How to choose air filters or air cleaners for effective improvement of indoor air quality
Table of types of particles in indoor air
How to use air filters or air cleaners to removing or keep out indoor contaminants
Best methods for cleaning & filtering indoor air
This article explains choosing and using different types of air filters or air cleaners to improve indoor air quality in homes. We include a table of the types of particles found in indoor air, particle sizes, and type of filter needed to remove them. We also include a quick guide to the common hazardous gases found in indoor air.
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We point out which filter types are effective for different indoor air particle or gas contaminants, and how to buy and use air cleaners. This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.
There are many types and sizes of air cleaners and filters
on the market, both stand-alone units and those integrated
with HVAC equipment. Different types of air cleaners work
on different types of pollutants and none handles everything.
The effectiveness of a device depends on a number
of factors including the type and efficiency of the filter,
how much air flows through it, how well the polluted air
reaches the filter, and how effectively the clean air is delivered
to occupied areas. (Some small units tend to draw
in the same air they just exhausted, creating a short circuit
with little impact on the larger space).
Also, with electronic
air cleaners, performance drops off rapidly if the filters are
not kept clean.
Another limiting factor is that many allergies are
linked to larger particles, such as pollen, house dust, animal
dander, and some molds, that are more likely found settled
on surfaces than suspended in the air. A high-efficiency vacuum
is needed for these, not an air cleaner.
Particles vs. Gases in Indoor Air
Some filters are effective with particles, such as dust and
pollen, and others are effective with gases, such as combustion
fumes and formaldehyde. Certain pollutants such
as tobacco smoke contain both gases and particles, so they
require two types of filters for effective removal.
Particles in Indoor Air - Particulates, Health Effects, & Air Filter Efficiency Chart
Sometimes called “particulates,” these are
small solid or liquid particles suspended in the air. They
can be captured in mechanical or electrostatic filter elements.
How many get captured depends on the size of
the particle along with the type, size, and efficiency of the
filter and the rate of airflow.
See the particle size and filter type efficiency chart at left.
Tiny respirable airborne particles, .01 to 5 microns in size, invisible to the naked eye, pose the greatest risk to health because they stay airborne for many hours, almost indefinitely when riding air currents, they move through a building much like a gas, passing through even very small openings where air leaks are present, and because they are breathed deeply into the lungs.
Larger inhalable airborne particles 10 microns and above, (more likely to be trapped in the nose) are more often irritants and allergens (such as pollen grains or insect fragments and fecal pellet fragments in dust).
HEPA filters provide the best filtering performance across all particle sizes. As a reference to size, the diameter of a typical human hair is 25 to 60 microns.
Best Practices Guide Source: reprinted in the original text cited above, with permission from Oikois.com (C) 1994 Iris Communications, Inc.
Respirable airborne particles. These are small, invisible particles,
typically ranging in size from 0.5 to 2.5 microns
(millionths of a meter) that can penetrate deep into the
lungs and cause acute or chronic illnesses. Examples
include asbestos, viruses, bacteria, and the particles in tobacco smoke.
Other sources include unvented
kerosene and gas space heaters, woodstoves,
fireplaces, poorly adjusted furnace flues, and cracked
heat exchangers. Health effects vary with the type of
particle, degree of exposure, and individual sensitivity,
and range from eye and respiratory irritation to
chronic diseases, such as cancer.
Inspirable airborne particles. Particles ranging in size from
about 2.5 to 10 microns include dust, pollen, animal
dander, and some mold spores. These can be inhaled,
but they generally do not penetrate deeply into the
lungs. They may cause allergic responses and other
health problems in some individuals.
Visible dust found in indoor air. Most particles over 10 microns get
trapped in the nose and upper airways and do not
generally cause health problems.
See CONTINUOUS BLOWER FAN OPERATION for a discussion of using the central air conditioning or heating blower and duct system combined with filters for reducing indoor dust and particle levels.
Quick Guide to Gases as Contaminants & Hazards in Indoor Air
Gaseous pollutants include combustion gases and
a huge array of organic chemicals that have been detected in homes. Gaseous organic compounds can originate indoors
from combustion appliances, cigarette smoking, cleaning and personal hygiene products, or hobby materials,
or can outgas from building materials, such as pressed wood products, paints, adhesives, and caulks. Others, such
as auto emissions and pesticides, originate out of doors and are drawn into the home with outside air.
Health effects vary with type of pollutant, level of exposure, and individual sensitivity, and range from eye
and respiratory irritation and allergic responses to cancer and other serious diseases affecting the respiratory, liver,
cardiovascular, and nervous systems.
Gaseous pollutants can be removed from the air by passing them through special adsorbents, such as activated
carbon, that adhere to the gas molecules.
Radon & Other Gas Hazards in Indoor Air
Radon is a radioactive gas that may enter a building from soil or groundwater. The gas breaks down
into short-lived particles, which can get trapped in the lungs and cause cancer. Although some adsorbents
can reduce radon gas levels and some high-efficiency filters can trap radon progeny, this has not been adequately
tested and is not currently endorsed by the EPA as a radon mitigation method.
For a detailed list of articles about various indoor gas hazards, gas testing equipment, and gas testing methods, see our GASES, EXPOSURE, TESTING article links below.
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
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