Mike Krause asked:


There as been a tremendous increase in air cleaners and filters for the home introduced into the market over the last several years that are being sold as ” Allergy Filters”. Once something only purchased by people with chronic health conditions, more and more people are aware of the need for cleaner indoor air and are buying them.

There are several different types of air cleaners and filters that are marketed by their manufacturers as allergy filters. Some of these are of benefit to those with allergies and some do not. In this article, we will explore allergy filters that are installed in the air conditioning or heating system, units that actually attach to the air conditioner or furnace, stand alone allergy filters, ionizers and ozone machines that are marketed as allergy filters, and allergy filters used to filter the fresh air coming into your home.

There are several different types of allergy filters to use in place of your regular ac filter on your air conditioning or heating unit (HVAC). This is in place of the current filter you are using on the return side of your system. Most home owners use a filter made from a blue, spun polyester media. Unfortunately, these trap very few particles and actually have to be loaded up with dust to start trapping large particles. It’s like throwing marbles at a tennis net. A step up from this level of filtration is the dry-tack polyester pad. These disposable ac filter pads fit into a permanent frame. Each change, you remove the old allergy filter pad and insert a new one. The pads are usually treated with an antimicrobial product to keep mold from growing on them.

The next step up in ability to trap particles is a pleated filter material. These ac filters are also disposable and the better made ones will last for up to three months at a time. The idea behind the pleated material to make the allergy filter is that by pleating the fabric you create more surface area. More surface area means more media to trap allergens. When looking at these filters, you want a MERV rating of 8 or better. The MERV scale ranges from 1 (least efficient) to 16 (most efficient), and measures a filter’s ability to remove particles in the 3 to 10 micron size range. The higher the rating, the more particles removed. Also, higher rated filters trap more of the smaller particles. Last but not least in the HVAC filter category is the permanent filter. These allergy filters create an electrostatic charge by the friction created as air moves across the filtering media. These filters are usually very high in efficiency (that means they trap many particles) but may restrict air flow. This is true especially for older HVAC equipment. Once a month the permanent allergy filter is cleaned with water and reinstalled in the unit.

The next category of allergy filter for the HVAC system is a whole house HEPA filtration unit that attaches directly to the HVAC system. These units can be either HEPA (high efficiency particulate air) or TFP (turbulent flow precipitators). They work by drawing a portion of the air (usually about 30%) from the return side and passing it through the filter and then returning it back into the system to be either heated or cooled. These machines allow you to use one filtration system to clean all the air in a home. However, the HVAC system must be on at all times for the air to be filtered. This may not be an option in parts of the area where the heating or air conditioning system is not used year-around.

The filter that most people are familiar with is the stand alone HEPA air cleaning filter. HEPA is a measurement. It means that 99.97% of particles as small as 0.3 microns are trapped by the filter. There are many makes and models on the market today. The most important consideration is getting the proper size allergy filter for the area to be cleaned. Most allergy filter manufacturers rate their machines in square footage. This is not a helpful measurement, as we live in 3 dimensions and not 2!

The first calculation to make is the volume of air you need to clean. This is expressed as the cubic feet of air in the room. Multiply length of the room in feet times the width of the room in feet times the height of the ceiling in feet to get the volume of air in cubic feet. (Length x width x ceiling height = volume). When you review the different makes and models of allergy filters, the most important number is the number of cubic feet of air moved per minute by the machines motor. The allergy filter can never clean any more air than the motor can move. You use this number to determine how quickly that machine can clean your volume of air. For example, if you have a larger bedroom that is 12 x 14 with 10 foot ceilings, you have a volume of air of 1,680 cubic feet (12 x 14 x 10 = 1,680). If you buy a machine that moves 100 cubic feet of air per minute, than it will take 16.8 minutes to turn over the air in that room one time. (1,680 / 100 = 16.8) That means that the air will pass through the filter less than 4 times an hour. You want a minimum of 6 and ideally at least 8 air exchanges an hour. In this instance, the machine is sorely undersized and would not be a good buy, no matter what the price. It is simply too small for the size of room. If you have such a large room, you would need to either purchase multiple units or look for an allergy machine with more power. Some of the best and most powerful units on the market are made by Austin Air and Airpura. They make several models that move 400+ cubic feet of air per minute. In our previous example, the higher powered machine would result in an air exchange every 4.2 minutes and over 14 air exchanges an hour.

There are also machines that are marketed as allergy filters that are not truly filters and most certainly are not for people with allergies. These “allergy filters” are ozone machines and ionizers.

Ozone units clean the air by utilizing a third molecule of oxygen as a free radical to destroy odors at their source. Ozone is not effective in removing particles from the air, but it is effective in controlling odors. Ozone is also a lung irritant and should not be used by people with allergies or asthma. Many of the manufacturers of ozone producing machines do not disclose this bit of information. Ionizers are also frequently marketed as allergy filters. They do not in fact contain a filter. Ionizers work off the basic principle that opposite charged particles attract. Some units work by the use of positive and negative charged rods. The particle enters the back of the unit, passes one rod and picks up a charge and then is drawn to the second rod with the opposite charge. The problem is once the second rod is covered in particles it no longer emits a strong enough charge to attract any more particles. The rods must then be cleaned, releasing particles back into the air where they are inhaled by the allergic person. The other type of ionizer uses radio frequency to send both positive and negative ions out into the room. The neutral particles pick up these charges, then stick to each other, and then fall to the first surface they hit in clumps. These machines work great to keep the particles knocked down, but as soon as there is any movement in the room, the particles are disturbed and become airborne again. Many manufactures of these types of ionizining machines fail to disclose they also produce trace amounts of ozone. One major manufacture was sued and lost over this issue.

The last type of allergy filter is not a traditional filter, but is a special screen used on your windows. These screens are called micro air screens and they can trap particles as small as 8 microns. These allergy filters fit into your windows and allow you to keep your windows open and bring in fresh outside air without bringing in mold, pollen, or other pollution.

So as you can see, there are many types of allergy filters made today. Which type of filter you use determines how clean your air will be.