Air cleaner installed in central air conditioning duct system (C) Daniel FriedmanIndoor Air Filtering Strategies

  • AIR FILTERING STRATEGIES - CONTENTS: Cleaning & Filtering Strategies to Improve Indoor Air Quality. Particles vs. Gases in Indoor Air vs. types of air filtration & air cleaning. Particles in Indoor Air - Particulates, Health Effects, & Air Filter Efficiency Chart. Guide to Gases as Contaminants & Hazards in Indoor Air
  • POST a QUESTION or READ FAQs about how best to filter indoor air to improve indoor air quality
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Indoor air filtering & cleaning methods: 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. We point out which filter types are effective for different indoor air particle or gas contaminants, and how to buy and use air cleaners.

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Air Cleaning & Filtering Strategies to Improve Indoor Air Quality

Room air conditioner filter cleanout (C) Daniel FriedmanOur photo (left) illustrates a simple pull-out washable foam air conditioner filter used on a portable room air conditioning unit. As noted in Best Practices Guide to Residential Construction:

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

Table of particle size vs air filter effectiveness (C) J Wiley, Steven  Bliss

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 (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.

Filter cascade on gas furnace (C) D Friedman

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.

Also see A Guide to Reducing Exposure to Formaldehyde Hazards in Indoor Air

see Radon Hazards in buildings: health effects, measuring, remediation guide for details.

see Carpeting and Indoor Air Quality, Health Effects for carpet, carpet padding, adhesive outgassing.

VOCs as indoor gas contaminants are discussed at Guide to Sources & Remedies for Volatile Organic Compounds VOCs as Indoor Air Contaminants.

-- Adapted with permission from Best Practices Guide to Residential Construction.


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