Air Purifiers - a Review of Technologies

With an astonishing variety of air purification options on the market, it can be hard to decide which one will work the best for you. We have developed this document to make it easier for you to compare air purification technologies.

Dangerous and Troublesome Household Air Contaminants

The EPA and other authorities have placed substances that cause indoor air concerns into three categories. You can find out more about particular airborne contaminants here.

Particles	Particles are very small (usually microscopic) solid or liquid substances. For clarity, we have broken down this single EPA category into two separate categories: Microbiologicals This category includes mold spores, bacteria and dust mites Non-Biological Particulates This category includes pollen, animal dander, particles from cigarette smoke, emissions from gas combustion appliances, viruses, etc.
Gaseous Pollutants	This category includes gases created through combustion (burning) and organic chemicals that are NOT associated with particles. Examples are Carbon Monoxide, Nitrogen Dioxide and other gases that come from cigarette smoking, combustion appliances, and vehicle exhaust, etc.
Radon and its Progeny	The Radon category includes radioactive pollutants which originate from natural sources such as rock, soil, groundwater, natural gas and mineral building materials.

The Air Purifier Lineup

HEPA Filters

Effective Against:	microbiologicals and non-biological particulates
How They Work:	HEPA (High Efficiency Particulate Air) filtration material was invented by the US government during WWII to prevent radioactive particles from escaping from labs. Filters are made up of boron silicate (a glass-like substance) microfibres that are formed into flat sheets in a process similar to paper making. The flat sheets are then pleated to increase surface area and the pleats are separated by baffles to direct airflow into the filter. HEPA filters are simple, mechanical filters that remove only particles from the air (not gases or vapours), but they remove particles VERY effectively - medical grade or "true" HEPA will remove 99.97% of particles that are 0.3 microns or larger from the air. It is important to realize however, that not all HEPA filters are created equally. Only filters designated "true HEPA" or "medical grade HEPA" can promise to deliver the results cited above. Consumers must be vigilant to purchase only products with this designation, as un-certified filters can have significantly poorer results. HEPA filtration is used in a variety of critical filtration applications, including those in nuclear, electronic, aerospace, pharmaceutical and medical fields. They are the only types of filters recommended by authorities such as the EPA and American Lung Association for use as in-home air purifiers.
Concerns	The only concern with these units is that because they require a fan to operate, there is some noise. Fan noise problems are limited through the use of multi-speed settings that are available on most models.
Maintenance	Filter replacement approximately every 1-5 years depending upon contaminant load.

HEPA Filters with Activated Carbon (Charcoal)

Effective Against:
How it works	This type of synergistic system combines the benefits of true HEPA filtration with those of activated carbon. Activated carbon is made by burning hardwood, nutshells, coconut husks, and other carbon-based materials at very high temperatures with steam, and in the absence of oxygen. This process removes any non-carbon elements from the source material and creates a porous internal structure that provides huge surface area. 5 grams of high quality activated carbon can have an adsorption surface area greater than a football field. This huge surface area is what allows your unwanted molecules to be adsorbed and trapped. When certain chemicals pass next to the carbon surface they attach to it and are trapped. Activated carbon can trap an extremely wide spectrum of chemicals because of the different types and/or sizes of pores in its structure. The best activated carbon is made from coconut husk, and it is ideal for the removal of odorous compounds, gases from volatile organic compounds and gases of low molecular weight from the air. Air filtration using activated carbon is very safe, and trouble-free.
Concerns	The only concern with these units is that because they require a fan to operate, there is some noise. Fan noise problems are limited through the use of multi-speed settings that are available on most models.
Maintenance	Periodic filter replacement (frequency depends upon contaminant load)

Ozone Generators

Effective Against
How They Work	Ozone generators create ozone gas - a volatile oxygen derivative that has three oxygen atoms instead of two. Because oxygen needs to have two atoms to be stable, it readily gives up the third - which combines with other molecules, turning them into other substances. The theory of ozone purification is that the ozone converts many chemicals in the air into other, harmless substances. In practice and research unfortunately, this is rarely the case. The Environmental Protection Agency (EPA) cites research on it's website that indicates that ozone is not effective in removing many of the problematic air pollutants from the home, and may even take some relatively harmless substances and convert them into ones that are more harmful than the originals. "Available scientific evidence shows that at concentrations that do not exceed public health standards, ozone has little potential to remove indoor air contaminants." Environmental Protection Agency (EPA)
Concerns	Authorities strongly recommend against deliberately using any ozone generating appliance in the home, and particularly against ozone generating air purifiers. "It (ozone) can have damaging health effects, especially for persons with asthma and other lung diseases, children and the elderly. It is produced by ozone generators and some other electronic air cleaners." American Lung Association "Health Canada warns the public not to use air cleaners designed to intentionally generate ozone (ozone generators) in their homes." Health Canada Be aware that many sales pitches will use the terms "energized oxygen" or "pure air" instead of ozone. According to the EPA these terms are misleading: "Ozone is a toxic gas with vastly different chemical and toxicological properties from oxygen." Environmental Protection Agency Harmful Effects of Ozone Include: When inhaled, even at low levels, ozone can cause respiratory problems in the elderly, children and even healthy adults. Long term exposure to ozone may lead to premature aging of the lungs and chronic respiratory illness. Relatively low exposure can cause chest pain, coughing, shortness of breath and throat irritation. Ozone is also extremely corrosive to plastics and some metals - meaning that it can result in damage to household items, computers, wire insulation, duct work, pipes, etc
Maintenance	None

Ion Purifiers

Effective Against
How it Works	Negative ion purifiers generate negative ions and distribute them into the room's air. The ions create a static charge which causes particles in the air to be attracted to the room's surfaces - walls, drapes, floors, table tops, curtains, occupants, etc. This process removes the irritating and dangerous particles from the air, but does not remove them from the room. A concern with the use of ion purifiers is that the process used to generate the ions that are disseminated into the room also produces the dangerous chemical, ozone.
Concerns	Authorities are concerned that the same electrical charge that attracts particles to a room's surfaces could also attract them to the lung tissue of the room's occupants. The same process that is used to generate the negative ions can also generate ozone. With particles being attracted to walls and furniture, there is some concern about these surfaces becoming soiled. Ion purifiers are also known to generate ozone which, in addition to it's previously mentioned health repercussions is also known to be extremely corrosive to plastics and some metals - meaning that it can result in damage to household items, computers, wire insulation, duct work, pipes, etc.
Maintenance	None

Electrostatic Precipitators

Effective Against	microbiologicals
How it Works	Electrostatic Precipitators (also called electronic filters) electrically charge airborne particles and then trap them on oppositely charged metal plates. They are available to be installed in central filtration systems and in portable units with fans. As with negative ion generators, they create negative ions within a space through which air flows, giving airborne particles a charge. The particles are then drawn back into the cleaner by a fan and collected on an electrostatically charged panel filter (or metal plates). Electrostatic Precipitators employ either a one or two-stage system, with the two-stage units being more effective than their one-stage counterparts. One stage systems use the same charged medium to charge the particles in the air and to collect them. Two stage systems use high-voltage electrodes or wires which place a charge on the incoming airborne particles, and then draws them between a series of oppositely charged metal plates which attract the particles from the air, causing them to precipitate onto the metal plates. Collection efficiency of these units varies widely depending upon the area of the collecting plates, air flow rate, and the strength of the electrical field. One disadvantage to these systems is that while the air flow rate will remain consistent throughout use, the particle capture efficiency will decrease with use (as the plates become coated with debris).
Concerns	Some systems can generate ozone gas. Charged particles can soil walls and room surfaces if they escape collection grids.
Maintenance	Collection plates have to be cleaned often to ensure particle capture efficiency, which decreases with use.

UV Sterilizers

Effective Against	microbiologicals
How They Work	UV light air purification products are basically specialized light-bulbs that produce UVC and sometimes UVV light. UV light is a well known and effective killer of biological organisms, and breaks down many organic gases into their harmless components. When organisms are exposed to UV light, the light enters the cell wall and alters its DNA, killing it. When UV light of a strong enough intensity is used (UVV), it can actually break down the chemical bonds in organic molecules, often removing organic gases from the air. The problem with UV light is that its effectiveness depends heavily upon the amount of time that cells and molecules are exposed to it. If they are not exposed to the direct light for a significant period of time, it will not work. In many cases, UV-type air purifiers will not be effective. Not because the UV light doesn't work, but simply because the particles of concern are not exposed to the light for an adequate period of time. UV products that produce UVC frequencies will only be effective against biological particles, whereas products that produce UVV frequencies will be effective against various organic compounds also.
Concerns	UV light generating products should have a built-in main shut-off system so that owners are not exposed to the light when servicing the system as UV light can cause blindness. Also, these systems will only work if they take into account the fact that the air needs to be in contact with the light for a significant period of time. UV light destroys plastics, so plastic ducting, coated wire, etc should never be exposed to the light. UV loses efficiency if the air moving past the light is warmer or colder than the temperature that the light is calibrated to be effective with. Generally UV systems are only recommended to individuals by experts in the field after those experts have fully assessed needs and determined that there is not another system that would be more effective for them. The Federal Trade Commission (FTC) says about UV light air purification products: "…Most household air cleaners move air too quickly to provide adequate exposure time." (to be effective)
Maintenance	UV light products require that the special bulb be replaced annually (at a cost of $100-150) and that the bulb be cleaned periodically.

Incineration-type Purifiers

Effective Against	microbiologicals
How they Work	The working part of an incineration-style air purifier is a ceramic incineration chamber (or a group of them). The chambers heat the air to temperatures of about 400 degrees Fahrenheit or 250 degrees Celsius, which then rises through them drawing new, cool air in by creating a convection current. While the air is passing through the chambers, particles in the air are incinerated (burned) and thereby removed from the air. Preliminary research shows that Incineration-type air purifiers are quite effective at removing biological particulate material from the air. They also have the advantage of silent operation since they rely upon convection to move the air through the chambers rather than fans, and are maintenance free.
Concerns	Because of the gentle, convection-based airflow there is some concern about a low "clean air delivery rate".
Maintenance	None