What’s in a chemical filter?

Current events certainly stimulate a lot of research, and the accidental release of toxic chemicals from storage and transportation units is no exception.  When a dangerous chemical is odorless and colorless, how can we possibly protect ourselves and the air in our homes?  Enter chemical filters, which are actually already available for purchase.

Many gas filtration media start with a base material and then add (impregnate) the base material with specialized materials.  Since all of these base materials have a multitude of miniature holes that can get “plugged up” with dust, it’s best to make sure the dust filter is clean and in place to preserve the life of the chemical filter.  The base material is incredibly light because of the vast amount of internal surface area it has in its many cavities and holes.  The way these materials  purify air is called “adsorption” (in contrast to absorption).  Adsorption and absorption both “suck up” liquids or contaminants, but there is a difference:  a sponge will absorb water, but eventually the water leaks out or evaporates.  Activated carbon, activated alumina and zeolites adsorb certain chemicals, some including water, but the adsorbed liquid or gas does not escape because a chemical bond is made, until the material is regenerated with high heat.  Since adsorption is a surface phenomenon, good adsorbents are those having large surface areas per unit mass and high attractive forces for the compounds to be adsorbed, called the solute. Although all molecules are adsorbed to a certain extent, those having high polarity are likely to be more strongly adsorbed than those having low polarity. Since water is a highly polar material, it is much more strongly adsorbed than hydrocarbons, for instance. (Air and Gas Drying with Activated Alumina)

Here are the most common base materials:

A main ingredient in these filters is activated carbon.  We’ve written about it here but when you have something more dangerous than sweaty socks or cat litter smells, we like to dig a bit deeper on its capabilities and limitations.  Activated carbon removes volatile organic compounds (VOCs) and acid gasses, which make them useful against a wide variety of pollutants.  To review, VOCs are compounds that have a high vapor pressure and low water solubility.  They are natural and human-made chemicals that are used and produced in the manufacture of paints, pharmaceuticals, and refrigerants. (EPA.gov)  An acid gas is any gaseous compound which, when dissolved in water, will form an acidic solution.  Many industrial compounds are acid gasses, but did you know that we actually exhale an acid gas?  Carbon dioxide (CO2) is an acid gas, and activated carbon is a method for removing excess CO2 in a space.  

Activated alumina is another base.  It’s a porous, solid form of aluminum oxide, otherwise known as Al2O3 or alumina. This is the same mineral that makes up the precious gems ruby and sapphire, with impurities being the source of the stones’ bright colors. After activated alumina has been evacuated of existing moisture by heating it, the high surface area and many pores of the material allow for the uptake of water and other molecules through adsorption.  (What is Activated Alumina?) At about 1.5 to 3 times the cost of activated carbon, activated alumina is pricier but very attractive for adsorbing different chemicals.

Zeolites are another base material for chemical adsorbents.  They can be naturally occurring or man-made.  Zeolite has an aluminosilicate framework. Which simply means minerals composed of aluminum, silicon, and oxygen, plus countercations. This framework provides exceptional strength and stability to the honeycomb structure. It also makes it very difficult for the positively charged atoms (cations) to leach from Zeolite. (What is Zeolite?)   Molecular sieves are a type of zeolite that are manufactured to precise frameworks and pore sizes for specific applications.

Now that you know some of the base materials, additives (also called “doping”) can enhance capture/filtration of specific contaminants.  

• Potassium Permanganate is added when sulfurous compounds may be present (such as hydrogen sulfide and sulfur dioxide).   According to this filter company, their molecular sieve impregnated with potassium permanganate oxidizes gaseous pollutants such as hydrogen sulfide, sulfur dioxide, formaldehyde, ethylene, mercaptans, and various aldehydes and alcohols.   Do you know what mercaptans are?  They are the sulfurous, rotten-egg smelling additives that are added to natural gas to help you know that there is a leak.

• Potassium Iodide:  some AustinAir filters are impregnated with potassium iodide, for increased removal of formaldehyde and ammonia.  They are best used in places like new homes, laboratories, beauty salons, funeral homes, etc. Sources of formaldehyde in homes can be carpets, wood panel, construction materials and furniture upholstery, if not an outright chemical leak. (achooallergy.com)

• Magnesium Dioxide and Copper Oxide: The air filter company Blueair adds these compounds in their activated carbon filters to remove carbon monoxide, ethylene oxide, and ozone.

So which filter is best for me?  It’s hard to plan for every scenario, but you may take into consideration the following:

• This article/table by the California Air Resource Board (CARB) in response to the Aliso Canyon gas leak (California, 2015) describes what types of filters are in a number of high-end air purifiers.  The models may have changed slightly but it can give you an idea of how air purifiers can be used to reduce specific chemicals (mercaptans were judged to be the source of most peoples’ health issues).

• If you live very close to a refinery or manufacturer of specific chemical products, you may want to find out what chemicals they manufacture, store and load, what VOCs/fumes are produced and what air filter is best to remove those VOCs. 

• If you are interested in a particular type of filter, look for (or ask for) test data that verifies it removes what it’s designed to remove.  In an MIT study of 4 consumer-grade air cleaners, only 2 of them removed the VOCs toluene and limonene effectively.  

• In the case of radon, activated carbon filters (and the others mentioned above) will not adsorb radon gas, but they will adsorb most of the radon decay products (termed “radon daughters or progeny”), which are actually the source of health issues associated with radon. (Reduction of Radon Working Level by a Room Air Cleaner).   Therefore, activated carbon is not recommended for first-line defense against radon infiltration into your home; a venting system is more appropriate and you should contact a professional if you have radon in your home.

• In the end, activated carbon is widely used for a reason: it removes a lot of VOCs!  If you don’t have a specific chemical that you’re concerned about, this type of filter is a readily available, broad spectrum weapon against many pollutants.  If your air is particularly dusty, you’ll want to make sure that it also has a HEPA pre-filter to protect the activated carbon from getting clogged with dust.

Photo by Paul Teysen on Unsplash