Home Air Filters

Why this Information Is Important: Outdoor air pollutants like coal dust can be removed from air indoors using an air filter. These are part of homes' air handling and ventilation systems and portable air filter devices. Air filters that remove coal dust can be expensive, especially when changed out often, and are limited by how much air moves through the air filter. Learn which air filters provide the most protection and how to receive financial support to offset high costs. Attention: Residents, policymakers See the highlights before jumping in. Hide Highlights

Outdoor Air Pollution Comes Inside

Outdoor air and air pollutants like fine particulate matter (PM_2.5) and dust (PM₁₀) come into our homes through windows, doors, and the building's air handling system.

One way to protect yourself and reduce the amount of PM_2.5 and PM₁₀ in our indoor air is by passing the air through an air filter. This is done using the air filter that is part of the HVAC (heating, ventilation, and air conditioning) system or a portable air filter device.

What Is an Air Filter?

An air filter is a physical material made of fine fibers woven together into a mesh. As air passes through the filter, the mesh traps PM_2.5, PM₁₀, and potentially even smaller particles, keeping them from moving throughout the home.

Air filters are made of different sizes of mesh. PM_2.5 and PM₁₀ also have a size and will be removed from the air by an air filter with a mesh size small than the particle size. For example, PM₁₀ means particles with a diameter equal to or smaller than 10 micrometers (µm) in diameter. PM_2.5 means particles with a diameter equal to or smaller than 2.5 µm in diameter. To remove smaller particles from the air, you need a filter with a finer mesh. Very-fine mesh filters can remove most PM_2.5 and PM₁₀, making the indoor air safer to breathe.

Fine mesh filters cost more to buy than coarser mesh filter that are less-protective.

Will Your Air Filter Remove Coal Dust?

All air filters have a MERV rating, which stands for Minimum Efficiency Reporting Values. The MERV rating can be used to determine the size of the filter mesh and, therefore, an air filter's ability to remove PM_2.5, PM₁₀, and particles as small as 0.3 µm in diameter, indicated as PM_0.3.

The higher the MERV rating, the higher the percentage of particles, especially smaller particles, the filter removes from the air. Studies have shown that using an appropriate filter can reduce indoor PM_2.5 levels by 50% or more, meaning residents can significantly lower the amount of harmful particles they are breathing every day.

The MERV rating is also often represented using symbols. Look for an air filter that remove "particles," "air pollution," or "smog."

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  Photograph: MERV 11 filter, front side, April 2026. Photograph by Jackie Hertz.
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  Photograph: MERV 11 filter, back side (see bottom of filter), April 2026. Photograph by Jackie Hertz.
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  Photograph: MERV 12 filter, front side, April 2026. Photograph by Jackie Hertz.
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  Photograph: MERV 11 filter, back side (see bottom of filter), April 2026. Photograph by Jackie Hertz.

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  Photograph: MERV 11 filter, front side, April 2026. Photograph by Jackie Hertz.
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  Photograph: MERV 11 filter, back side (see bottom of filter), April 2026. Photograph by Jackie Hertz.
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  Photograph: MERV 12 filter, front side, April 2026. Photograph by Jackie Hertz.
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  Photograph: MERV 11 filter, back side (see bottom of filter), April 2026. Photograph by Jackie Hertz.

HVAC Air Filters

Homes can have an HVAC system that works by pulling outside air into the home, passing it through an air filter, heating or cooling the air, and then circulating the air within the home through vents in some or all rooms. Even if your home doesn't have air conditioning, it likely has some ventilation system that is still called an HVAC system. An air filter that is part of the HVAC system has the potential to remove particles from the air in many rooms at the same time.

The HVAC system only passes indoor air through the air filter when it is on. The air filter can also get clogged with dust and debris, which reduces the air flow and, therefore, the amount of PM_2.5 and PM₁₀ removed from the air. This is why it is recommended that we change the air filter at least every three months.

Portable Air Filter Devices

Portable air filter devices work differently. Instead of connecting to the home's HVAC system, portable air filter devices sit in one room and pull air through the air filter with a fan. This makes portable air filter devices useful for targeting one specific room, for example, a bedroom while sleeping, but they do not remove particles from the air of the whole home. An additional downside is that the fan can be noisy.

High Efficiency Particulate Air (HEPA) Filters. Many portable air filter devices use HEPA filters. HEPA filters are a pleated mechanical air filter that can remove as much as 99.97% of dust, pollen, mold, bacteria, and particles as small as PM_0.3. Not all HVAC systems, especially older systems, are able to accommodate HEPA filters.

Make Your Own Portable Air Filter Device: Corsi-Rosenthal Box. A Corsi-Rosenthal box is a low-cost alternative to a commercial air filter device that you can build at home in under an hour. Corsi-Rosenthal boxes have been widely used in schools, community centers, and homes.

Corsi-Rosenthal boxes are built from the exact same MERV-rated filters as used for HVAC systems. The difference is that Corsi-Rosenthal boxes use a standard box fan to create air flow. Corsi-Rosenthal boxes can be built with 1–5 MERV-rated air filters.

What You Will Need:

• Four MERV 13 or 14 air filters, all the same size, commonly 20 × 20 × 2 inches but based on the box fan you have.
• One 20-inch box fan, the same width as your filters.
• A piece of cardboard large enough to cover the bottom of the box (you can use the box the fan came in).
• Tape, strong packing tape or duct tape works the best.

How to Build It:

• Step 1. Check the air flow direction on your air filters. There will be an arrow printed on the cardboard edge of the air filter indicating the direction of air flow. Arrows should point inward, toward the center of the box.
• Step 2. Form a box with the four filters. Tape the four corners where the filters meet so the box holds its shape securely.
• Step 3. Seal the bottom. Trace the bottom opening of your filter box onto a piece of cardboard and cut it out. Tape this cardboard securely to the bottom of the box, sealing off any openings completely. Check all the edges for gaps and seal them with tape. This forces air to flow through the filters.
• Step 4. Attach the fan to the top. Place the box fan on top of the filter box so that it covers the top opening. The fan should be oriented to blow air upward and out. Tape the fan securely.
• Step 5. Turn it on. The higher fan speed, the more air passes through the air filters and the more particles are removed from the air.

For removing PM_2.5 and PM₁₀, a well-built Corsi-Rosenthal box using MERV 13 or 14 air filters performs comparably to a commercial air filter device that cost hundreds of dollars more. A Corsi-Rosenthal box is safe to use.

Portable Air Cleaning Devices Can Be Dangerous

There are a wide variety of devices available to purchase in stores and online. Please be aware, some of these devices can actually be dangerous.

Portable air filter devices are safe because they just passed air through an air filter with a fan. This is the only type of device that is safe to buy.

Do not use devices that: (1) produce a gas called ozone or (2) use an ultraviolet (UV) light to "purify" or "clean" air. These produce an array of toxic gases such as ozone and other oxidants that are very toxic to breathe.

Buying Guidance

Air filters that remove PM_2.5 and PM₁₀ are more expensive than air filters that do not. Residents of Southeast Newport News and Lambert’s Point have reported changing their HVAC air filters monthly because the filters quickly appear black and clogged with coal dust. This leads to a substantial financial burden on residents.

Air filter costs are tabulated below, based on the lowest prices found on Lowes.com in April 2026 for the popular and reputable brand Filtrete, a 3M company. This is not an endorsement.

Air filter costs are tabulated annually, assuming air filters are replaced every three months, as the packaging directs, and monthly, as is common in Southeast Newport News and Lambert’s Point.

Key: the symbol $\le$ means "less than or equal to."

MERV Rating	5	8	11
PM10 Removed from Indoor Air (PM3–PM10)	$\le$20%	$\le$70%	$\le$85%
PM2.5 Removed from Indoor Air (PM1–PM3)	–	$\le$20%	$\le$65%
Smaller Particles Removed from Indoor Air (PM0.3–PM1)	–	–	$\le$20%
Protects against Coal Dust	No	No	Some
Repair Lab Recommendation	No	No	No
Cost per Filter	$4.76	$9.89	$10.99
Annual Cost at 4 per Year	$19.04	$39.56	$43.96
Annual Cost at 12 per Year	$57.12	$118.68	$131.88
Specific Air Filter the Estimate Is Based on	Filtrete 16 $\times$ 25 $\times$ 1 AC Furnace Air Filter MERV 5, MPR 300	Filtrete 16 $\times$ 25 $\times$ 1 AC Furnace Air Filter MERV 8, MPR 700	Filtrete 16 $\times$ 25 $\times$ 1 MERV 11 1085 MPR Allergen Defense Extra Electrostatic pleated Air Filter 4-Pack

MERV Rating	12	13	14
PM10 Removed from Indoor Air (PM3–PM10)	$\le$90%	$\le$90%	$\le$95%
PM2.5 Removed from Indoor Air (PM1–PM3)	$\le$80%	$\le$85%	$\le$90%
Smaller Particles Removed from Indoor Air (PM0.3–PM1)	$\le$35%	$\le$50%	$\le$75%
Protects against Coal Dust	Yes	Yes, and also protects against smaller particles from traffic pollution on I-664 and smoke	Yes, and also protects against smaller particles from traffic pollution on I-664 and smoke
Repair Lab Recommendation	Yes	Yes	Yes.
Cost per Filter	$16.48	$23.97	$25.97
Annual Cost at 4 per Year	$65.92	$71.91	$103.88
Annual Cost at 12 per Year	$197.76	$287.64	$311.64
Specific Air Filter the Estimate Is Based on	Filtrete 16 $\times$ 25 $\times$ 1 MERV 12 1500 MPR Allergen, Bacteria and Virus Electrostatic pleated Air Filter 2-pack	Filtrete 16 $\times$ 25 $\times$ 1 MERV 13 1900 MPR Premium Allergen, Bacteria and Virus Electrostatic pleated Air Filter	Filtrete 16 $\times$ 25 $\times$ 1 MERV 14 2500 MPR Premium Allergen Ultrafine Particles Electrostatic Pleated Air Filter

Resources and Materials: See for Yourself and Dig Deeper

Corsi-Rosenthal Box, University of California Davis, Undated

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How to Build a Corsi-Rosenthal Box, University of California Davis College of Engineering, Undated

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The Front Porch and Community Action

Have a question? Want to start a conversation or share an observation? Join the discussion: Front Porch

Have you made a records request that you want to add to this page or another page? Add Your Voice

You can also email the Repair Lab at editor@voicesinthedust.org or call, text, or send a voice memo to (757) 317-0356.

Do You Need Additional Air Filters to Protect Your Home?

Voices in the Dust Air Filter Fund: Are you a resident of Southeast Newport News or Lambert's Point who is experiencing coal dust impacts in your home? Could you use some support offsetting the high costs of protective air filters? Write the Repair Lab and East End Civic Association at editor@voicesinthedust.org to request air filters for your HVAC system. We can drop them off at your home or leave them at the East End Civic Association offices for you to pick up.

Do you want to donate to the Voices in the Dust Air Filter Fund? Your donation is tax deductible and 100% of your donation will be used to buy air filters that are protective against coal dust for residents living in Southeast Newport News and Lambert's Point.

Support the the Voices in the Dust Air Filter Fund: Donate

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