Allergens from mold and bacteria (microbes) are probably spreading throughout your house right now. If you have central air-conditioning or a forced-air heating, ventilation and air-conditioning (HVAC) system, inadequate air filtration could be allowing mold and bacteria (microbial) growth inside your HVAC system. Fewer than 10% of residential HVAC systems have filters capable of preventing microbial growth.

Microbial contamination can trigger or exacerbate respiratory problems, including allergies and asthma. Certain molds even produce toxins that are linked to cancer.

Paying big money for high-end air-filtration devices does not necessarily solve this problem. Many expensive filtration products do little to improve air quality. Here’s what works and what doesn’t work…


The cheapest, easiest way to improve your home’s air quality is to improve the quality of the disposable filter you use in your forced-air system.

Most home owners use fiberglass panel filters that have a Minimum Efficiency Reporting Value (MERV) of just three or four. To avoid microbial growth, instead use a “pleated” filter rated MERV 8 or higher.

Buying advice: MERV 8 filters are available at home-improvement stores, such as The Home Depot and Lowe’s, and many hardware stores for between $5 and $10 apiece. Don’t worry about the filter’s brand—focus instead on its MERV rating, which usually is clearly marked on the package. Using the correct size is equally important—even the best filter will not improve air quality if air can get around it in your system’s filter compartment or if the compartment does not close properly with the filter inside. Measure your system’s filter compartment before buying. Then make certain that the filter compartment has an airtight cover—magnetic covers are available online and in stores.

Do not pay extra for expensive “electrostatic” filters. Manufacturers claim that these are washable and reusable, but in my experience, there is no way to clean them adequately.

Downside: Switching from a one-inch-deep MERV 3 to a one-inch-deep MERV 8 filter is likely to reduce your system’s airflow by up to 10%, possibly increasing your cooling and heating bills slightly. (Deeper filters—two-inch, four-inch, six-inch—do not reduce the airflow as much but are too deep to fit most units.) Replacing filters every three months will minimize the energy consumption increase—dirty filters force heating and cooling systems to work even harder.


If you have used filters rated lower than MERV 8 in the past, it is likely that mold and bacteria may already be growing on your air conditioner’s blower and coil unless you live in an arid region. Installing a filter with a higher MERV rating will not kill existing microbial growth, nor will it prevent microbial allergens from spreading throughout your home—the air conditioner’s blower and coil are located after the filter. The only reliable way to remove existing contamination is to hire a duct-cleaning service.

Contact the National Air Duct Cleaners Association (NADCA) to find a pro in your area. The duct-cleaning sector is rife with scammers, but NADCA members usually are legitimate. Expect to spend around $1,000 to clean the ducts of an average-sized home of about 2,350 square feet. Any company that promises to clean ducts for significantly less is likely to either do a slipshod job or find excuses to pad your bill during the job. Ducts should be cleaned once every five years or so.

Most important, before signing a contract, confirm that the duct cleaner will clean your air conditioner’s blower, blower cabinet and cooling coil—some duct cleaners ignore these components and clean only the ducts.


If someone in your household has severe allergies or asthma, you might be tempted to invest in an air-filtration or purification system. While some high-end products truly deliver cleaner air, others are a waste of money…

MERV 11 or higher pleated filters do a wonderful job of trapping particulates and preventing microbial growth—but there’s a catch. Deep filters with MERV ratings above 8 or so tend to be too deep to fit in the filter compartments of standard residential HVAC systems. Installing a deeper filter holder is likely to cost $700 to $1,000.

MERV 11 replacement filters typically cost $40 to $60 each and last around six months.

Verdict: This is probably the most cost-effective high-end filtration option, even if you have to retrofit your system to accommodate the filter.

Whole-house air cleaners that accommodate high-MERV filters include: Aprilaire Models 2210/2310/2410/2250, which ship with a MERV 11 but have the option to upgrade to a MERV 13Honeywell F100 Whole-House Media Air Cleaner.

High-Efficiency Particulate Air (HEPA) whole-house filters have impressive MERV ratings of 16 and up. Unfortunately, residential whole-house HEPA filters are “bypass filters,” which means that they subject only a small percentage of the air that passes through the HVAC system to extreme filtration on each circulation. Bypass systems do a fine job of removing particulates from the air, but they do little to prevent microbial growth within the HVAC system, which is the greater threat to residential air quality.

Verdict: Not worth the cost (around $2,000 to $3,000) except, perhaps, for those with severe allergies to particulates, such as pollen and pet dander.

Electronic filters attach an electrical charge to particles that pass through, then collect the charged particles on an oppositely charged metal plate. In theory, that should be effective. In practice, these systems must be cleaned at least once a month to remain effective. Few home owners do this, so their electronic filters soon become useless.

Verdict: Don’t bother. I’ve yet to see a residential electronic filter that’s still functioning properly after a year.

Ultraviolet-light air purifiers supposedly kill germs by irradiating the air that passes through the HVAC system. The technology works well in hospitals and other large industrial applications, but residential UV purifiers are so small that they are essentially worthless.

Verdict: A total waste of $900!

A hot water–circulating heating system reduces the odds of respiratory problems by perhaps 50%, compared with forced-air heating systems—even if central air-conditioning is still used a few months of the year. Unfortunately, making this change in an existing home could cost tens of thousands of dollars.

Verdict: Switching to a water-circulating heating system is worth considering if you are remodeling or building a home and a family member suffers from serious respiratory problems.


These do an effective job of removing particulates, such as pollen, from the air, but they will not prevent microbial growth inside your home’s central-air system.

Buying advice: Choose a model with a “Clean Air Delivery Rate” (CADR) of at least 100. Avoid those with electronic filters regardless of their CADRs, however. Not only do electronic filters become useless if not cleaned frequently, they often produce ozone, a lung irritant, so they can create respiratory problems. Also, check portable air cleaners’ decibel rating before buying—you don’t want one much over 50 decibels. Noisy portable air filters are a common complaint. IQAir just started selling the Atem, a small desktop HEPA filtered purifier with directed air flow that can be great for allergy sufferers at work.

Verdict: At about $400, this could be a lifesaver for an allergy-sufferer forced to work in a space contaminated with particulate allergens.


Window air-conditioner units tend to use filters rated MERV 3 or lower.

Buying advice: A company called WEB Products makes a MERV 7 “Washable Electrostatic Filter” specifically for window air conditioners. It’s the best filter on the market for window units—though I recommend disposing of these filters each cooling season rather than washing and reusing them. (, search for “WRAC,” $7.34 each). No filter should ever touch the cooling coil, so if the filter does not fit inside the air conditioner, tape it so that it covers the entire intake grille (not the narrower supply grille at the top).

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