Section 2: Types of filters

Having identified various kinds of pollutants, you're now ready to learn about filters. Some filters work better for various kinds of pollutants than others. There are other advantages and disadvantages to each kind of filter.

HEPA filter
Removes: pollen, pet dander, mold spores, dust, dust mites, and other airborne allergens.
HEPA stands for High Efficiency Particulate Air. A HEPA filter has the highest efficiency available for removing small particles. According to the Institute of Environmental Science, a certified HEPA filter must capture a minimum of 99.97% of particles 0.3 microns and larger. This means that for every 10,000 particles which pass through the filter, only 3 can be allowed to escape.
The first HEPA filters were developed in the 1940's by the United States Atomic Energy Commission to filter radioactive dust. HEPA filters are recommended by the U.S. Department of Homeland Security.

These high-efficiency filters are the most effective way to remove airborne particles – backed by both the air cleaning industry and the U.S. government.
HEPA filters are not cheap, and can be expensive to replace.

Things to consider:
Not all HEPA filters are created equal. Size matters: the more square feet of HEPA filter, the more particulates it will be able to remove. HEPA filters can be made from thin paper, synthetics, and a variety of materials; the quality of the pleats in the filter media also influence the performance of the filter.
So, the size, material, and construction of the actual filter media all play a role in the air purifier's performance, and may account for why one HEPA filter is more expensive than another.

Activated Carbon
Removes: gases, odors, and other toxins.
Activated carbon has been treated with oxygen, opening up millions of pores in the carbon to create a large surface. There are so many of these tiny pores that one pound of activated carbon has a surface area of 60 to 150 acres. This huge surface area makes it ideal for adsorbing gases and odors. "Adsorb" is not a typo: adsorption is when materials attach through chemical attraction. These chemicals and gases are too small to be trapped by a HEPA filter, but they bond to the enormous surface area in the activated carbon.
The bigger the carbon filter, the more chemicals it will be able to absorb and the longer it will keep on working. When it's full, it can't adsorb any more.

Removes gases and odors, which are too small to be trapped by a HEPA filter.
When it's full, it's full. Replacing carbon filters can be costly.

Things to consider:
Size also matters when it comes to carbon filters. The more carbon you have, the more chemicals you will be able to absorb. The price and the effective life of the air purifier are highly dependent on the amount of carbon. Some off-the-shelf air purifiers have a few ounces of carbon sprayed onto a mesh filter; some air purifiers have 30 lbs or more.

Impregnated Activated Carbon
Removes: gases, odors, volatile organic compounds.
Impregnated carbon undergoes the same process as activated carbon, and then is blended with an additional chemical. This additional chemical is known as a "chemisorbent" and makes the activated carbon better able to handle certain chemicals and gases, like volatile organic compounds and reactive gases. Typical chemisorbents are potassium iodide and potassium permangate. Air purifiers which are recommended for VOC's or gaseous contaminants generally contain this specially blended carbon.

UV Filter
Removes: airborne micro-organisms, viruses, bacteria.
The UV filter is not exactly a filter, but instead a germicidal UV lamp. As antigens and pathogens pass over the lamp, it emits ultraviolet waves, which alter their DNA. This renders them sterile, so they are no longer harmful. The UV light is combined with other types of filters, as it is only capable of killing microorganisms.
Air purifiers with UV filters are often used in sterile environments such as hospitals, kitchens, daycares, and labs. In residential use, they are great for controlling mold. And, if you are someone who gets sick a lot, an air purifier with a UV filter could be a great way to give your immune system a little outside help.

Things to consider:
Consider number of watts. Germicidal UV lamps can have different wattages. Also consider number of UV bulbs: 2? 4? 6? The longer an individual micro-organism is exposed to the UV rays, the more likely it is to be destroyed.

Pre-filter
Removes: large particles.
Pre-filters come in almost every air purifier. They can be made out of a variety of materials – polyester, carbon or charcoal, or cotton. Some air purifiers have activated carbon or charcoal in the prefilter to help with odors, but the main purpose of a prefilter is to trap large particles – dust, pet hair, etc – before they enter the air purifier. This protects the HEPA filter and makes it last a lot longer. Usually the prefilter is less expensive to replace or can even be vacuumed and re-used, extending the life of the HEPA filter and cutting down on your filter replacement costs.

Electrostatic precipitators
Removes: airborne particles.
In an electrostatic precipitator filter, particles are ionized (electrically charged) by passing over an electronic cell, then are attracted and trapped by oppositely charged collector plates. There are no expensive filters to replace – all you have to do is wash the collector plates. Sounds like a great deal, right? Well, sort of. The ability of the plates to attract particles decreases quite rapidly as they fill up, so you have to clean them often to maintain any efficiency with the air purifier. Also, these types of air purifiers often generate ozone, which can be very damaging to human health, and aggravate asthma and other respiratory conditions. If you do decide to purchase this type of air purifier, you will want to make sure that the levels of ozone generated are safe – and not just according to the manufacturer.

No expensive filters to replace.
Some types generate unsafe levels of ozone, a known lung irritant. It can also be a hassle to continually keep the collector plates clean.

Things to consider:
The concerns with ozone aside, simply consider whether the cost efficiency of hand-washing the collector plates is worth the time and hassle. If you have to clean the collector plates weekly or even daily to get efficient performance, you may decide that expensive filter replacements are worth the money. ConsumerReports has more information on which types of electrostatic precipitators are better than others, so investigate a model before you decide to buy it.

Charged-Media Filter
Removes: airborne particles.
These types of filters work similarly to the electrostatic precipitator filters, except instead of collecting the particles on a plate, charged-media filters collect the particles on the fibers in a filter. The advantage to these types of filters is that they are able to collect very small particles, sometimes as small as 0.1 microns, through a combination of a filter and an electrostatic charge. The disadvantage is that like the electrostatic precipitator filters, they also lose their efficiency fairly quickly, and can require much more frequent filter replacements than a HEPA filter. These types of units can produce ozone, but the better ones on the market do not.

Initially very effective in collecting small particles. Can be quieter than an air purifier which uses solely mechanical filtration (a HEPA or HEPA type filter).
Gradually loses efficiency, more frequent filter replacement with higher costs.

Things to consider:
If you are going to purchase this type of air purifier, make sure that it does not produce ozone. The best air purifiers in this category are Blueair air purifiers, which produce negligible ozone; the filters need to be replaced every 6 months, however, which can get pricey.

Ionization air cleaner
Removes: airborne particles.
These types of air cleaners use static charges to remove particles from the air. An ionizing air cleaner draws in particles, where they are ionized (negatively charged). Then, they are sent back into the room, where they are attracted to positively charged surfaces: walls, floors, furniture, curtains, people, etc. That's right: the particles are removed from the air, not the room. So, while your air is cleaner, your room is dirtier. Unless you want to spend a lot of time vacuuming the carpets and cleaning the walls, it is likely that the particles will simply be dislodged and spread into the air again. Or, your walls and floors will continue to look really dirty.
The advantage of an air cleaner like this is that it has no fan, and it has no filter. That means no noise, and no replacement cost. The obvious disadvantage is that it makes your home dirtier and has limited effectiveness unless you are willing to clean a lot to remove the accumulated particles from the surfaces in the room. In addition, should you inhale an ionized particle, there is evidence that it will actually be more harmful to you – ionized particles are more likely to stick to the walls of your respiratory system, causing allergic reactions and lung irritation.
Many ionizing air cleaners also generate ozone, a documented health hazard which can damage lungs, worsen chronic respiratory conditions, and aggravate asthma.

No filter cost. No noisy fan.
Soot-like deposits on walls, floors, other surfaces. Ionized particles are more harmful if inhaled. Can generate ozone, which is harmful.

Ozone generators
As you've probably guessed by now, these types of machines are highly controversial and not recommended. According to manufacturers, ozone renders chemical contaminants harmless, through a safe chemical reaction which alters their chemical composition. However, according to the EPA, these claims are very suspect. Obviously enough, ozone generators generate ozone – which can damage lungs, aggravate asthma, cause chest pain and throat irritation, and other health problems. Studies have shown that the ozone concentrations produced by ozone generators can exceed health standards, even if you follow manufacturer's instructions. In fact, scientific evidence shows that ozone is generally ineffective at controlling indoor air pollution unless it is used in quantities above public health standards. In laymen's terms, if the machine isn't generating enough ozone to make you sick, it's not generating enough ozone to clean the air. Even if used in high enough concentrations to break down harmful chemicals, it can leave behind other irritating and corrosive chemicals. For more information, you can visit the EPA online, which has published extensive information about the risks of ozone generating air cleaners.

Now that you have all that information, here's a quick reference chart. And if you're still not sure, go with a combination HEPA/activated carbon filter. HEPA filters are the most consistent and reliable way to remove airborne particles and allergens, and activated carbon will remove household odors, gases, and chemical toxins.