Frequently Asked Questions

Frequently Asked Questions (FAQ)


How do I select a portable air cleaner for my home?

In the selection of a portable air cleaner, sometimes called an air purifier, you should consider 1) the air cleaner’s clean air delivery rate (CADR) for particles; 2) whether the air cleaner removes gaseous pollutants as well as particles; 3) the level of noise produced by the air cleaner; 4) electricity cost, initial cost, filter replacement cost; 5) whether the air cleaner produces ozone or other undesirable pollutants.

The CADR is a measure of the rate at which the air cleaner removes particles, usually expressed in cubic feet per minute of (CFM). The Association of Home Appliance Manufacturers (AHAM) recommends different minimum CADRs depending on the size of the room.

Some air cleaners only remove particles from the air, other air cleaners remove both particles and gaseous air pollutants (some of these gases are odorous), and some air cleaners are designed to only remove gaseous pollutants. In most buildings, particles pose a greater health risks than gaseous pollutants; thus, in general, particle removal should be the primary consideration. Many people purchase air cleaners to remove allergens and these allergens are particles. However, in some situations, for example when odors are objectionable and odor sources cannot be removed, removal of gaseous pollutants by an air cleaner will be desirable. Air cleaners with media to remove gaseous pollutants will generally be more expensive to purchase, and the cost of periodically replacing their filtration media will be higher than the cost of just replacing particle filters. The effectiveness of portable air cleaners in removing gaseous pollutants, for example, how long they remain effective, is not well understood. Effectiveness will vary among air cleaners and independent data on effectiveness for gaseous pollutants are not available for most air cleaners. Some air cleaners marketed for gaseous pollutant removal, both with and without particle removal filters, cost hundreds of dollars and remove gaseous pollutants at a rate too low to have any benefit. Some similarly expensive particle-removal air cleaners remove particles at too low a rate to provide any significant benefits.

The fans and moving air of air cleaners create noise causing people to sometimes turn off air cleaners because they are too noisy. Technical literature for some air cleaners includes a noise rating, usually in A-weighted decibels or dBA. Lower dBA values correspond to a quieter air cleaner. The noise produced by an air cleaner will increase with fan speed and air flow rate. One of the quieter portable air cleaners with good particle removal has noise rating ranging from about 21 dBA at the lowest fan speed to 46 dBA at the highest fan speed. The noise level considered acceptable varies among people. Also, when background noise is low, such as at night in a quiet bedroom, air cleaner noise will be more noticeable. It would be best to listen to an air cleaner operating in a quiet space before making a purchase, but this step may not be practical.

There is a large variability in electricity use among air cleaners, even for devices with similar CADRs. Electricity use will increase with fan speed or air flow rate. The cost of electricity used by air cleaners can be significant. For example, continuous operation of a 200 Watt air cleaner for a full year will cost $200 at a typical residential electricity price of 11.3 cents per kilowatt hour. By choosing an energy efficient air cleaner, long term operation costs can be reduced. In addition to energy cost and first cost, the cost of replacing filtration media and the required frequency of replacement should be considered when contemplating a purchase.

Some air cleaners intentionally release ozone because ozone can react with and break down some gaseous air pollutants, often these devices are called ozone generators. The amount of destruction of gaseous pollutants by ozone released to indoor air is generally insignificant and the ozone produced poses respiratory health risks. Some air cleaners can produce ozone, sometimes only a small amount, because of their design; however, these units do not intentionally release ozone to the indoor air for purposes of air cleaning. California certifies air cleaners that produce only a small amount of ozone, see the following web site. Some air cleaners do not produce significant ozone; however, they incompletely break down gaseous pollutants yielding new air pollutants that may pose greater health risks than the original pollutants. Some ultraviolet photocatalytic air cleaners and plasma air cleaners have this problem; however, available data are very limited.

For more information, see:

a) the section of this web site on air cleaning

b) the U.S. Environmental Protection Agency site

c) a position document on air cleaning from the American Society of Heating, Refrigerating, and Air Conditioning Engineers

How can I obtain copies of the papers cited on your web site?

We have made a few key papers available for download from the web site via the downloadable papers links for each topic. Due to copyright restrictions, we cannot provide copies of papers written by others. However, many papers are available to the public, free of charge, via the web. You can use a web browser to search, using the paper’s title and/or authors names as search terms. Often, the search will get you to a web site where the paper can be viewed or downloaded for free, or purchased. Google Scholar is publically available and was specifically developed to facilitate the finding of published scholarly documents. Another publically available online tool focusing on health-related papers is Pub Med. You can also contact an author and request a copy of the paper. Most papers list a corresponding author and their email address and often this information, plus an abstract, can be found at the journal’s web site, even when the full paper is not available at no charge.

How do I select a low volatile organic compound (VOC) paint?

Many manufacturers offer “low VOC” or “no VOC” interior paints and finishes. A document from the U.S. Environmental Protection Agency identifies several organizations that certify or otherwise identify low VOC interior paints and finishes.

How can I find a low volatile organic compound (VOC) carpet?

You should consider emissions of VOCs not only the carpet, but also the underlying carpet pad or adhesive. A document from the U.S. Environmental Protection Agency provides guidance and links to the Carpet and Rug Institute’s Green Label program for carpets, carpet cushions, and adhesives.

Can you recommend an IAQ consultant or IAQ testing company?

We cannot recommend specific indoor air quality (IAQ) consultants or testing companies. You can find some guidance for selecting companies providing IAQ services at web sites listed below.

Guidance from American Industrial Hygiene Association

Guidance from Minnesota Department of Health

IAQ services are expensive, particularly for home owners, and we recommend that you first become informed about IAQ issues and possible remedial measures. This IAQScience website provides much useful information to help you learn about IAQ problems and solutions. Web based information from the US Environmental Protection Agency is another excellent source. Many states also have indoor air quality information on the web sites of their Health Departments or Environmental Agencies. A website of the California Air Resources Board provides much helpful information.

What type of furnace filter should I use in my home?

You should first determine how thick of a filter your forced air heating or heating and air conditioning system can accommodate. Many residential systems have space for only a one inch thick filter. The see-through flat panel filters often used in these locations are not recommended because they remove very few of the small particles that pose health risks.

Various rating systems are used to indicate the particle removal efficiency of filters sold for use in residential heating and cooling systems. In the U.S., these filters may have a MERV, MPR, or FPR rating. We recommend use of a filter with a particle removal efficiency rating of MERV 11, MPR 1000, or FPR 7 or higher; however; even a MERV 8 rated filter which is roughly equivalent to a MPR 600 or FPR 5 filter is substantially better than the common see-through flat panel filter. The use of more efficient filters is more important when indoor particle levels are high because of high outdoor air particle levels or strong indoor particle sources, and indoor particle sources should be reduced when possible. Also, high efficiency filters are more important when the occupants are highly susceptible to adverse health effects from particles, for example, when occupants have respiratory and cardiac disease.

Normally, the filters with higher particle removal efficiency ratings are pleated, i.e., they have a folded or accordion-like filter media, to maintain a low resistance to airflow. Sometimes, a higher efficiency filter will have too much resistance to airflow for the forced air heating and cooling system of a home. You might want to engage a heating and air conditioning contractor who can determine what type of filter you can use in your home. Filters that are more highly pleated will often have a lower airflow resistance than a less pleated filter with the same efficiency rating. Some heating and cooling systems can accommodate a pleated filter that is more than one inch thick, for example a two inch thick pleated filter. A thicker filter provides more space for pleated filter media to maintain a low airflow resistance. A heating and air conditioning contractor can usually modify your forced air heating and cooling system so that it can accommodate thicker filters. The airflow resistance of a filter increases over time during use, so frequent changing of filters will help to avoid periods of high airflow resistance.

When installed in your forced air heating and cooling system, there should be minimal gaps between the filter and the filter’s housing. Gaps allow unfiltered air to flow around the filter, sometimes substantially reducing the overall particle removal by the filter system.

The filter in a residential forced air heating and cooling system only removes particles when there is airflow through the system. When providing heating or cooling, many systems run only 10% to 20% of the time. In mild climates, operation times can be even less. Also, in homes without air conditioning, months can pass without system operation. Often a heating and air conditioning contractor can add controls so that the fan of your forced air system heating and cooling system can be programmed to operate a portion of each hour, or even continuously, regardless of the need for heat or air conditioning. However, use of portable air cleaners will, in some cases, be a lower cost option.

For more information, see the section of this web site on air cleaning.

What are some common asthma triggers?

Asthma triggers are exposures that cause or worsen asthma symptoms. Indoor asthma triggers include allergens, airborne non-allergenic particles, and various gaseous air pollutants. Tobacco smoke, which contains particles and many gaseous pollutants is an important asthma trigger. Allergens from house dust mites, pets, molds, cockroaches, rodents, and plants such as grasses, weeds, and trees can trigger asthma, these allergens are present in particles too small to see. Particles as well as gaseous pollutants, such as ozone, from outdoor air can trigger asthma. Various chemicals found in workplace air can also trigger asthma. Air pollutants are not the only triggers for asthma. Other important asthma triggers include exercise, cold air, stress, and respiratory infections. Sources of additional information include:

a web site from the U.S. Environmental Protection Agency;

a web site from the Centers for Disease Control and Prevention;

the section of this web site on dampness and mold;

the section of this web site on volatile organic compounds.

What is formaldehyde and is it a problem?

Formaldehyde is an organic air pollutant containing one atom of carbon, one atom of oxygen and two atoms of hydrogen. It has the chemical formula HCHO or CH2O. Formaldehyde is present in outdoor air but indoor air formaldehyde concentrations are usually much higher because of indoor formaldehyde sources. Manufactured wood products that use formaldehyde-containing resins to glue together pieces or particles of wood are often the dominant indoor source. These manufactured wood products include particle board, plywood, oriented strand board, fiberboard and similar items often used in walls, floors, furniture, cabinets, doors, and modern manufactured-wood beams. There are many other possible indoor formaldehyde sources including but not limited to tobacco smoking, cooking, and some paints, fabrics, and insulation materials.

Formaldehyde can be an irritant of skin, eyes, nose, and throat and some authorities consider formaldehyde to increase the risks of certain types of cancer in people. Also, there is limited evidence that higher levels of formaldehyde, or increased sources of formaldehyde, increase the risk of allergies and asthma. However, there is a debate about the levels of formaldehyde necessary to cause these health effects.

For more information see:

a) the section of this web site on volatile organic compounds 

b) the U.S. Environmental Protection Agency site

c) a web site of the California Air Resources Board

d) a document from the International Agency for Research on Cancer

What is a volatile organic compound (VOC)?

Indoor volatile organic compounds, or VOCs, are carbon-containing organic chemicals present in indoor air. They come from a large number of indoor sources including building materials, furnishings, consumer products, tobacco smoking, cooking, people and their activities, and indoor chemical reactions. VOCs from attached spaces such as garages may also enter indoor living spaces. Outdoor air is also a source of indoor VOCs. Indoor air typically contains many VOCs, but most are present at low concentrations. Formaldehyde is one of the most common indoor VOCs. VOCs may be odorous and some VOCs are known or suspected to cause a variety of adverse health effects. For more information on VOCs, see the related section of this web site.

Does mold really pose a health risk?

Mold is always present in the air and on indoor surfaces, thus, total avoidance of mold is impossible and may not be desirable. However, visual evidence or history of dampness in buildings and substantial visible mold or mold odor in buildings have been consistently associated with increases in symptoms of asthma and other respiratory health effects. There is also substantial evidence that dampness and visible mold are associated with increases in acute bronchitis and respiratory infections. Because of increases in other contaminants, such as bacteria and some chemicals, when buildings are damp the available research does not prove that mold is the cause of the observed adverse health effects. However, mold is a possible and biologically plausible direct cause of the adverse health effects and avoidance of the dampness problems that lead to mold and other pollutants is clearly advisable. For more information see the section of this web site on dampness and mold. 

What are the main ways to control dampness in a home?

The first priority is to prevent or fix leaks of liquid water into the building or building envelope. Water entry through the above-grade roof and walls and water entry through surfaces in contact with the soil should be prevented. For above-grade leaks, normal maintenance measures, such as replacing or repairing the roof will often be all that is needed; however, professional assistance may be needed if there were errors in design or construction of the home. Below grade water entry is addressed by directing downspouts away from the foundation, grading of soil to slope away from the building (when possible), and having functional below-grade foundation water draining systems. In some situations a sump and sump pump may be required. Water leaks also occur from plumbing, both water supply and sewage plumbing, and these leaks need to be immediately fixed.

Cooking and bathing are key indoor sources of water vapor. Bathroom exhaust fans and range hoods that vent to outdoors should be used to vent much of the generated water vapor to outdoors. If exhaust fans are not available, window opening may help, except when it is hot and humid outdoors. Clothes dryers must also be vented to outdoors.

When it is hot and humid outdoors, the outdoor air that enters a home is an important source of water vapor or indoor humidity. Operation of air conditioning systems or dehumidifiers may be necessary to control indoor humidity. An air conditioning system condenses water vapor when it cools the air and the liquid condensate is drained to outdoors. High rates of outdoor air ventilation will make it more challenging to control indoor humidity when it is hot and humid outdoors.

When it is cold and dry outdoors, the outdoor air will have less water vapor than the indoor air. Indoor humidity can usually be maintained at an acceptable level by providing recommended amounts of outdoor air ventilation, preventing water leaks, and using bathroom fans and range hoods that vent to outdoors.

Even when the indoor air humidity is not elevated, when it is cold outdoors water may condense on the inside surfaces of windows, particularly energy inefficient windows, such as single pane windows. Water may also condense on walls or inside walls if there is insufficient thermal insulation or if the walls have been poorly designed or improperly constructed. The best solution is to fix the underlying building envelope deficiency, for example, by adding insulation or installing energy-efficient windows. Operation of bathroom and kitchen fans will help and dehumidifier use will help in some situations.

For more information see the references and links at the following location.

How can I determine the ventilation rate in my building?

If your building is mechanically ventilated using a fan to supply outdoor air through a duct system, instruments can be used to measure the rate of mechanical outdoor air supply, although sometimes the measurement uncertainty will be high. In general, only suitable contractors have the necessary instruments and skill in instrument use. The measured outdoor air supply rate can be divided by the indoor volume, floor area, or number of occupants to determine different types of ventilation rates. Your building’s total ventilation rate may be higher than the mechanical ventilation rate due to air entry via leakage through the building envelope. Measurements of indoor air carbon dioxide concentrations are frequently used to provide a rough indication of ventilation rate per person, although there are several common sources of error. A maximum indoor carbon dioxide concentration of 1000 parts per million is often used as an indicator of adequate ventilation but this employs a set of assumptions that are not true in every situation. Suitable carbon dioxide instruments cost a few hundred dollars. For homes, contractors sometimes employ a fan system, called a blower door, installed in a doorway together with pressure sensors to measure the effective total size of leaks in the home’s envelope. A mathematical model and weather data are then used to estimate the ventilation rate. Researchers sometimes inject a tracer gas into the indoor air and measure its concentration decay over time to determine the ventilation rate; however, this measurement method is rarely used except for research. Various technical papers provide more information on measurement of ventilation rates, for example this article. A standard test method for use of blower doors is available for purchase.

How can I determine if my house is too humid?

You can purchase a humidity sensor and measure the humidity at various locations in your home. A relative humidity consistently above approximately 70% indicates an increased risk of indoor mold growth. The U.S. EPA recommends maintaining the indoor humidity below 60% to decrease mold growth. A relative humidity above 50%, although common, increases the survival of house dust mites which produce allergens to which many people are sensitized. You should measure the humidity in multiple indoor locations and at multiple times, as humidity can vary with location, as weather changes, and as the indoor temperature changes.

While harder to measure, the humidity at surfaces, such as walls and floors, is more important than the humidity of room air. The humidity at the surface of a cold wall or cold floor can be high enough to promote mold growth and dust mite survival, even when the humidity of the air in the center of the room is moderate. If you can see significant mold growth or moisture condensation in your home, you have a dampness problem that should be addressed. If the air smells moldy, you may have a dampness problem. High indoor humidity could be the cause, or contribute to the mold growth. However, leakage of water from outdoors, leakage of water from plumbing, transport of water from soil through below-grade walls and floors, and other conditions can cause damp building materials and mold growth even when the indoor humidity is low. The building envelope design and construction is important. For example, without suitable vapor barriers, warm humid outdoor air entering through walls can the contact cooled wall surfaces of an air conditioned home leading to high humidity and mold growth inside the walls. Thus, measurement of consistently high indoor humidity values can indicate problems, but the absence of high indoor humidity values does not assure the absence of dampness and mold problems. For more information, see the section of this website on dampness and mold and the ASHRAE Position Document on Limiting Indoor Mold and Dampness in Buildings.

Should I use an ozone generator?

Here is a statement about the health effects of ozone from the U.S. Environmental Protection Agency:

“Breathing ozone can trigger a variety of health problems including chest pain, coughing, throat irritation, and congestion. It can worsen bronchitis, emphysema, and asthma. Ground level ozone also can reduce lung function and inflame the linings of the lungs. Repeated exposure may permanently scar lung tissue.”

Ozone generators increase indoor air ozone levels, sometimes to well above the regulated levels in outdoor air. Ozone generators are also relatively ineffective in destroying airborne pollutants and can increase indoor air concentrations of very small particles that penetrate deep into the lung. Given the clear evidence of adverse effects of ozone, ozone generators should not be used in occupied spaces. 

For more information, see:

a) a related section of this web site

b) a position paper on air cleaning from the American Society of Heating, Refrigerating, and Air Conditioning Engineers

c) the following web site from the California Environmental Protection Agency

What are sick building syndrome symptoms?

Sick Building Syndrome (SBS) symptoms are acute symptoms, such as irritation of eyes, nose, and throat, headache, fatigue, cough, and tight chest, that occur at work and improve when away from work. These symptoms can have multiple causes, thus, they do not indicate a specific type of disease or a specific type of pollutant exposure. SBS symptoms have been widely reported by occupants of offices and schools, and in a few studies by occupants of homes. Some occupants in every office building will report some SBS symptoms, but indoor environmental factors that are known or suspected to lead to increased SBS symptoms include a lower ventilation rate (throughout the normal ventilation rate range encountered in buildings), strong indoor pollutant sources, air conditioning, and higher indoor temperatures. The fraction of occupants experiencing SBS symptoms is often called the symptom prevalence or symptom prevalence rate. Some SBS symptoms may be allergy symptoms; however, SBS symptoms can occur in people without allergies. For more information see:

the section of this web site on building ventilation;

the section of this web site on volatile organic compounds.

How much ventilation is needed in my building?

This question is not easily answered because the desirable amount of ventilation (outdoor air supply) varies with the situation. Research has found that occupants of commercial buildings with higher ventilation rates generally perceive the indoor air quality to be better and have fewer sick building syndrome symptoms. With increased ventilation, aspects of human performance generally increase by a small amount. In schools, there is evidence that higher ventilation rates reduce absence rates and increase aspects of performance. Much less research is available on the health consequences of ventilation rates in homes. There are limited data suggesting improvements in allergy and asthma health outcomes with increases in home ventilation. However, increase in ventilation rates generally increase building energy consumption and energy costs, and increases in energy consumption lead to emissions of carbon dioxide that contribute to climate change. In addition, increased ventilation rates can increase indoor air concentrations of some pollutants from outdoor air, such as ozone and particles, and these pollutants pose risks to health. Thus, the benefits of higher ventilation rates need to be balanced by the costs of ventilation and risks of higher indoor levels of outdoor air pollutants. Many factors may influence the balance. For example, when a building has strong indoor sources of pollutants, more ventilation is desirable, and when the outdoor air is highly polluted less ventilation is desirable. If the incoming outdoor air is well filtered to remove particles, the risks of increased ventilation from indoor air concentrations of particles from outdoor air will be greatly diminished. If the climate is severe, ventilation will increase energy use and energy costs by a larger amount. Considering many of these factors, the American Society of Heating, Refrigerating, and Air Conditioning Engineers has published standards for minimum ventilation rates that vary with building type. These standards are available for purchase at this site. It is notable; however, that research applicable to offices indicates benefits of increasing ventilation rates to well above the minimum rates recommended by ASHRAE.

For more information see the section of this web site on building ventilation.

I purchased an indoor air quality (IAQ) sensor and measured the concentration of volatile organic compounds (VOCs) in my home. The concentration was 250 micrograms per cubic meter. Is this a safe VOC level?

The indoor air contains many different VOCs and the concentrations vary among the VOCs. Also, the types of VOCs present indoors and the relative abundance of VOCs vary among buildings. Some VOCs are basically benign at the concentrations found in buildings. Other VOCs may, at concentrations found in some buildings, be a source of irritation or pose a significant cancer risk. Some VOCs have a very low odor threshold, thus, they are smelled even when indoor concentrations are low. Other VOCs have high odor thresholds and, at the concentrations encountered in normal buildings, are not detectable by smell. The VOC sensors marketed for general consumers respond to a subset of all of the VOCs present in indoor air and report a total volatile organic compound (TVOC) concentration. These sensors do not measure the concentration of individual VOCs. Therefore, with only a TVOC concentration, we cannot determine if there are VOCs present indoors at concentrations that may cause irritation, or significant cancer risks, or significant odors. TVOC concentrations are generally not considered to be a reliable indicator of potential health risk. This is particularly true for consumer-grade TVOCs sensors. Thus, we cannot determine whether your home has a “safe” level of VOCs. We can suggest one possible use for consumer-grade VOC sensors. If you find that the indicated TVOC concentration in your building is consistently far (several fold) higher than the TVOC concentration measured using the same device in several other buildings, it may be worthwhile to engage an IAQ expert who can measure the concentrations of individual VOCs and advise you about the potential health risks. Unfortunately, such a service will often cost more than $1000. If you are concerned about VOCs, we suggest that you read the section of this web site on VOCs, including the sub section on Implications for Good Building Practices, and consider whether you can identify and reduce indoor sources of VOCs.

How does indoor air temperature affect worker performance?

Most research has found that office workers perform best when they are thermally comfortable, which typically occurs when indoor air temperatures are 71 to 72 oF. As the indoor temperature rises or falls above or below this range, the available data suggest that performance decreases about 0.3% to 0.4% for each 1 oF change in temperature. Many other factors, such as the education and training of the work force, have a larger effect on worker performance; however, the economic benefits of maintaining thermal comfort to increase worker performance appear to far outweigh the costs. For more information, see the section of this website on human performance.

Does dampness and mold in schools cause health problems?

Dampness and mold in homes has been associated with increases in a range of respiratory health effects including increases in asthma. Less research has been performed on the health consequences of dampness and mold in schools. Most individual studies have failed to find statistically significant increases in respiratory health symptoms in occupants of schools with visible dampness or mold.  However, in a large majority of cases, there were non-statistically-significant increases in respiratory health symptoms among occupants of schools with visible dampness and mold. The combined data from multiple studies strongly suggest increased respiratory health symptoms with dampness and mold in schools. The evidence of adverse health effects is most convincing for cough, wheeze, and nasal symptoms or rhinitis. There is much less evidence that measures of lung function, such as forced expiratory volume, are affected by dampness and mold in schools. For more information, see the section of this website on Indoor Air Quality in Schools.

What are the effects of low ventilation rates in schools?

Many school classrooms have ventilation rates that do not meet minimum ventilation standards. In all types of buildings, lower rates of ventilation with outdoor air lead to higher indoor concentrations of air pollutants emitted from indoor sources. At the same time, lower ventilation rates can reduce indoor concentrations of ozone and particles from outdoor air and lower ventilation rates will usually save energy. High ventilation rates of schools in hot and humid climates can lead to an elevated indoor air humidity if the school’s ventilation and air conditioning system are not designed to accommodate high ventilation rates.

Research has shown that, in general, low ventilation rates adversely affect student performance. Low ventilation rates are also associated with increases in student absence. There is less certainty about the effects of low ventilation rates on student health; however, the available research suggests that low ventilation rates will sometimes adversely affect respiratory health.

For more information, see the section of this website on IAQ in schools.