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Filters are everywhere in our life; from the oil filter in your lawn mower to the air filter in your vacuum, car, or furnace. For the most part, we just accept that they do their basic job and filter bad stuff out and let good stuff through. We follow the recommendations of the manufacturer and replace them when we remember to with a HEPA, foam, carbon, or MERV filter. But what do these terms mean, and how do they impact our lives?

In general, a filter has to be tight enough to block any contaminant, but not so tight that it significantly impedes what you want to get through. For instance, it does no good to have an air filter that filters everything but doesn’t let any air through. That’s why there are different grades for different jobs. A prefilter for a vacuum cleaner serves a completely different job than a furnace filter, so they are constructed in different ways.

vacuum filter black

HEPA Filters

HEPA-rated filters are the best-known type of premium filtration. After all, if you look at any air purifier or allergen vacuum, you’re going to see the HEPA logo proudly displayed. And rightly so; HEPA filtration is a standard set by the United States Department of Energy. To pass muster, a HEPA filter must remove at least 99.97% of particles that have a size of 0.3 micrometers or larger.

HEPA filters don’t operate like a sieve with really tiny openings. That sort of construction would make it difficult to push air through the openings and the filter would get clogged very easily. Instead, the filter has a series of layers made up of very thin fiberglass fibers that are arranged in random directions. These fibers are usually between 0.5 and 2 micrometers thick. This random array of fibers lets the HEPA filter do its job.

How they accomplish this is through three different filtering mechanisms built into one HEPA filter. Some contaminants get smashed right into the filter material in a direct impact filtration. Others get snagged by a fiber as they whoosh by and are intercepted. The last type of filtering mechanism is diffusion, where a particle gets swept around randomly by air currents and meanders through the filter until one of the other two filtering types happens.

Wear and tear on a HEPA filter comes from all the air being forced through it. Eventually, instead of particles being forced around the fibers and trapped, passages are formed that go straight through. Once that happens, a HEPA filter stops filtering and needs to be replaced.

Keep in mind that HEPA filters don’t filter out odors or smells. In order to do that, an activated charcoal filter is needed. Usually, a charcoal filter is added as a mesh prefilter. Prefilters are usually made of foam or some other airy material. Their job is to trap the larger stuff so your HEPA filter doesn’t have to deal with it. Things like hair, larger bits of pollen, and shreds of paper get caught by the prefilter.

man with air filter

MERV Filters

MERV stands for Minimum Efficiency Reporting Value, and this is how filters in air circulating systems are rated. A HEPA filter only has one pass to clean all of the contaminants out of the system; a furnace or HVAC filter deals with recirculated air. That means that your furnace filter has multiple chances to clean your air to the standard you want.

MERV values are rated by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. They rate a filter based on how much of a specific sized contaminant it will remove at each pass. MERV ratings start at 1 and go up to 20, increasing in efficiency as the number goes up.

To give you an idea of what each rating means, a MERV-1 filter is the kind you find in a window air conditioner, with the filter only capable of removing about 20% of particles from 3 to 10 micrometers in size. A MERV-1 filter will capture dust, hair, carpet fibers, and some pollen.

A typical furnace filter for residential use must be rated at least MERV-6, but the better ones are MERV-8. These filters catch about 70% of larger air contaminants per pass but may leave allergy sufferers feeling a little in the lurch, as they aren’t as good at handling smaller things like pet dander.

On the other hand, a MERV-11 rated filter is typically considered a superior residential filter. These filters are pleated with a rigid frame and are between 1 and 6 inches thick. They are capable of removing over 85% of particles from 3 to 10 micrometers per pass, and about 50 to 65 percent of particles from 1 to 3 micrometers. These filters are considered allergen-reducing filters, removing things like mold spores, dust mites, pet dander, and even pudding mix easily. They are also capable of capturing larger bacteria, such as those that cause Legionnaires’ disease.

Filters that are rated above MERV-12 are generally reserved for extreme commercial uses, such as hospital rooms and electronic clean rooms. These filters are capable of removing over 99 percent of contaminants as small as a particle of smoke or most viruses.

When you look at a HEPA filter for your vacuum or air purifier and compare it to a furnace filter, the furnace filter is going to look less dense. That’s because the filters in your air system have to let a much higher volume of air through without a huge drop in pressure. The more restrictive a filter is, the more energy it takes to push air through; correspondingly, the more air that gets pushed through, the faster the filter gets worn out.

It’s a constant trade-off between efficiency, longevity, and cost. As the saying goes, pick two. A long-lasting, highly efficient filter will cost more, whereas an inexpensive and highly efficient filter will need to be replaced more often. If you’re being cost-conscious, the best way to save money on filters comes from finding a good generic brand that you can trust, such as ThinkCrucial. In most cases, generic brands are manufactured with the same stringency and standards as OEM equipment. Original Equipment Manufacturer (OEM) parts have increased consumer costs simply because of the name brand. This is the same in pretty much any aftermarket part system, whether it’s filters or car parts. The important part is knowing that you don’t have to pay through the nose for excellent quality and customer service.


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