Chemical air purifiers (gas phase purifiers) use adsorbents to capture molecular sized pollutants, odors and non-particulates such as cooking gas, out gassed paint and building material vapors, and vehicle exhaust gas.
Adsorption is the adhesion of particles from a gas, liquid, or solid to a surface.
Absorption, in which a fluid permeates or is dissolved by a liquid or solid is the common misperception.
We are talking about molecular scale here, gas molecules are 0.001 micron and smaller and cannot be removed by even the best HEPA filter alone.
They are called Volatile Organic Compounds (VOC).
VOC’s are carbon-based airborne chemicals vaporized from an enormous list of common household products and furnishings.
No standards for room air purifier gas removal have been developed, but the government of China has begun researching for new guidelines.
These units must be combined with a mechanical filter if a single air purifier is to remove both particulate allergens and gasses.
Adsorption, as distinct from absorption, is the physical process of binding a thin film of gas molecules to a large surface area.
Activated carbon is the most common adsorption medium in chemical air purifiers.
A gram of activated carbon has 10,000 square feet of internal surface area. One pound of activated carbon has a surface area equal to about 125 acres.
Activated carbon is manufactured by heating bituminous coal, wood or coconut shell to form a carbonized char, then oxidizing to create a highly porous adsorbent material.
Activated carbon is best for removal of high molecular weight compounds: volatile organic compounds (VOC's) like benzene, toulene and xylene.
These dangerous solvents are found in too many household products, and also in processed foods.
Carbon filters are more expensive, adding significantly to the price of an air purifier.
Varying grades of activated carbon are available, with price based on the source.
Coconut shell is a premium source of carbon.
In toxic environments filter material may need to be replaced more frequently than the manufacturer recommends.
Carbon gets saturated at different rates for different chemicals, so a highly toxic environment weighted with one pollutant will require even more fresh filters.
Polar molecules, like the common sweet-smelling aldehydes, will adsorb with weak bonds.
This allows some to escape from the filter, which may continue to work well for other odors.
Filters designed last several years may produce odors after only months of service.
If a home environment is toxic enough to clog carbon filters quickly, source removal is needed!
With activated charcoal type filters, more weight, measured in pounds, is usually better.
These filters expose moving air to a large surface area.
We’re talking about 15 to 20 pounds just for the carbon.
The main exception is the industry leader in quality, IQAir, which uses a bit less mass (5 lbs carbon and potassium permanganate) in a technically sophisticated synergism.
Zeolites, naturally occurring clay minerals derived from volcanic ash, are another type of adsorbent, often mixed with carbon in chemical air purifiers.
Zeolites can be produced synthetically, and are cheaper than good carbon.
One pound of Zeolite can have 20 to 30 acres of surface area.
The mineral is capable of removing V.O.C.'s and ammonia compounds such as pet urine.
VOC gasses are trapped in the pores of Zeolite’s structure.
Carbon air purifiers, and especially those with zeolite, must be kept dry!
Zeolite is a clay mineral which swells up when damp, blocking air passage. They cannot be used in bathrooms, or left outside.
As one exasperated retailer I interviewed snarled:
“They ran it in the sauna and then brought it back, (multiple expletives deleted).”
High humidity is conducive to mold growth, dust mites, and allergy triggering in general.
While adsorption technology is central to toxic gas and odor control in chemical air purifiers, chemisorbents are a complementary gas removal technology found in top end chemical air purifiers.
They work through a two step process, binding gasses to a carrier media, and then, chemically breaking them down by oxidation.
The usual carrier media is activated alumina, which is made by treating aluminum ore so it becomes porous and highly adsorptive.
The alumina is impregnated with Potassium compounds to create a powerful oxidizing media.
The best VOC gas cartridges have a carbon/potassium permanganate mix.
For high-aldehyde situations, the oxidizing media may be necessary. Some manufacturers offer variable mix canisters by special order.
Sometimes, relatively small quantities of activated carbon, as found in inexpensive light weight granulated carbon-impregnated mesh filters, can reduce odors in a residence to low levels.
However, because many chemicals produce health effects at levels below those where odors are perceived, removal of odors alone is not a foolproof indicator of a healthful environment.
Of course, these “odor” oriented carbon screens will only work for a very short time.
If replacing a saturated gas phase filter seems expensive, consider that your lungs have approximately the surface area of a small bedroom floor.
These organs are designed for gas transfer into your blood.
Toxic gasses filling the pores of a large charcoal element could be poisoning your liver instead!
Solvents like toluene and benzene are liver-Kryptonite: please, please, please remove them from your home.
Too many air purifier buyers are expecting too much from their machine: no chemical air purifier will save you from a totally polluted home!
A clogged pre-filter will dramatically reduce efficiency, frequent washings are recommended.
Many disappointed users of purifiers fail to maintain (clean) their prefilters frequently enough.
If there is an inefficient pre-filter ahead of the gas filter, the expensive carbon will collect particulates instead of VOC gas, resulting in degraded performance and short carbon media life.
No chemical air purifier eliminates 100% of gaseous pollutants and odors.
Different adsorbents are needed to remove low molecular weight gasses, including formaldehyde, hydrogen sulfide and ammonia.
Chemical air purifiers cannot reduce carbon monoxide or carbon dioxide.
It is a common mistake to close up the house permanently and turn on the air purifier.
The rule of three applies especially to toxic VOC’s;
1. remove the source,
2. ventilate your living space,
3. buy a quality air purifier properly sized for your room.
Real air purifiers, in my humble opinion, are heavy-weight-carbon VOC chemical air purifiers with sealed and gasketed HEPA filters.
Air Purifiers at Amazon.com.