Air purifiers evolved in response to people's reactions to allergens like pollen, animal dander, dust, and mold spores. Reactions (sneezing, runny nose, scratchy eyes, and even more severe consequences such as asthma attacks) are the result of antigens found in the home. These antigens are major triggers of asthma, and there are more than 17 million asthmatics in the United States alone. Air purifiers remove a portion of these particles, thus reducing allergic-type responses.
Due to their extremely small size, allergens are able to pass through a standard vacuum cleaner bag and redistribute into the air where they stay for days. Even a single microgram of cat allergens is enough to invoke an allergic response in most of the six to 10 million Americans who are allergic to cats. Other airborne particles—such as bacteria and viruses—can cause illnesses, some of which are fatal. There are many reasons—allergies, asthma, fatal illnesses—that millions of air purifiers are sold in the United States every year.
There are two common types of air purifiers that can remove some or all of the disease and allergy-causing particles in the air: mechanical filters—the most effective are classified as High Efficiency Particulate Air filters (HEPA filters)—and electrostatic precipitators.
HEPA filters are made out of very fine glass threads with a diameter of less than 1 micron (a micron is 0.00004 in, 0.001 mm). By comparison, a human hair has a diameter of about 75 microns (0.003 in, 0.07 mm). The fine glass threads are tangled together and compressed to form a filter mat. Because the individual threads are so microscopic, most of the mat consists of air. The openings in the mat are very small, generally less than 0.5 micron (0.00002 in, 0.0005 mm). HEPA filters will collect particles down to 0.3 microns (0.00001 in, 0.0003 mm) in diameter. Even though the filter may only be 0.10 in (2.5 mm) wide, it would consist of 2,500 layers of glass threads.
Electrostatic precipitators rely on electrostatic forces to remove particles from the air. They work by creating a cloud of free electrons through which dust particles are forced to pass. As the dust particles pass through the plasma, they become charged, making them easy to collect. Electrostatic precipitators can collect particles down to a diameter of 0.01 microns (0.00001 mm).
Neither HEPA filters nor electrostatic precipitators can remove volatile organic compounds from the air, therefore do nothing to reduce odors. For this reason, most air purifiers are equipped with a pre- or post-filter composed of activated carbon. Activated carbon is produced by heating a carbon source (coconut shells, old tires, bones, etc.) at very high temperatures in the absence of oxygen, a process also known as pyrolysis or destructive distillation. Pyrolysis separates the pure carbon from the other materials contained in the raw material. The pure carbon is then exposed to steam at 1,500°F(800°C). The high temperature steam activates the carbon. The activation process forms millions of cracks in the carbon grains. These cracks have diameters of about 0.002 microns (0.000002 mm). Because there are so many cracks, the activation process provides the carbon with an enormous surface area per weight—about 6.5 acres/oz (1,000 m 2 /g). The millions of cracks provide locations where organic compounds can be adsorbed. In addition, the surface of the carbon carries a residual electrical charge that attracts non-polar chemicals (chemicals that do not have separated positive and negative charges) to it. Activated carbon is very effective at adsorbing odor producing compounds.
Air purity has been a concern as long as human beings have lived in groups. One of the reasons that hunter-gatherers are nomadic is that they periodically need to move away from their garbage dumps and latrines. In A.D. 61, the Roman philosopher Seneca complained about the miasma of chimney smoke that constantly hung over Rome. In 1306, King Edward I of England banned the burning of coal in London due to the heavy pollutants left in the air.
The Industrial Revolution of the eighteenth and nineteenth century only worsened the problem. Burning coal to produce electricity and fuel trains produced a dark cloud of smoke over every major center of industry in the world and covered entire cities with soot. To deal with this problem, engineers built higher smoke stacks to move airborne waste further away from the source. Regardless of how high the stacks got, the people down wind complained about the ashes and the acid gases from coal combustion (the source of acid rain) destroying their crops. Air pollution took another turn for the worse after World War II when automobiles became the primary means of transportation in the industrialized world. Automobile smog has provided Los Angeles with the worst air quality in the world.
The materials that go into both HEPA filters and electrostatic precipitators are: a case made out of plastic, an electric fan to induce air flow through the filter, the filter itself, and control switches to control the speed of the fan and turn the air purifier on and off. The HEPA filters are made of borosilicate glass fibers or plastic fibers (e.g., polypropylene) bound together with up to 5% acrylic binder (the same compound that binds latex paint to a house). Electrostatic precipitators generate ions by running extremely high positive direct current voltages through steel wires set between grounded steel charging plates. Cases are almost universally made from plastic, usually high-impact polystyrene, polyvinyl chloride, high-density polyethylene, or polypropylene. Most air purifiers are also usually equipped with a post-filter composed of activated carbon.
HEPA filters are designed based on the size of particles to be removed and the required air flow rate. The finer the pores in the HEPA material, the finer the particles removed from the air. However, collecting finer particles means the filter material will clog sooner and need replacing on a more frequent basis. The designer will specify the diameter of the glass fibers and the mat density of the filter fabric that fixes the filter pore size. HEPA filters can contain binders that provide additional strength, but this also produces a filter that clogs sooner.
Design of an electrostatic precipitator is considerably more complex. Home electrostatic precipitators usually are designed to have two components, an ionizing component (where the electron cloud is created) and a collecting component (where the charged dust particles are pulled out of the air). The collecting component consists of a series of parallel steel plates—half are grounded and half carry a positive direct current voltage—thus alternate plates are either positively or negatively charged. The ionizing unit consists of thin wires strung between a separate set of grounded steel ionizing plates parallel to, but set in front of, the collector plates. The thin wires carry a very high positive voltage direct current (up to 25,000 volts in a home air purifier). The positive charges in the wires induce a flow of electrons between the wires and the adjacent ionizing plates. Because there is a very high voltage on the wire, electrons are pushed toward it by an acceleration of around 1,000 times the acceleration of gravity, which accelerates the electrons to very high velocities. For example, as a particle of dust mite excrement floats past the wire, the high-speed electrons collide with the electrons in the molecules of the particle, knocking
- 1 Pellets of the raw material (high-impact polystyrene, polyvinyl chloride, high-density polyethylene, or polypropylene) are fed into a hopper and heated to the melting point, 300-590°F (150-310°C).
- 2 The molten plastic is injected under high pressure into a mold of the case. The mold is usually made from tool steel by a highly skilled mold maker. Vents in the mold allow the entrained air to escape as the plastic enters. The mold designer must assure that the mold fills evenly with plastic and that all of the entrained air is allowed to escape, otherwise the final part might contain small air bubbles or even voids.
- 3 Water is forced through channels built into the mold to transfer heat from the molten plastic into the environment. Once the part is sufficiently cool, which can take up to two minutes, the mold opens. Hydraulically-operated pins push the part out of the open mold into a receiving bin.
- 4 An electric fan is used to pull air through the air purifier. The fan is usually purchased from a small-parts supplier. The fan consists of a small electric motor with metal fan blades attached to the motor's power take-off. The fan blades are usually spot welded to a collar, which is slipped onto the power take-off and bolted in place.
- 5 The fan is usually attached to the case with steel screws.
- The glass fibers that make up a HEPA filter are created by passing molten glass or plastic through very fine pores in a spinning nozzle. The resulting glass fibers cool and harden almost instantly because of their tiny diameters.
- The spinning nozzle moves back and forth (causing the glass fibers to form a web) above a moving conveyor belt onto which the fibers are collected. The speed of the conveyor belt determines the thickness of the filter material—a slow conveyor belt allows more glass fibers to build up on the belt.
- The melting and cooling of the fiber produces some bonding of the fibers. As the conveyor progresses, a latex binder is sprayed onto the fabric to provide additional strength. The fabric can be any width up to the practical size of the machinery and can be cut down to the size specified by the customer before the fabric is taken up on rollers.
- Once the HEPA mats are formed, they are folded into an accordion pattern in an automatic folder. The accordion pattern allows up to 50 ft 2 (5 m 2 ) of filter material to be enclosed in a small space.
- The accordion-shaped filter is then enclosed in a filter case, usually consisting of an open wire grid. The purpose of the filter case is to support the filter.
- The electrostatic precipitator collection system is manufactured by enclosing steel plates into a plastic casing, often by hand assembly. The plates are arranged parallel to each other in the case.
- Wires are then connected to alternate plates through which the high voltage positive direct current will be applied to the plates. The other plates are grounded.
- The ionizing unit is constructed by running small diameter wires in front of the collector plates.
- A voltage transformer, which is used to convert 115 volt household alternating current into high voltage direct current, is fixed to the precipitator case. This voltage is run to both the positively charged collector plates and the ionizing wires.
The activated carbon filter
- 1 The activated carbon filter (for odor reduction) usually consists of carbon-impregnated cloth or foam. This is manufactured by infusing the raw material with powdered activated carbon.
- 2 The carbon filter is then wrapped around the inside or outside of the HEPA filter, or stretched in a frame at either the inlet or outlet of the electrostatic precipitator.
- 3 There are very few components in an air purifier. For this reason, they are usually bench assembled. In bench assembly, moving conveyors bring the individual components or sub-assemblies (e.g., the fan already attached to the case) to a bench where a person then hand assembles them. In a typical HEPA air purifier, there may only be five components that require assembly: casing, fan, particulate filter, carbon filter, and the on/off switch.
Filter efficiency is the most important quality control test for air purifiers. The American Society for Testing and Materials (ASTM) publishes quality control tests that filters must meet before they can be used in certain applications or be marketed as HEPA filters (e.g., ASTM-F50: Standard Practices for Continuous Sizing and Counting of Airborne Particles in Dust Controlled Areas and Cleanrooms Using Instruments Capable of Detecting Single Submicrometer and Larger Particles). The United States Department of Defense has promulgated a standard in which dioctylphthalate (DOP) particles are blown through a filter. To pass, the filter must remove 99.97% of the influent DOP.
The byproducts of manufacturing include the non-carbon materials that are distilled from the manufacture of activated carbon, specification filter material, and excess material that must be discarded in the production of HEPA filters. Most of the other manufacturing wastes, plastic runners from the injection machines and excess sheet metal, can be recycled.
Additional wastes are produced during the operation of air filters. The ions produced by electrostatic precipitators interact with oxygen in the air to produce ozone. At high concentrations, ozone is poisonous. The ozone levels produced in a home electrostatic precipitator are unlikely to reach dangerous levels, but some people are sensitive to even low levels of ozone. The collector plates in an electrostatic precipitator need to be cleaned periodically.
HEPA filters have limited lifetimes, depending on the amount of air that is filtered through them and the amount of particulates in the air. Most manufacturers recommend that they be replaced every few years. The used filters cannot be recycled and thus end up in landfills.
Activated carbon can be recycled, but the cost of handling the small amount of carbon contained in a home air purifier would be prohibitive. Generally, it also ends up in landfills after it is used completely.
As scientists learn more about environmental pollutants and their impact on human health, the need to provide cleaner air in homes and offices will only grow. The current generation of HEPA filters can only remove particles down to 0.3 microns (0.00001 in, 0.0003 mm) in diameter while it is believed that particles down to 0.1 microns (0.0001 mm) in diameter can cause mechanical damage to lung tissue. Viruses can be as small as 0.02 microns (0.00002 mm) in diameter. Clearly, there is still progress that can be made in controlling indoor air pollution. The current direction of technology is toward ever finer filter materials. The new standard in filtration is the ULPA filter, which stands for Ultra Low Penetrating Air. An ULPA filter is required to be able to remove particles down to 0.12 microns (0.00012 mm) in diameter, about one third of the diameter of the smallest particle a HEPA filter can remove.
Where to Learn More
Cooper, David C., and F. C. Alley. Air Pollution Control: A Design Approach. Prospect Heights, IL: Waveland Press, Inc., 1994.
Godish, Thad. Indoor Environmental Quality. New York: Lewis Publishers, 1999.
Mycock, John C., et al. Handbook of Air Pollution Control Engineering and Technology. New York: Lewis Publishers, 1995.
Christiansen, S. C., et al. "Exposure and Sensitization to Environmental Allergen of Predominantly Hispanic Children with Asthma in San Diego's Inner City." Journal of Allergy and Clinical Immunology (August 1996): 288-294.
"Electrostatic Precipitation." Tin Works, Inc. 3 June 2001. < http://www.tinworks.com >.
"Filter Media." Mac Equipment. 3 June 2001. < http://www.macequipment.com >.
"Plastic: Injection Molding." Industrial Designers Society of America. 17 June 2001. < http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm >.