozone Generators

Unlike Ionic air cleaners, which produce small amounts of ozone as a by-product, some devices are marketed which intentionally produce large amounts of ozone. Ozone is a highly-reactive oxidizer which is extremely effective in destroying strong odors, as well as airborne bacteria and certain chemical pollutants. Ozone Generators, however, have no effect at all on allergens and other particle pollutants. By and large, ozone generators are used for very specific applications, such as smoke removal and restoration work, and are not suitable for use in occupied areas. Even the manufacturers of ozone generators recommend avoiding their use when people or pets are present.

Other Technologies

Besides the three main categories of air purifier technologies, there are other technologies which warrant a mention, as many of these are used as supplements to both HEPA filters and ionizers to increase their effectiveness for different types of applications.

Since neither HEPA filters nor ionic devices are very effective against gases and odors, they often add a gas-phase filter, such as Activated Carbon to filter bad odors from the air. Activated carbon and other gas-phase filters use pellets or plates of carbon or some other material covered in microscopic pores which react chemically with odor-causing molecules in the air. While activated carbon filters are not designed to trap pollutant particles or allergens, they are an effective add-on to HEPA filters or electrostatic air cleaners when it comes to odor control.

Ultraviolet (UV) light is known to be an effective sterilizing agent, and UV lamps are commonly used in hospitals and lab environments to kill germs including bacteria, mold, and viruses. UV treatment is also used in water treatment. The limitation of UV treatment is that it requires a sufficient amount of time for the dosage to do its job, and there is some question about whether a UV lamp in an air purifier can be effective against airborne germs passing rapidly through the unit. Some HEPA filter air cleaners, however, achieve good results by using a UV lamp to kill microorganisms on the filter, thus improving the effectiveness of the filter against disease-causing microorganisms.

Conclusion

Each of the three major types of air cleaner technologies have their benefits for specific applications, and supplemental technologies like gas-phase filtration and UV treatment may be combined with these processes for added effectiveness against different types of indoor air pollution. The science behind HEPA filtration, ionic air purification, and ozone can be complicated, and is still the subject of lively debate in many air quality and health arenas, but it all essentially boils down to one concept: some air cleaning technologies perform better than others at certain tasks. The following table presents the removal capabilities of each type of air purifier process, helping you decide which type is right for your home or office:

Air Purifier and Air Cleaner Technologies

HEPA Filters

Ionic Air Purifiers

Ozone Generators

Other Technologies

Conclusion

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