The Dirt on Air Filters

There are three methods that help to control indoor
pollution: source control, ventilation and air cleaning.
Source control (removing the source of the allergens) is
the most cost-effective; unfortunately, not all pollutants
can be identified or eliminated. Pet dander, for instance,
is one of the most prevalent sources of indoor pollution,
but getting rid of Fido is more than most pet owners can
bear. Ventilation (bringing outside air inside) helps, but
most of us don’t live in a climate where leaving the
windows open year-round is an option. In addition, leaving
the windows open allows outside allergens to enter the
house. But cleaning the air with filters, even though it’s
more costly than the other two methods, is probably the
best way to reduce the allergens in your house. Combine all
three methods and you’ll breathe easier.

Filter Functions

There are three general types of filters: mechanical, ion
generators and electronic air cleaners. In addition, there
are “hybrid” systems that combine mechanical, ion and/or
electronic features.

Mechanical filters are the models found in most homes with
central heating and/or air conditioning. They are also
found in portable fan-forced units and serve as register
covers where an air duct enters a room. The standard
mechanical filter is a flat filter that contains coarse
fibers (typically fiberglass, aluminum or synthetic
material) held in place with a cardboard or plastic frame.
Other models use an “electret” media, which is a
permanently charged plastic film or fiber. A third type is
a panel filter, which has a pleated or an extended surface.

Generally, mechanical filters are efficient at collecting
large particles, but remove a small percentage of smaller
particles (the panel filter is somewhat more efficient at
small-particle capture). With mechanical filters, the more
dense the filter material or the greater the filter surface
area, the better it will remove pollutants. Whole-house
mechanical filters, at $1 to $15 each, are a bargain in the
air-cleaner world, but they are also the least effective,
stopping only 10% to 40% of pollutants. They also need to
be replaced on a monthly or bimonthly basis.

Electronic air cleaners are the most efficient
cleaners of indoor air, removing up to 95% of dirt, dust
and smoke.

Ion generators, which come as portable units only, use
electrostatic charges to remove particles from indoor air.
The charged particles in a room are then attracted to
walls, floors, tabletops, draperies and occupants, which
often results in a noticeable accumulation of soot. In some
cases, these devices contain a collector to attract the
charged particles back to the unit. They may also have a
fan and mechanical filter. Ion generators take second place
in the price hierarchy, at $50 to $150 apiece, but you
usually need a unit for each room. Also most systems
require either filter replacement or periodic cleaning.
They can remove 25% to 94% of the pollutants.

Electronic air cleaners use an electrical field like a
powerful magnet to trap charged particles. Whole-house air
cleaners are most often installed in house ducts between
the cold air return and the fan, but they can also be in
portable units with fans. Electronic air cleaners are
usually either electrostatic precipitators or charged-media
filters. Electrostatic precipitators collect particles on a
series of flat plates, or cells, enclosed in a metal frame.
There are nonelectronic electrostatic precipitator filters,
but these are not as effective. Charged-media filters, on
the other hand, collect the particles on the fibers in a
filter.

Electronic air cleaners are the most efficient cleaners of
indoor air, removing up to 95% of dirt, dust and smoke.
They are also the most expensive, ranging in cost from $150
to $600, with some units costing upwards of $1,000. One big
advantage to electronic air cleaners is that they can be
reused almost indefinitely, needing only to be washed in
the dishwasher or tub.

Finally, hybrid filters — those that combine all three
types of cleaners — may be whole-house or portable, and can
combine different methods such as mechanical filters and
passive electrostatic filters. These can range in price
from $50 to $200 and often require expensive filter
replacements on a periodic basis. Their effectiveness
varies depending on the combination of filters.

How Effective are They?

No air cleaning system can remove all pollutants from
indoor air. In addition, there are no uniform standards for
comparing systems, although most filters are tested and
given an ASHRAE (American Society of Heating, Refrigeration
and Air-Conditioning Engineers) rating. Portable filters are sometimes given a Clean
Air Delivery Rate (CADR) by the Association of Home
Appliance Manufacturers (ARAM). This refers to the rate at
which air is moved through the filter, measured in cubic
feet per minute. In both ratings, the higher the number,
the better.

Under the right conditions, the better filters can
effectively remove many household pollutants. In addition,
air cleaners may also contain absorbents (charcoal) and/or
reactive materials to remove cigarette smoke and odors.
Ozone generators designed to eliminate odors and kill
bacteria are becoming increasingly popular, but, curiously,
ozone in sufficient quantities is itself a pollutant and
can cause shortness of breath and chest pain if allowed to
accumulate. It is also important to understand that air
cleaners cannot remove pollutants such as dust, dander and
pollen once they settle. Another important consideration is
that whole-house air cleaners usually only work when the
heater/air conditioner is on; therefore, you may not have
air cleaning when the temperature is mild.

Portable Filters Versus Whole House Filters

All filter materials come in either a portable or whole
house system — if you don’t have a forced-air heating
system, you are limited to the portable kind. Portables are
generally cheaper than whole-house units, costing $80 to
$500 depending on the size of the room you’re filtering.
They can be focused in high-risk areas such as the bedroom
or living room. Look for units that have a “high-efficiency
particulate air (HEPA) filter” or an “ultra-low penetration
air (ULPA) filter.” These filters work well for removing
dust and smoke. Brand names, which include Honeywell,
Panasonic, Holmes, Hunter and Kenmore, are available from a
large variety of retail stores.

Whole-house air cleaners, on the other hand, filter
all of the air in your house (as long as the
windows are closed) and they can vary in price from as
little as $1 to as much as a $1,000 with installation. A
good, pleated electrostatic filter that replaces the
ordinary fiberglass filters standard in most central
heating units can provide reasonably good filtration for a
very small price, about $10 to $15 (these filters must be
changed periodically). It’s important to note that washable
electrostatic filters may not work as well as replaceable
ones. For the most effective cleaning of indoor pollutants,
however, you will probably need to spend at least $400 for
an electronic precipitator unit. Brand names include
Honeywell, Research Products, 3M, Purolator, Precisionaire
and American Air Filter. While consumers can easily install
small filters, larger units generally require a heating
contractor, which may add significantly to the cost.

Some so-called “personal” or “micro” air
cleaners… may promise “sparkling clean air” or even an
“increase in your energy level.” These claims are unproven
and probably false.

Misleading Claims

Since the business is not well-regulated, you will run into
a lot of hype, including questionable products and claims.
Some so-called “personal” or “micro” air cleaners (small
individual cleaners that you wear or put on your desk), may
promise “sparkling clean air” or even an “increase in your
energy level” in exchange for $50 to $200. Don’t bet on it.
Simply put, machines with a tiny CADR will have a tinier
effect on the air you breathe.

That said, not even the massive units can provide complete
protection from airborne pollutants. Nevertheless,
electronic air cleaners, although initially expensive, are
probably the best bet over the long run. If you have
allergies during the spring or fall, you may want to
supplement a whole-house air cleaner with a portable in
your bedroom, since your heater or air conditioner will not
be running enough during these times. The bottom line? In
concert with source control and ventilation, air cleaners
can be an effective weapon in the fight against
allergies.

Allergies & Allergy Control

The US Environmental Protection Agency
The Air Conditioning & Refrigeration Institute
The American Academy of Allergy, Asthma & Immunology
The Association of Home Appliance Manufacturers
The American Society of Heating, Refrigeration and Air-Conditioning Engineers