This section addresses strategies for preventing indoor air contamination and the effectiveness and appropriate uses of air cleaning devices. Devices addressed include portable, single room air cleaners and whole-house air filters, such as those typically located in the return air duct of a central HVAC system. Various standard air filter test methods are described.

Also see the Green Building Factsheets for introductions to this and other green building topics.

The EPA divides air pollutants into three categories: particulate matter, gaseous pollutants, and radon and its progeny. Particulate matter includes any substance that is light enough to be suspended in the air. Gaseous pollutants include combustion gases, vapors emitted by household cleaning products, personal hygiene products, carpets, paints and sealants, and various newly-manufactured goods ranging from toys (plastics) to magazines (ink) to furniture (pressed wood, adhesives, synthetic materials), etc. Radon is a gas that is emitted from traces of uranium in natural sources such as rock, soil, groundwater, natural gas, and mineral building materials. Radon progeny are the various isotopes of elements formed as radon radioactively decays.

Filters, Fig. 1

EPA's radon map of Texas; areas in orange have a higher radon concentration than those in yellow.

Of concern from a health standpoint are small, invisible respirable-size particulate matter, having a high probability of penetrating deep into the lungs, where they may stay a long time and cause acute or chronic effects. Large particulate matter such as some molds, pollen, animal dander, and house dust allergens, do not penetrate as deeply, but may cause an allergic response. Respirable-size particles include, but are not limited to, those from cigarette smoke; unvented combustion appliances such as gas stoves and kerosene heaters; viruses, bacteria, and some molds; and fragments of materials which, when whole, would be considered larger than respirable size particles. Health effects from exposure to respirable size particles in the air depend on the types and concentrations of particles present, the frequency and duration of exposure, and individual sensitivity. Health effects can range from irritation of the eyes and/or respiratory tissues to more serious effects, such as cancer and decreased lung function. Biological particles, such as animal and insect allergens, viruses, bacteria, and molds, can cause allergic reactions, infectious diseases, and/or can produce toxic products that may be released into the air.

Health effects from exposure to gaseous pollutants in the air may vary widely depending on the types and concentrations of the chemicals present, frequency and duration of exposure, and individual sensitivity. Adverse effects may include irritation of the eyes and/or respiratory tissues; allergic reactions; effects on the respiratory, liver, immune, cardiovascular, reproductive, and/or nervous system; and cancer.   Radon progeny may deposit in the lungs and pose a health risk, including lung cancer. Only smoking causes more lung cancer deaths than radon progeny, and smoking in high radon areas multiplies the risk of cancer from either cause alone. The EPA recommends testing all homes below the 3 rd floor level for radon.

The most effective way to maintain good indoor air quality is to remove indoor sources of pollution. Store household cleaning products, gas tanks, automotive lubricants and fluids, glues, paints, and any other solvents outside in a storage shed, and make sure that they are well sealed in their containers. Do not allow smoking indoors.   Maintain clean bathroom and kitchen surfaces in order to prevent the growth of mold and bacteria. Keep pets outdoors whenever weather permits. Lastly, perform tasks that involve scraping, grinding, and sanding outside when possible.

Ventilation is another key to maintaining good air quality. Introduce outdoor air into the indoor environment. Open doors and windows when the weather allows, and/or use mechanical ventilation devices. Be aware that the outdoor air in your area may be more polluted than your indoor air. Take notice of any possible local sources of contaminants that may affect the air quality in your immediate area.

Air filters are a secondary form of maintaining good indoor air quality. Three questions to ask about whole-house air filters are:

1. To what extent does the filter remove contaminants from the air supply?
2. To what extent does the filter restrict the air supply's flow?
3. What is the long-term cost of the filter?

There are many filter comparison tests, as summarized below. The closest thing to an "apples to apples" test currently available for most panel air filters is the ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) Standard 52.2 MERV (Minimum Efficiency Reporting Value). Under this test, a filter can achieve a score between 1 and 16, 16 being the most effective at removing contaminants. 2

While cleaning the air, filters also restrict the airflow. When upgrading to a more efficient whole house filter, modifications to the air handler and/or ductwork may be necessary in order to overcome this added restriction. Most HVAC systems in central Texas already have an overloaded air handler blower motor due to poor design. Modifications to the blower motor and/or ductwork may be necessary before upgrading to a more restrictive air filter.

When comparing air filters for prices, consider the cost of using a particular filter over a one to two year period. Some filters must be replaced monthly, some every few months and others may last several years. Calculate the cost for the number of filters that would be needed over an appropriately chosen amount of time.

Air filters only clean the air that flows through them. Most air filters do not have a gasket to seal the air filter frame to the air return duct housing. Sometimes air is allowed to bypass the filter. This "blow-by" can significantly reduce the effectiveness of a filter. If space permits, install thin weather stripping around the return duct housing to create a gasket at the duct housing/filter frame interface. 3

Room air purifiers (air cleaners) are comprised of a box containing a fan and a filter that uses one or more of the same technologies as whole-house air filters. These air purifiers can be effective for a single room. They are not recommended for whole houses or apartments, especially in those that use a central HVAC system. Location of an air-cleaning unit can greatly impact its effectiveness. The unit should be placed close to any known source of air pollution. Otherwise, the best place for a portable air cleaner is near the indoor occupants. Any object which blocks the path of air emitted by an air purifier may cause the unit to continuously cycle the same air through it, rather than clean all the air in a room. A common method used to compare portable air-cleaning devices is the Clean Air Delivery Rate (CADR), which is obtained by multiplying a unit's filtering efficiency by its air flow rate, and is given with respect to filtering out a specific pollutant. The CADR essentially gives an air purifier with a fair cleaning efficiency, but which works at a very fast rate, an equivalent score to one that is extremely efficient at cleaning, but works very slowly. Some air purifiers may be made of materials which off-gas, and may themselves add to the gaseous indoor air pollution.

Filters, Fig. 2

Air cleaner

High voltage electronic filtering devices such as ionizers and precipitators may produce ozone (O 3 ) in small amounts. Ozone generators may be sold as air cleaners, but are not approved by the federal government for use in occupied areas. These generators are ineffective at removing indoor contaminants while maintaining safe levels of ozone, which is both hazardous to human health and destructive to many natural materials. Relatively low amounts of ozone can cause chest pain, coughing, shortness of breath, and throat irritation. Ozone may also worsen chronic respiratory diseases such as asthma and compromise the ability of the body to fight respiratory infections. People vary widely in their susceptibility to ozone. Electronic precipitators and ionizers should not produce dangerous levels of ozone when properly used. Devices that ionize using a positive current produce less ozone than models that use a negative current.

Activated carbon is sometimes used as a chemical reaction filter, adsorbing some high molecular weight gaseous pollutants. However, the carbon may quickly reach the same pollution concentration level as that of the air stream. Once this occurs, the activated carbon will no longer adsorb pollution, but will emit the pollutant whenever the level in the air stream fluctuates below that of the carbon. In order to be effective, activated carbon must be replaced or regenerated frequently.

Panel Filters

These filters are usually of one-inch nominal thickness and work as a direct replacement in most circumstances. Simple filters are standard filters made of one-inch thick, loose packed, spun or matted fiberglass (or other synthetic material) surrounded by a paperboard frame. This type of filter performs very well with regards to not restricting airflow. However, these filters are ineffective in removing all but the largest airborne particles. These filters contribute very little to air quality improvements and protection of the HVAC system. Cost ranges from $1-$5 per unit.

Electrostatic washable filters perform similarly to simple fiberglass filters. They are reusable, reducing landfill. Cost ranges from $15-$60 per unit.

Pleated media filters of one-inch thickness are generally decent with respect to restriction of airflow and removal of particulate matter. The electrostatic charged models perform somewhat better with regards to removing smoke and smaller particulate matter. Cost range $5-$80 per unit.

Filter, Fig. 3

Typical pleated media panel filter

Box Type and Other Large-Size Filters

The following filtering devices are much larger than panel filters and usually require professional installation. They may benefit from the use of a pre-filter, which is essentially a lower efficiency filter intended to catch large particles before reaching a high efficiency filter. Such a strategy will prolong the life of the higher efficiency filter. Extended surface filters are either folded accordion-style, multiple bag, or v-cell filters. These filters vary in performance from as good as to significantly better than pleated media filters in terms of both air cleaning and flow restriction. Extended surface filters usually last three to five times as long as panel filters before replacement is necessary. Electronic Precipitation filters come in two variations, positive and negative ion generators. Both types of precipitators ionize particulate matter in the air stream, and then collect the ionized particles on grounded plates. These filters require periodic cleaning, with the duration between cleaning dependent upon the inlet air quality. Negative Ionizers have no collector plates. Each negative ion particulate is emitted, and then sticks to the first grounded object that the particulate encounters (possibly your lungs). This type of air cleaning device may need a pre-filter in order to effectively remove large particulate matter. Indoor surfaces of walls, drapes, furniture, etc. may develop a residue of particulate matter when using a negative ionizer.

HEPA and ULPA (High-Efficiency Particulate Air and Ultra Low Penetration Air) filters are often used in hospitals and manufacturing or laboratory clean rooms. HEPA filters achieve 98-100% efficiency results on the DOP penetration and atmospheric dust-spot efficiency tests.

ASHRAE 52.1 TEST METHODS

Arrestance Test

The object of this test is to show how much (by weight) of a prescribed synthetic dust blend a given filter prevents from passing though itself. This test is only good for judging a filter's ability to catch large particulate matter. Atmospheric Dust-Spot Efficiency Test In this test, atmospheric air is run through the filter being tested and then a micro-fine white filter paper. Any particulate matter that passes through the filter being tested, in theory, will be caught on the micro-fine filter paper. The test measures the difference in light transmittance through the white filter paper both before and after the air is circulated. Any particulate matter caught in the micro-filter paper will help block light transmittance. This test is good for judging a filter's effectiveness in trapping small particulate matter.

Dust-Holding Capacity Test

A prescribed synthetic dust blend is administered to the air-stream until the air flow resistance of the test filter reaches the manufacturer's recommended maximum, or for a prescribed arrestance value for filters that do not reach their maximum resistance in a reasonable amount of time. The dust-holding capacity for a filter is then defined as the total weight of dust retained by the filter. This test measures the amount of filtration a given filter will perform before replacement or cleaning of the filter is necessary.

ASHRAE 52.2 TEST METHODS

Particle Size Removal Efficiency Test

This test is used to calculate the Minimum Efficiency Reporting Value (MERV) - a filter's effectiveness at removing several tested sizes of particulate matter with respect to the filter's restriction to air flow for each given size group of particulate matter. DOP Penetration Test

This test, prescribed in US Military Standard MIL-STD-282 (1956) and US Army document 136-300-175A (1965), measures the percentage of dioctyl phthalate (DOP) that penetrates past a test filter. HEPA and ULPA filters are usually measured using this test, which is considered a good test for particulate matter of 0.3-micron diameter.

2000 ASHRAE Handbook: HVAC Systems and Equipment Inch-Pound Edition, Chapter 24, American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, GA

Breathe Easy , Consumer Reports pg. 42-46, January 2000

Canadian Coalition for Nuclear Responsibility website:www.ccnr.org

OZONE GENERATORS THAT ARE SOLD AS AIR CLEANERS: An Assessment of Effectiveness and Health Consequences: www.epa.gov

Residential Air Cleaning Devices- A Summary of Available Information: Office of Air and Radiation (OAR), United States Environmental Protection Agency, February 1990

US EPA IAQ-Radon website www.epa.gov/iaq/radon