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It’s a commonly used acronym, and a typical way to purify indoor air, but what exactly is a HEPA filter?

HEPA stands for high efficiency particulate air, and it’s a way of categorizing the quality of air filters built to remove dust, pollen, pet dander, mold, bacteria, and other airborne particles from indoor air.

A filter of HEPA quality used within heating, ventilation, and air conditioning (HVAC) systems can greatly improve the quality of indoor air by filtering small particles of greatest health concern.

Air filters work by trapping pollutants in the fibers of their paper-like material. A HEPA grade filter can, theoretically, trap particles as small as 0.3 microns (for reference a human hair may be between 17 and 180 microns in width.) That means that when placed in a filtration system, HEPA filters can remove 99.97 percent of particles from the air.

Though HEPA filters are a highly effective way to clean the air, there are other types of filters, including electrostatic filters and electrostatic precipitators, that use a small charge to trap particles.

There’s also ionization, a process that charges the particles in a room so that they are attracted to walls, floors, tabletops. Some devices using ionization can attract the charged particles back into the unit. Ionization is not recommended by the Environmental Protection Agency (EPA), especially for people with respiratory diseases, as potentially irritating ozone can be released in the ionization process.

Newer filtering systems use ultraviolet (UV) irradiation light to kill germs, notes Jake Loiko, owner of Hawks Mountain Home and Property Services in Springfield, Vermont. “A lot of systems are going UV now,” he says. “It’s not just a filter, it kills bacteria, and that’s why those are becoming hugely popular.”

Keep in mind, says Loiko, that UV filters would need to be used in tandem with an air filter that takes care of the particles themselves. “If you’ve got cat hair flying around, UV’s not going to take care of that,” he says.

There are many options on the market, and understanding the ins and outs of the various air filters can be tricky. We focused on HEPA air filters that could be used for a whole house or HVAC system, and used the following criteria for selection:

  • Non-ozone producing. In some air filtration systems, ozone can be introduced into the indoor air at levels “significantly above levels thought harmful to human health” says to the EPA.
  • Non-ionizing. In addition to the concerns about ozone production, ionizing air purifiers are not deemed by the EPA to be as effective. “They do not remove gases or odors, and may be relatively ineffective in removing large particles such as pollen and house dust allergens,” according to the EPA website.
  • A MERV rating of 8 or above. MERV stands for Minimum Efficiency Reporting Value, which reports a filter’s ability to capture larger particles. The higher the MERV rating, the better it is at trapping specific types of particles.

As you consider filter options, Loiko suggests first ensuring your HVAC system has a filter. “If you don’t have a filter, contact your local HVAC company and have them install a filter rack,” he says.

Key questions to ask when deciding on a filter, says Loiko, is the size filter you need and what you want to filter out. That’s why most of the filters in this list are categorized by what kind of pollutants they filter out.

Best filter for whole home or HVAC system

Aerostar 20x20x1 Pleated Air Filter

Price: $

With an Amazon rating of 4.5 stars out of more than 18,000 ratings and the most economical filter on our list, this synthetic electrostatic filter is popular for a variety of uses.

With a MERV rating of 13, it can filter out microscopic particles of lint, dust mites, spores, pollen, pet dander, fine dust, smoke, viruses, and bacteria. The manufacturer suggests changing the filter every 60 to 90 days — more frequently during the summer and winter months.

Best filter for smoke and smog

Filterbuy 16x25x4 Air Filter

Price: $$

This is a relatively economical choice, and its MERV 8 rating will work well to filter out smoke and smog. The electrostatically charged, pleated synthetic filter can trap 90 percent of airborne particles without impacting air flow, according to the manufacturer.

Rather than just straight-up cardboard, it uses industrial-grade beverage board frames for greater longevity. However, the manufacturer still recommends replacing these every 90 days.

Best filter for pollen

Filtrete Healthy Living 4-Inch 20x25x4 Air Filter

Price: $$

This electrostatic air filter has a MERV rating of 12, which makes it plenty sufficient for common allergens such as mold spores, pollen, and pet dander. The real benefit to this filter, and the reason for the slightly higher price tag, is that it can be used for up to a year before being changed out (depending on usage). That’s because it’s 4 inches thick, though keep that in mind when determining if your system can support a filter of that size.

Best filter for flu season

Filtrete 1900 Premium 20x25x1 Air Filter

Price: $$

With a high MERV rating of 13, this 1-inch electrostatic pleated filter can attract and capture microscopic particles. That means it can filter a wide variety of pollutants, including smoke, cough and sneeze debris, bacteria, and viruses. Loiko says keep in mind that with the high the MERV rating, you’ll need to replace this filter more often.

Best reusable air filter

K&N 20x20x1 HVAC Furnace Air Filter

Price: $$$

With a 4.5-star rating on Amazon, the K&N is a popular and well-reviewed air filter. With a MERV of 11, it can remove dust, pollen and other particles. However, its biggest claim to fame is that it’s washable and reusable — saving you from repeated purchases of disposable filters. (This makes it a more environmentally conscious choice, as well.)

The filter made of a pleated synthetic material is designed to fit directly into your HVAC filter register and comes in multiple sizes and dimensions. Though this filter has the highest price point on the list, the ability to wash and reuse it will save you money in the long term.

Best portable air purifiers

Medify MA-112 Air Purifier with HEPA Filter

Price: $$$

If you don’t run your HVAC system often or you don’t trust that it’s completely sealed up, you may want a portable, standalone air purifier for your home. In that case, this Medify air purifier is a solid and a relatively cost-effective option.

Most portable air cleaners come with a clean air delivery rating (CADR). The higher the CADR, the greater the size of the area that will be served. This air purifier has a high CADR rating of 950 and can cover 2,500 square feet. It can also remove 99.9 percent of particles, including odors, smoke, pet dander, and dust down to 0.1 microns in size, according to the manufacturer.

Visit local home improvement stores

Your local hardware store will have most of your basic filters, and can be a good place to compare prices and see your options firsthand.

Have the HVAC system checked

Loiko’s biggest tip for homeowners is to make sure that your HVAC system is well maintained. “If your heating system is not taped at every seam when it’s sucking in the air, that means it’s drawing air from everywhere in your house. So if that’s not all coming through the filter, you can put the most efficient filter on in the world and it won’t mean anything,” he says.

That’s why he recommends a professional deep-cleaning service to clean ducts and tape any areas that are not sealed up properly.

All air filters require cleaning or replacement to work as promised. “You don’t need a professional to do it,” says Loiko. “Just find your filter, pop the housing off, and swap the filter out.”

How often to clean or replace the filter will depend on the season or region, but Loiko recommends a minimum of every 3 months. “If you’re living in a highly forested area, your pollen season is going to be wicked, and you may need to replace it more often,” he says. And keep in mind, says Loiko, that the higher the efficiency, the more often you will have to clean or replace it.

Though no HEPA filter will remove all the indoor air pollutants, a high-efficiency filter, combined with a well-maintained HVAC system, can go a long way. This roundup of tips and recommended filters is a good first step towards cleaner indoor air.