Indoor air quality

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Indoor Air Quality (IAQ) deals with the content of interior air that could affect health and comfort of building occupants. The IAQ may be compromised by microbial contaminants (mold, bacteria), chemicals (such as carbon monoxide, radon), allergens, or any mass or energy stressor that can induce health effects. Recent findings have demonstrated that indoor air is often more polluted than outdoor air (albeit with different pollutants) although this has not changed the common understanding of air pollution. In fact, indoor air is often a greater health hazard than the corresponding outdoor setting. Using ventilation to dilute contaminants, filtration, and source control are the primary methods for improving indoor air quality in most buildings.

Techniques for analyzing IAQ include collection of air samples, collection of samples on building surfaces and computer modelling of air flow inside buildings. The resulting samples can be analyzed for mold, bacteria, chemicals or other stressors. These investigations can lead to an understanding of the sources of the contaminants and ultimately to strategies for removing the unwanted elements from the air.

Risk Assessment


Radon is the invisible, radioactive atomic gas that results from radioactive decay of some forms of uranium that may be found in rock formations beneath buildings or in certain building materials themselves. Radon is probably the most pervasive serious hazard for indoor air in the United States and Europe, probably responsible for tens of thousands of lung cancer deaths per annum. There are relatively simple tests for radon gas, but these tests are not commonly done, even in areas of known systematic hazards. Radon is a very heavy gas and thus will tend to accumulate at the floor level. Building materials can actually be a significant source of radon, but very little testing is done for stone, rock or tile products brought into building sites. The half life for radon is 3.8 days indicating that once the source is removed, the hazard will be greatly reduced within a few weeks.

Molds and other Allergens

These biological agents can arise from a host of means, but there are two common classes: (a) moisture induced growth of mold colonies and (b) natural substances released into the air such as animal dander and plant pollen. Moisture buildup inside buildings may arise from water penetrating compromised areas of the building envelope or skin, from plumbing leaks, from condensation due to improper ventilation, or from ground moisture penetrating a building slab. In areas where cellulosic materials (paper and wood, including drywall) become moist and fail to dry within 48 hours, mold colonies can propagate and release allergenic spores into the air.

In many cases, if materials have failed to dry out several days after the suspected water event, mold growth is suspected within wall cavities even if it is not immediately visible. Through a mold investigation, which may include destructive inspection, one should be able to determine the presence or absence of mold. In a situation where there is visible mold and the indoor air quality may have been compromised, a mold remediation may be needed. Indoors, mold growth can be inhibited by keeping humidity levels below fifty percent and by eliminating leaks or moisture condensation and accumulation.

There are some varieties of mold that contain toxic compounds (mycotoxins). However, exposure to hazardous levels of mycotoxin via inhalation is not possible in most cases, as toxins are produced by the fungal body and are not at significant levels in the released spores. The primary hazard of mold growth, as it relates to indoor air quality, comes from the allergenic properties of the spore cell wall. More serious than most allergenic properties is the ability of mold to trigger episodes in persons that already have asthma, a serious respiratory disease.

Carbon Monoxide

One of the most acutely toxic indoor air contaminants is carbon monoxide (CO), a colorless, odorless gas that is a byproduct of incomplete combustion of fossil fuels. Common sources of carbon monoxide are tobacco smoke, space heaters using fossil fuels, defective central heating furnaces and automobile exhaust. Improvements in indoor levels of CO are systematically improving from increasing numbers of smoke-free restaurants and other legislated non-smoking buildings. By depriving the brain of oxygen, high levels of carbon monoxide can lead to nausea, unconsciousness and death.


Legionellosis or Legionnaire's Disease is caused by a waterborne bacterium that grows best in slow moving or still warm water. The primary route of exposure is aerosolization, most commonly from evaporative cooling towers or showerheads. A common source of Legionella in commercial buildings is from poorly placed or maintained evaporative cooling towers, which often release aerosolized water that may enter nearby ventilation intakes. Outbreaks in medical facilities and nursing homes, where patients are immuno-suppressed and immuno-weak, are the most commonly reported cases of Legionellosis. More than one case has involved outdoor fountains in public attractions. The presence of Legionella in commercial building water supplies is highly under-reported, as healthy people require heavy exposure to acquire infection.

Legionella testing typically involves collecting water samples and surface swabs from evaporative cooling basins, shower heads, faucets, and other locations where warm water collects. The samples are then cultured and colony forming units (cfu) of Legionella are quantified as cfu/Liter.

Legionella is a parasite of protazoans such as amoeba, and thus requires conditions suitable for both organisms. The bacterium forms a biofilm which is resistant to chemical and antimicrobial treatments, including chlorine. Remediation for Legionella outbreaks in commercial buildings vary, but often include very hot water flushes (160°F), sterilization of standing water in evaporative cooling basins, replacement of shower heads, and in some cases flushes of heavy metal salts. Preventative measures include adjusting normal hot water levels to allow for 120°F at the tap, evaluating facility design layout, removing faucet aerators, and periodic testing in suspect areas.

Asbestos Fibers

The U.S.Federal Government ( and some States have set standards for acceptable levels of asbestos fibers in indoor air. Many common building materials used before 1975 contain asbestos, such as some floor tiles, ceiling tiles, taping muds, pipe wrap, mastics and other insulation materials. Normally significant releases of asbestos fiber do not occur unless the building materials are disturbed, such as by cutting, sanding, drilling or building remodelling. There are particularly stringent regulations applicable to schools. Inhalation of asbestos fibers over long exposure times is associated with increased incidence of lung cancer. While smokers have a greater risk of lung cancer than asbestos workers that do not smoke, smokers that are exposed to high levels of asbestos over many years have a much greater risk of developing lung cancer than either smokers that have not been exposed to asbestos, or persons that have been exposed to high levels of asbestos that do not smoke.

Carbon Dioxide

Carbon dioxide is a surrogate for indoor pollutants emitted by humans and correlates with human metabolic activity. Carbon dioxide at levels that are unusually high indoors may cause occupants to grow drowsy, get headaches, or function at lower activity levels. Humans are the main indoor source of carbon dioxide. Indoor levels are an indicator of the adequacy of outdoor air ventilation relative to indoor occupant density and metabolic activity. To eliminate most Indoor Air Quality complaints, total indoor carbon dioxide should be reduced to below 600 ppm above outdoor levels. NIOSH considers that indoor air concentrations of carbon dioxide that exceed 1,000 ppm are a marker suggesting inadequate ventilation (1,000 ppm equals 0.1%). ASHRAE recommends that carbon dioxide levels not exceed 700 ppm above outdoor ambient levels. The UK standards for schools say that carbon dioxide in all teaching and learning spaces, when measured at seated head height and averaged over the whole day should not exceed 1,500 ppm. The whole day refers to normal school hours (i.e. 9.00am to 3.30pm) and includes unoccupied periods such as lunch breaks. Canadian standards limit carbon dioxide to 3500 ppm. OSHA limits carbon dioxide concentration in the workplace to 5,000 ppm for prolonged periods, and 35,000 ppm for 15 minutes.

Debating the Definition of Indoor Air Quality in HVAC Design

There is a movement in the commercial and residential HVAC industry to pay more attention to the issue of indoor air quality (IAQ) throughout the design and construction stages of a buildings life. The "Green Design" movement also places emphasis on IAQ. Indoor air quality must be understood in order to discuss ventilation as a method of maintaining high IAQ, and ways to introduce demand controlled ventilation (DCV).

For the past several years, there have been many debates among indoor air quality specialists about the proper definition of indoor air quality and specifically what constitutes "acceptable" indoor air quality. Consequently, it is probably best to reference the currently accepted definition shown in the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) ventilation standard.

HVAC engineering issues

A basic way of maintaining the health of indoor air is by the frequency of effective turnover of interior air by replacement with outside air. In the UK, for example, classrooms are required to have 2.5 outdoor air changes per hour. In halls, gym, dining, and physiotherapy spaces, the ventilation should be sufficient to limit carbon dioxide to 1,500 ppm. In the USA, and according to ASHRAE Standards, ventilation in classrooms is based on the amount of outdoor air per occupant, not air changes per hour. Since Carbon dioxide indoors comes from occupants and outdoor air, the adequacy of ventilation per occupant is indicated by the concentration indoors minus the concentration outdoors. The value of 650 ppm above the outdoor concentration indicates approximately 15 cubic feet per minute of outdoor air per adult occupant doing sedentary office work. In classrooms, the requirements in the ASHRAE standard 62.1, Ventilation for Acceptable Indoor Air Quality, would typically result in about 3 air changes per hour, depending on the occupant density. Of course the occupants aren't the only source of pollutants, so outdoor air ventilation may need to be higher when unusual or strong sources of pollution exist indoors.

The use of air filters can trap some of the air pollutants. The Department of Energy's Energy Efficiency and Renewable Energy section wrote "[Air] Filtration should have a Minimum Efficiency Reporting Value (MERV) of 13 as determined by ASHRAE 52.2-1999."[1] Air filters are used to reduce the amount of dust that reaches the wet coils. Dust can serve as food to grow molds on the wet coils and ducts and can reduce the efficiency of the coils.

Moisture management and humidity control requires operating HVAC systems as designed. Moisture management and humidity control may conflict with efforts to try to optimize the operation to conserve energy. For example, Moisture management and humidity control requires systems to be set to supply Make Up Air at lower temperatures (design levels), instead of the higher temperatures sometimes used to conserve energy in cooling-dominated climate conditions. However, for most of the US and many parts of Europe and Japan, during the majority of hours of the year, outdoor air temperatures are cool enough that the air does not need further cooling to provide thermal comfort indoors. However, high humidity outdoors creates the need for careful attention to humidity levels indoors. High humidities give rise to mold growth and moisture indoors is associated with a higher prevalence of occupant respiratory problems.

The "dew point temperature" is an absolute measure of the moisture in air. Some facilities are being designed with the design dew points in the lower 50's °F, and some in the upper and lower 40's °F. Some facilities are being designed using desiccant wheels with gas fired heater to dry out the wheel enough to get the required dew points. On those systems, after the moisture is removed from the make up air, a cooling coil is used to lower the temperature to the desired level.

Commercial buildings, and sometimes residential, are often kept under slightly-positive air pressure relative to the outdoors to reduce infiltration. Limiting infiltration helps with moisture management and humidity control.

Dilution of indoor pollutants with outdoor air is effective to the extent that outdoor air is free of harmful pollutants. Ozone in outdoor air occurs indoors at reduced concentrations because ozone is highly reactive with many chemicals found indoors. The products of the reactions between ozone and many common indoor pollutants include organic compounds that may be more odorous, irritating, or toxic than those from which they are formed. These products of ozone chemistry include formaldehyde, higher molecular weight aldehydes, acidic aerosols, and fine and ultrafine particles, among others. The higher the outdoor ventilation rate, the higher the indoor ozone concentration and the more likely the reactions will occur, but even at low levels, the reactions will take place. This suggests that ozone should be removed from ventilation air, especially in areas where outdoor ozone levels are frequently high. Recent research has shown that mortality and morbidity increase in the general population during periods of higher outdoor ozone and that the threshold for this effect is around 20 parts per billion (ppb).

Institutional Roles

The topic of IAQ has become popular due to the greater awareness of health problems caused by mold and triggers to asthma and allergies. Awareness has also been increased by the involvement of the United States Environmental Protection Agency. They have developed an "IAQ Tools for Schools" program to help improve the indoor environmental conditions in educational institutions (see external link below).

A variety of scientists work in the field of indoor air quality including chemists, physicists, mechanical engineers, biologists, bacteriologists and computer scientists. Some of these professionals are certified by organizations such as the American Industrial Hygiene Association and the American Indoor Air Quality Council. AIHA and A2LA both offer laboratory accreditation programs that relate to indoor air quality.

On the international level, the International Society of Indoor Air Quality and Climate (ISIAQ), formed in 1991, organizes two major conferences, the Indoor Air and the Healthy Buildings series. The next conference, Indoor Air 2008, will take place in Copenhagen, Denmark. Healthy Buildings 2009 will be held in Syracuse, New York. ISIAQ's journal Indoor Air is published 6 times a year and contains peer-reviewed scientific papers with an emphasis on interdisciplinary studies including exposure measurements, modeling, and health outcomes.

How To Improve Indoor Air Quality

According to the National Safety Council, Americans spend about 90 percent of their time indoors, and 65 percent of that time at home. Thus poor indoor air quality can have a significant impact on people’s lives, especially those who are most vulnerable: infants and children, pregnant women, the elderly, and those who have chronic illnesses.

Steps can be taken to help improve indoor air quality at home, the workplace, and in other indoor environments. Some of those steps include the following:

  • Understand the role of Green Cleaning. Many "green" cleaning products contain chemicals that react with ozone to form toxic and irritating chemicals, as described above, through indoor air chemical reactions. The result is air that may contain more hazardous chemicals than were present before the green cleaning products were used. In particular, citrus based solvents and pine-based solvents have been shown to react with ozone and form formaldehyde, higher molecular weight aldehydes, acidic aerosols, and fine and ultrafine particles. Ozone can be removed from outdoor air by filtration with carbon, but large amounts of carbon are required to remove a high fraction of the ozone present in most urban and many ex-urban areas.
  • Install an air purifier, or an air purification system. Caution is advised. Ozone-based air cleaners are not advised and both the U.S. EPA and the California Air Resources Board have issued advisories on the use of ozone air cleaners. California is currently proposing to regulate ozone air cleaners.
  • Introducing plants to reduce the levels of indoor air pollution is only minimally effective. Many advocates of plants have advised introducing "toxin-consuming plants" to your home, office, and other indoor environments. Certain plants, including philodendron, spider plants, golden pothos, peace lilies, bamboo palms, mums, and english ivy remove dangerous toxins (e.g., formaldehyde, carbon monoxide, benzene, and others) from the air, but at extremely low rates. Even minimal ventilation with outdoor air removes them more quickly. In fact, the plants themselves do not play a major role in the removal of pollutants whereas the soil or other medium in which they are planted have been shown to play the removal role. The amount of pollutants removed by plants is actually minuscule compared to the removal that occurs even with very low rates of outdoor air ventilation. There is also a risk of increasing indoor humidity levels and of promoting mold growth when indoor plants are not properly maintained. See the article "Can house plants clean indoor air? at [Can_House_Plants_Solve_IAQ_Problems?
  • Use natural household cleaning products and reduce exposure to potentially toxic airborne substances. Be wary of so-called "green" cleaning products if outdoor ozone concentrations are above 50 ppb.
  • Keep your plumbing traps filled with water to help prevent sewer gas from entering the building.
  • Regularly clean the vents in your kitchen, bathroom, and dryer, and make sure they operate properly.
  • Do not smoke or allow smoking in your home, and avoid indoor areas where smoking takes place.
  • Avoid or reduce biological contaminants by maintaining humidifiers, dehumidifiers, and air conditioners; emptying water trays in dehumidifiers, air conditioners, and refrigerators frequently; drying or removing any water-damaged carpets or furniture; and routinely cleaning bedding and items used by pets.
  • Prevent carbon monoxide exposure by keeping gas appliances properly serviced, having your central heating system inspected and cleaned yearly, and never idling your car inside an attached garage. Install an automatic door closer on the door connecting the garage to the house.
  • Change filters on central cooling and heating systems and air cleaners according to the manufacturer’s directions.
  • Test your home for radon. Radon can seep into the house from contaminated earth and rock under the home, or from well water and/or building materials. Easy, do-it-yourself kits can be purchased at hardware and other retail outlets.
  • Ensure good ventilation.
  • Have any air-contioning systems inspected regularly to verify that there is no internal contamination build up contributing to poor indoor air quality. see


  1. [1] DOE EERE Indoor Air Quality - MERV 13 Air Filters

See also

External links

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