Airborne Contaminant Glossary

Minimum Efficiency Reporting Value (MERV)

Under a microscope, an air filter looks a lot like a slice of Swiss cheese - where the "cheese" is usually woven fabric or pleated paper, and the "holes" are just the gaps between the fibers. The holes in an air filter are big enough to let tiny air molecules pass through, but small enough to prevent allergen particles from getting through.

A MERV 8 filter can trap particles as small as 3 Microns - like dust & pollen, while the smaller holes in a MERV 11 filter can trap particles as small as 1 Micron - like smoke & bacteria.

The bottom line... a higher MERV rating means better filtration.

MERV Rating
Particle Size
0.3 - 1.0 Microns 1.0 - 3.0 Microns 3.0 - 10.0 Microns
1 - 4
Less than 20%
20 - 35%
35 - 50%
50 - 70%
70 - 85%
Less than 50%
More than 85%
50 - 65%
More than 85%
65 - 80%
More than 85%
80 - 90%
More than 90%
Less than 75%
More than 90%
More than 90%
75 - 85%
More than 90%
More than 90%
85 - 95%
More than 90%
More than 90%
More than 95%
More than 95%
More than 95%

Read the chart from left to right as follows:

A filter with a MERV rating of 14...

  • Will capture 75 - 85% of particles that are between 0.3 and 1 micron in size
  • Will capture more than 90% of particles that are between 1 and 3 microns in size
  • Will capture more than 90% of particles that are between 3 and 10 microns in size

Microparticle Performance Rating (MPR)

The 3M Filtrete MPR focuses on the smallest, and most troublesome particles - those between 0.3 and 1.0 Microns in size. Generally speaking, a filter with an MPR of 1000 is twice as effective at removing those tiny particles as a filter with a 500 MPR.

Clean Air Delivery Rate (CADR)

The CADR is a measure of an air purifiers ability to deliver particle-free air to your room - as determined by the Association of Home Appliance Manufacturers (AHAM). For each certified air purifier, the AHAM issues three CADR numbers - one for tobacco smoke particles, one for dust particles, and one for pollen. The actual numbers (ranging from 10 - 450) indicate the volume of filtered air delivered by the air cleaner. In general, an air purifier with a higher CADR number will clean a larger room (or clean a smaller room faster). The AHAM also issues a maximum recommended room size for each air purifier it certifies - i.e. "This air cleaner is suggested for use in a single closed room up to xxx square feet."

In selecting a purifier, you should first look for the AHAM seal. That way, you can be sure the claims made by the manufacturer have been independently verified. Once you've found the seal, you can easily compare ratings from brand to brand and select the air purifier that works best for you.


One micron is one millionth of a meter or approximately 1/25,000 of an inch. For comparison, a human hair is about 70 microns thick.

Typically, air filters are rated by the size of the particles they can remove. Most decent furnace filters can easily remove particles larger than 10 microns in size, but the best filters are able to remove particles smaller than 3 microns.

Electrostatic or Electrostatically Charged

At this moment, you're reading this web page through a thin layer of dust on your computer screen, and no matter what you do, or how many times you clean it off, the dust will always come back. Why? It's a simple matter of Physics. Because of its design, a computer screen has a positive electrical charge, and the dust you're reading through happens to be negatively charged. And remember... when it comes to electrical charges, opposites attract!

An electrostatically charged air filter uses this principle to capture even more dust than a regular filter - in effect, the electrostatic filter becomes a "dust magnet". 3M Filtrete air filters take this concept a step further than competing filters by including both positively-charged and negatively-charged fibers, so the dust will stick to the filter regardless of which charge it happens to be.

Filter Life

Over time, the holes in an air filter slowly become clogged with tiny particles of dust, pollen, mold, etc. In a typical home, a filter will become completely "full" after about 3 months of use. Certain conditions can add even more particles to the air, and reduce the life of the filter to less than 3 months.

Change your filter more often if you have:

  • Pets
  • Dirty ductwork
  • Recent construction work or a sanding project
  • Smoke from cigarettes, a fireplace, a wood-burning stove, or candles

Why should you change your filter on time?

As the filter reaches the end of its life, the clogging makes the fan work harder and harder to push the air through the filter. This restriction can reduce the airflow through the house, and may affect the performance of the heating & cooling system.


Pollen is a tiny oval grain-like structure manufactured by flowers for reproduction. Some plants fertilize themselves using pollen from their own flowers, while others require pollen from another plant of the same species. Many of these plants rely on wind to carry their small, light, and dry pollen grains from plant to plant - and it's that type of pollen that causes us the most trouble.

When an allergen like pollen finds its way into the human body, our immune system starts producing large amounts of antibodies, which cause our cells to release powerful inflammatory chemicals like histamines or leukotrienes. It's these chemicals that produce the allergy symptoms we all know so well... sneazing, runny nose, and itchy-watering eyes.

Mold Spores

Molds are microscopic organisms that belong to the fungi kingdom, so they are neither plant nor animal. Molds keep the natural world livable by decomposing leaves, wood, and other plant debris. Unfortunately, mold can also find its way into our homes because of its taste for things like wood, ceiling tiles, wallpaper, paint, carpet, sheet rock, and insulation. Molds reproduce by means of tiny (between 3 and 40 Microns in size), seed-like mold spores. When we disturb molds in our homes, these mold spores are released into the air.

Like other allergens, mold spores can cause an immune system response which leads to allergy symptoms such as sneazing, runny nose, and itchy-watering eyes.

Dust Mite Debris

Dust Mites are probably the closest thing to "bed bugs" that you'll find - if you have a microscope, that is. Invisible to the naked eye, dust mites are tiny creatures closely related to spiders & ticks. The dust mite's favorite food is the dead skin that constantly falls off of our bodies, and they prefer to live where the food is - in our beds, a comfy chair, or our favorite stuffed animals. As dust mites grow, they shed their skin, and just like every other creature in the world, they poop. Dust mite debris is just a combination of shed skin and feces. Every time you hop in bed, this debris makes it's way into the air, and that's where it becomes troublesome to humans.

Dust mite debris is second only to pollen in causing allergic reactions. Like other allergens, dust mite debris can trigger an immune system response which leads to symptoms such as sneazing, runny nose, and itchy-watering eyes.

Household Dust

Household Dust is made up of a wide variety of things including dead skin from humans & pets, finely ground plant & insect parts, minute particles of sand & soil, and fabric fibers from clothes, carpets, and upholstery. Dust typically accumulates in carpets, on horizontal surfaces, computer & TV screens, and sometimes clumps into dirty balls of fabric fibers, otherwise known as "dust bunnies".

It's virtually impossible to completely rid your house of dust, and nearly everything you do tends to stir it up into the air - where you can't help but breathe it. Like other allergens, dust can trigger an immune system response which leads to symptoms such as sneazing, runny nose, and itchy-watering eyes.


Lint is nothing more than tiny bits of fiber from fabrics, plants, paper, etc. Typically, these fibers are found clumped together in the clothes dryer or maybe even in your belly button. Lint that doesn't end up in navel nirvana, can be found floating around in the air.

Although not nearly as troublesome as asbestos fibers, lint fibers can sometimes cause an allergic reaction in people.


Things like cigarettes, cigars, wood burning stoves, and fireplaces can all affect the level of smoke in our homes. In addition to several known carcinogens, smoke contains fine particles composed of wood tars, gases, soot, and ashes.

Because of its composition, smoke is particularly troublesome for people. In addition to causing the typical allergic reaction like sneazing & runny nose, exposure to smoke can cause burning eyes, bronchitis, and can even trigger asthma attacks.

Pet Dander

You might be surprised to find out that most pet allergys have nothing to do with pet hair. It's the pet dander, or tiny flakes of skin, that cause allergic reactions in millions of people across the country. And, even if you send the cat to "live on the farm", the dander can remain in the house, causing trouble for as long as 6 months. That's because some flakes are small and light enough to stay suspended in the air, while others tend to stick like glue to whatever they land on.

As you might suspect, the more your dog scratches itself, the more dander is released into the air. And a scratching dog doesn't always mean fleas... it can also mean that your dog itself is allergic to something floating around in your home. Removing these allergens from the air may be the best way to help everyone involved.

People allergic to pet dander can suffer from swollen, itchy, watering eyes, runny nose, sneazing, coughing, wheezing, shortness of breath, and sometimes even asthma attacks.


The word smog, coined by Dr. Henry Antoine Des Voeux in 1905, is literally the combination of the words smoke and fog. More specifically, smog is the result of a chemical reaction between sunlight, oxides of nitrogen (NOx), and volatile organic compounds (VOC's). This reaction leaves behind a cloud of tiny particles in the air - all too familiar if you've ever been to Los Angeles, London, or Mexico City. Fortunately for those big cities, and unfortunately for the rest of us, smog particles are small enough to remain suspended in the air, and are gradually spread throughout the world - and into your house - by the wind.

Exposure to smog can cause respiratory problems such as wheezing, coughing, and shortness of breath, as well as irritation of the eyes, nose, and throat.


Bacteria are everywhere. They are on everything we see and touch, and even in the air we breathe. In fact, right now, the bacterial cells on (and in) your body actually outnumber your own cells. Much of this bacteria is beneficial, helping us to digest our food, fight off sickness, and even keep our breath fresh. "Good bacteria" also helps to displace, or crowd out, the bad bacteria that can cause things like pneumonia, syphilis, cholera, and infections like Strep and Staph.

You can significantly reduce the amount of airborne bacteria in your home with a high quality air filter. The Filtrete Ultra Allergen filter is effective at capturing even the smallest bacteria, and the unique filtering material won't support the growth of bacteria once it's trapped in the filter.

Particles that can carry Viruses

A virus is an incredibly small & simple "object" consisting of a piece of genetic material (DNA or RNA) wrapped in a layer of protein. It's not really a living organism, in that it doesn't have a cellular structure, and it can't metabolise food or reproduce itself outside of a host cell - which is where we come in.

Once inside our cells, viruses can cause diseases like the common cold, flu, chickenpox, Ebola, and AIDS - just to name a few. Like bacteria, viruses can be found nearly everywhere - particularly in our modern, tightly sealed homes. Unlike bacteria, viruses are rarely, if ever, helpful to humans so avoiding them is always a good idea. Unfortunately the tiny particles floating around our houses are perfect for carrying viruses from place to place, and from person to person.

Particles that can carry Tobacco Odors

Tobacco smoke typically consists of particles smaller than 1 micron in size. The finest elements of tobacco smoke (the part that we smell) can sometimes stick to other tiny microparticles in the air. Once airborne, these particles can carry the tobacco odor throughout the house for days or even weeks, eventually settling on your furniture, carpets, bedding, etc.