How to Evaluate Allergy Filters
There as been a rising interest in allergy
filters for the home over the last several years. Once something only purchased
by people with chronic health conditions, more and more people are aware of the
need for cleaner indoor air.
There are several different types of 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 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 homes 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. A step up from this
level of filtration is the dry-tack polyester pad. These disposable allergy
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 to keep mold from growing on them.
The next step up in ability to trap particles is
a pleated filter material. These allergy 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 allergy filter and then returning it back into the system
to be either heated or cooled. These machines allow you to use one allergy
filter 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 allergy filter that most people are familiar
with is the stand alone HEPA 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. One of the most powerful units on the market is made by
Austin
Air. 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 ionizers also produce trace amounts of ozone.
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.
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