Simply put, HEPA is a category of materials used in air filters. The acronym stands for High Efficiency Particulate Arresting, with “arresting” being used in the “stopping the progress of” sense. These materials were developed by the U.S. Atomic Energy Commission to protect its lab workers against radioactive particles. In order to qualify as true HEPA material by federal standards, the material must undergo a laser testing process to ensure it removes at least 99.7% of all particles larger than three-tenths of a micron across. A micron, by the way, is a distance equal to a thousandth of a millimeter. For comparison, the average human hair is roughly a hundred microns across, as is a grain of table salt. The human eye can see down to about thirty-five microns. An E. coli bacterium is from one to four microns across, but there are particles even smaller than a bacterium that can cause major damage. A particle of coal dust, for example, is so small that it takes a parade of two hundred and fifty of them to equal a single micron. In order to filter out harmful coal dust and prevent such minor inconveniences as fatal black lung, a HEPA filter must use “working parts” as small as the coal dust itself, plus the properties of chemistry and physics that regulate objects and interactions at the sub-molecular level.
How HEPA Air Purifiers Work
In some HEPA filters, it’s all about the carbon. Think back to bygone days of tedious misery in high school chemistry. Atoms can only bond in certain ways, which are determined by the way electrons are stacked up in concentric flight paths around atomic nuclei. Some atoms, like the atoms of inert gases, are reluctant to bond with any other material at all; but carbon atoms are arranged in such a way that they can link with many other kinds of atoms. This turns out to be surprisingly useful, especially when it comes to building large, complex molecules. Without large, complex molecules, it would be almost impossible to form compounds that can do the kinds of things organic material has to do. In other words, if not for the Tinker Toy properties of carbon, it would be difficult to make something like a human being. In a (vastly oversimplified) way of looking at it, carbon enjoys making friends. It bonds easily with hydrogen, oxygen, nitrogen, and other atoms common in airborne particulate matter, and it grabs those atoms as they pass and removes them from general circulation. It’s a non-metallic element, ergo less poisonous, and it can absorb up to three-fifths of its own weight in other gases.
Another natural material used in HEPA filters is a zeolite. Zeolites are compounds made of various minerals including aluminum, silicon, and oxygen. They’re either mined from the ground or created synthetically and look like, well, crystalline rocks. Fortunately, these rocks are capable of grabbing molecules carbon generally won’t, so they work well in conjunction with carbon filters. The molecules get trapped inside empty spaces in the crystalline structure. Zeolites can also remove water to very low partial pressures, which makes them useful as “desiccants” or dehumidifying agents. They can hold more than a quarter of their weight in captive water, along with volatile organic chemicals and mixtures of gases. Zeolites have a negative electrical charge (due to aluminum tetroxide molecules), so they function well in ion filtering processes. And most importantly, they’re non-toxic by oral, dermal, ocular, or respiratory transmission, so they’re safe for people and the environment as a whole.
These filtering processes work on the principle of ionization, which means adding a positive or negative charge to an atom. Negative ions, such as zeolites, attach themselves to airborne particles in much the same way static cling causes a sock to adhere to a sheet in the dryer. Those newly “sticky” particles include smoke, pollen, dust, animal dander, bacteria, viruses, and molds, the exact substances we don’t want floating around in our airspace. Negatively charged ions attach to particles as small as a thousandth of a micron across. Once that happens, the resulting conglomeration is big enough to be grabbed by a chemical collection sheet. It may even fall to the floor and stay there until it can be swept or vacuumed up—anything to keep it out of human lungs.
Another process used in HEPA filters is UV, or ultraviolet, light. What we think of as ordinary white or sunlight is really a spectrum of colors, as listed in the famous “Roy G. Biv” mnemonic: red, orange, yellow, green, blue, indigo, and violet. Those are the colors we can see, but there are other colors we can’t see beyond each end of the “visible spectrum.” We call those two bands of colors the infrared (below red) and ultraviolet (above violet) spectra. The color of a beam of light is determined by the frequency of its vibrations; i.e., how far apart the crests of each wave are. The crests of violet light waves are closer together than the crests of red light waves. We perceive some infrared waves as heat, like the warmth emanating from an oven; but a light wave more energetic than violet light whips back and forth so fast that it tears certain molecules apart. The ultraviolet portion of sunlight, for example, is what causes the damage to skin cells that we call sunburn. UV light also causes lethal damage to bacteria and viruses, so if we find a happy medium and deploy the waves carefully, we can filter the air of pathogens (i.e., infectious biological agents) without damaging the tissues of animals or people.
Incidentally, the list of contaminants in ordinary room air is longer and more complex than most people think. Some of those contaminants fall into a broad category of materials called volatile organic chemicals, or VOCs. They include compounds that are sometimes used as herbicides, pesticides, and cleansers. How did those find their way into family living areas? They may have “snuck in” inside other materials and escaped later in a process called “out gassing.” For example, some carpets and mattresses contain harmful materials, but the materials are bound up with other materials in harmless ways—harmless, that is, until the compounds break down over time. That allows the toxic fumes to escape, literally under our noses. Not all out gassed compounds are dangerous or fine enough to require HEPA filtering, but many do.
HEPA Filter Reviews
According to allergy sufferer Judy Tidwell of About.com, it’s possible to buy an effective HEPA air filter for less than a hundred dollars online. Here are the three brands she recommends:
The HEPA Air Purifier Quiet Care #17000 from Honeywell is effective in rooms of up to 168 square feet. Honeywell describes the product as the “#1 choice of physicians” and claims it’s thirty percent quieter than similar air cleaners from other manufacturers. It includes a three-speed control and electronic filter change indicator. It also guarantees six air changes per hour (ACH) and comes with a five-year manufacturing warranty.
The Panasonic F-P20HU1 costs about the same as the Honeywell but uses an infrared optical system to detect particulate matter passing through the filter. This allows for an automatic mode that adjusts the speed of the fan as required by the air quality at any given moment, which prolongs fan life, saves electrical energy, and reduces filter noise. The F-P20HU1 also includes an easily cleaned “pre-filter layer” that can prolong the life of the HEPA filter for up to three years. It’ll clear the air of a room up to 320 square feet in area.
Third on Tidwell’s list is the Hunter 30125 HEPAtech 125 Air Purifier with Ionizer, featuring Hunter’s “Whisper-Quiet” fan motor. This model can clear about 300 square feet of air. All three of the popular air filtration units listed by Tidwell weigh between ten and fifteen pounds and sell for a little over $100 online, perhaps even less during special promotions.
For a bit more—around $150—there’s the Vornado VAQS35 Air Purifier, which has five different settings and can clear rooms of up to 400 square feet. An Epinions.com reviewer described the Vornado by saying, “For the price, this is the best HEPA filter on the market.” Compare that to top-of-the-line models that cost up to $500, but offer only a marginal improvement in air quality over the four models listed above.
They say we can’t put a price on good health. Perhaps that isn’t true, but even if it isn’t, then at least the price of good respiratory health is within most people’s budgets. The trick is to look in the right places online. Believe it or not, eBay.com is a great place to look first. A recent search found Honeywell filter units starting at twenty-five bucks, with replacement filters at five dollars each. AllergyBegone.com includes a full list of replacement filter products from every major manufacturer, including Duracraft, Hamilton Beach, and Honeywell. A similar list, along with consumer advice, can be found at PurityPlanet.com, which adds brands from Bionaire, Holmes, and Oreck.
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