About Your House
Your Furnace Filter
What A Furnace Filter Can Do For You
Traditionally, furnace filters were designed to protect the
furnace and fans. With increased air quality awareness, some
filters are now being installed to reduce exposure to
particles which can affect your health.
There is a wide variety of furnace filters available.
However, you may find it difficult to select one that suits
your needs and know what to expect since there is no common
rating system.This purpose of this document is to provide you
with guidance when selecting your furnace filter.
What Airborne Particles Are Found In Your Home?
The particles you breathe in your home come from a variety
of sources including:
- dust on floors or other surfaces that is disturbed by
activity in the house;
- dust generated by smoking, burning candles, cooking,
doing laundry, etc.;
- hair and skin flakes from humans or pets;
- and particles from the outside air which come into your
home with infiltrating air.
Some particles are so small that they are inhaled and then
exhaled without being trapped in your lungs. Some larger
particles are trapped in your nose and throat and never reach
your lungs. Still other particles are too large to be
inhaled.The particles most dangerous to you are those that
enter your lungs and lodge there.
You can see the particles of dust which accumulate on your
television screen, shelves, and furniture. But you can't see
the respirable particles. Respirable particles can be easily
inhaled into your lungs and provoke respiratory illness.
Although you would probably like to keep visible dust out of
your home, the main health risk comes from respirable
particles, which include tobacco smoke, spores, bacteria, and
The activity levels of the people in your home can affect
the air you breathe. Activity such as vacuuming and cooking
can create or stir up particles. On the other hand, during
periods of inactivity such as the middle of the night,
particle concentrations tend to be much lower.
CMHC conducted a study to verify filter manufacturer claims
and to determine whether good filters will significantly
reduce your exposure to airborne particles. All results are
compiled and discussed in the research report: Evaluation of
Residential Furnace Filters (1999). You can obtain a copy of
this report by calling the Canadian Housing Information Centre
(CHIC) at 1 800 668-2642. A summary of the results of this
The CMHC study first tested ten filter types in a single
home and then the following filters in 5 additional homes:
- i) 25 mm (1") premium media filter
- ii) Charged media type electronic
- iii) 100 mm (4") pleated media filter
- iv) High efficiency bypass filters, such as a HEPA (high
efficiency particle arrestor)
- v) Electronic plate and wire (ESP)
Air in the houses was tested when these higher efficiency
filters were in use. The results were compared to when no
filter was used.
The electronic plate and wire filter (ESP) produces some
ozone during its operation. Exposure to elevated ozone can
irritate your lungs. Separate testing was done to verify
whether the amount of ozone produced by the ESP could affect
the occupants of the home.
Each filter was in use in each house only for one or two
days. The effects of dust accumulation on filter performance
could not be evaluated in these tests. If a filter actually
cleaned dust out of a house by cleaning house air, these tests
were too brief for such effects to be seen.
The research showed that exposure of the house occupants to
airborne particles appears to be directly linked to their
activities when they are in the home. The furnace filter
appears to have only a moderate effect on the exposure of an
individual to respirable particles in the home.
Consider each member in your home to be followed by a cloud
of dustólike "Pig Pen" in the "Peanuts" comic strip by Charles
Schulz. When occupants are moving around, they stir up the
dust.The dust in this cloud is usually not affected by the
quality of the furnace filter because the filter is far away
down a duct.
The table below shows the percentage of improvement
provided by each filter versus having no filter. The
improvements are greater when there is no activity in the
home, but particle levels were quite low in the test houses
during these periods whether or not the air was being
The Cost Of Clean Air
For a furnace fan filter to be effective, your furnace fan
would have to run almost all the time. Unless you already have
your furnace fan operating all the time, this additional fan
use can add up to $200 per year to your electric bill. The
following table shows the cost, including maintenance, of each
filter over a period of 15 years compared to the cost per unit
of clean air they provided.
||Maintenance and capital costs, per
year, over 15 years ($)
||Amount of clean air produced
||Cost of clean |
air per year
|25 mm pleated
|25 mm premium
|100 mm pleated
The following table shows the cost, including maintenance,
of each filter over a period of 15 years compared to the cost
per unit of clean air they provided. The table shows that
filters which cost the least produced very little clean air.
The 25 mm pleated filter actually had the greatest cost per
unit of clean air. The ESP filter was the most cost effective
because it produced the most amount of clean air, and cost
very little to do so.
||% improvements during
active periods in
|% improvements during
non-active periods in |
|25 mm premium
|100 mm pleated
What About Ozone?
Despite being the most effective filter in the tests, the
ESP produces small amounts of ozone during operation. In the
research project, a survey of fifteen homes with ESP filters
showed that all ESPs created ozone in the air stream of the
duct. None of these raised ozone levels in the house air above
the safe concentrations recommended by health guidelines.
During the test period, ozone levels were always higher in the
outside air than in house air, despite the ozone production by
the ESP filters.
This research showed that the particles in the duct air can
be reduced when an upgraded filter is installed.The results
also showed that this reduction will only moderately reduce
indoor exposure to respirable particles.
So... How Do You Reduce Levels Of Respirable
Our best current guess is to reduce dust entry by:
- removing footwear on entry;
- keeping major dust generators (smoking, pets, etc.) out
of the house;
- reducing dust collecting surfaces (open shelves,
carpets, upholstered furniture, etc.);
- diligent and frequent vacuuming with an efficient vacuum
- reducing the entry of particle-laden outdoor air by
closing windows, improving house airtightness, and
installing an intake filter on the air supply;
- using a good furnace filter.
Most of these recommendations will also reduce the amount
of visible dust in your house.
Canada Mortgage and Housing Corporation offers a wide range
of housing-related information. For details, contact your local
CMHC office or call 1 800 668-2642.