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IDPH Guidelines for Indoor Air Quality

Awareness of indoor air quality issues continues to increase throughout Illinois and the country. Studies conducted by the U.S. Environmental Protection Agency (USEPA) that compare risks of environmental threats to public health consistently rank indoor air pollution (including secondhand smoke, radon, organic compounds, and biological pollutants) among the top five risks. Indoor air quality problems can be very complicated due to the complexity of the buildings and ventilation systems, highly charged emotions from occupants, and the fact that standard epidemiology and industrial hygiene evaluation techniques can be inconclusive.

The Illinois Department of Public Health (the Department) has developed guidelines for acceptable indoor air quality, recommendations for achieving acceptable indoor air quality, and a basic approach for indoor air quality investigations. The Department defines acceptable indoor air quality as air in which there are no known contaminants at harmful levels as set forth in this document. The Department guidelines are not enforceable by law, but are designed to help individuals or groups improve the quality of indoor air in their homes and workplaces.

(I) Criteria for Acceptable Indoor Air Quality

Some guidelines cited in this document refer to standards promulgated by the American Society of Heating, Refrigerating and Air-conditioning Engineers Inc. (ASHRAE). These are found in the ASHRAE documents Ventilation for Acceptable Indoor Air Quality(ASHRAE 62-2001) and Thermal Environmental Conditions for Human Occupancy(ASHRAE 55-2004). The Department adopts these two documents as the basis for guidelines set forth in this document. If the ASHRAE guidelines are changed, the Department will review the changes and amend this document accordingly.

The Department’s levels and ranges for indoor air contaminants listed here are deemed protective of the general population, but may NOT be appropriate for hypersensitive individuals. Additionally, he Department is aware that many contaminants may be responsible for indoor air quality problems. This document lists the most common contaminants responsible for occupant complaints in buildings. If a contaminant is not listed in this document, we recommend contacting the Department for consultation.

For reference, the Department has included in Table 1 some occupational standards for the parameters discussed in this document. These occupational standards were

developed for healthy working-age adult males and are based upon exposures of eight hours per day, five days per week. A small percentage of workers, because of age, sex, genetic factors, personal habits (such as smoking, alcohol consumption, and drug use), medication, or previous exposures, may experience adverse health effects from exposure to some substances at levels lower than these occupational standards. Additionally, these occupational standards are NOT appropriate for the home environment. People spend more time in their homes and more susceptible persons, such as children and the elderly, may be present.

Humidity and Temperature

There is no "ideal" humidity level and temperature suitable for all building occupants. Many factors, such as personal activity and clothing may affect personal comfort. Acceptable relative humidity levels should range from 20 percent to 60 percent year-round. Levels less than 20 percent in the winter and greater than 60 percent in the summer should be considered unacceptable. Elevated relative humidity can promote the growth of mold, bacteria, and dust mites, which can aggravate allergies and asthma.

The Department concurs with the ASHRAE guideline that indoor temperatures in the winter be maintained between 68 degrees and 75 degrees, with a relative humidity level between 30 percent and 60 percent. Temperatures in the summer should be maintained between 73 degrees and 79 degrees, with a relative humidity level between 30 percent and 60 percent. These ranges should be acceptable for sedentary or slightly active persons.

Carbon Dioxide

Carbon dioxide is a normal constituent of exhaled breath and is commonly measured as a screening tool to evaluate whether adequate volumes of fresh outdoor air are being introduced into indoor air. The outdoor level of carbon dioxide is usually from 300 parts per million to 400 parts per million (ppm). The carbon dioxide level is usually greater inside a building than outside, even in buildings with few complaints about indoor air quality. If indoor carbon dioxide levels are more than 1,000 ppm, there is probably inadequate ventilation; and complaints such as headaches, fatigue, and eye and throat irritation may be prevalent.

Carbon dioxide itself is not responsible for the complaints; however, a high level of carbon dioxide may indicate that other contaminants in the building also may be present at elevated levels and could be responsible for occupant complaints.

Properly ventilated buildings should have carbon dioxide levels between 600ppm and 1,000 ppm, with a floor or building average of 800 ppm or less. If average carbon dioxide levels within a building are maintained at less than 800 ppm, with appropriate temperature and humidity levels, complaints about indoor air quality should be minimized. If carbon dioxide levels are greater than 1,000 ppm, complaints may occur. Therefore, 1,000 ppm should be used as a guideline for improving ventilation. If a building exceeds this guideline, it should NOT be interpreted as a hazardous or life-threatening situation. An elevated carbon dioxide level is only an indication of an inadequate amount of outside air being brought into a building. The level cited in this document should only be used as a guideline to determine the amount of fresh outside air entering a building.

In building areas where there are potential sources of carbon dioxide other than exhaled breath, the guidelines above cannot be used. Other sources can include exhaust gas from kilns, internal combustion engines, dry ice, etc. Under these conditions, the Occupational Safety and Health Administration (OSHA) standard for carbon dioxide should be used. The OSHA standard is an eight-hour time-weighted average (TWA) of 5,000 ppm with a short-term 15-minute average limit of 30,000 ppm.

Carbon Monoxide

Carbon monoxide is colorless and odorless and is a normal constituent of exhaust gases from incomplete combustion. Potential sources inside a building that may generate carbon monoxide include gas heating systems, gas stoves, gas hot water heaters, cigarette smoke, and portable kerosene heaters. For office areas, levels of carbon monoxide are normally between 0 ppm and 5 ppm. Levels greater than 5 ppm may indicate the presence of exhaust gases in the indoor environment and should be investigated. Levels of carbon monoxide inside buildings should not exceed 9 ppm. Exposure to carbon monoxide at levels as low as 35 ppm may cause mild fatigue.

If levels inside a building are detected greater than 100 ppm, the building should be evacuated until the source is identified and corrected. Adverse health effects such as headache and dizziness may occur after two-hour exposures to carbon monoxide levels as low as 100 ppm. The Department recommends that all buildings with indoor combustion sources install carbon monoxide detectors approved by Underwriters Laboratories (UL) to continuously monitor for this gas.

Hydrogen Sulfide

Hydrogen sulfide is a normal constituent of sewer gas. It is colorless and has an odor similar to rotten eggs. Sources of hydrogen sulfide in buildings usually arise from dry

drain traps or broken sewer lines. The level at which people can begin to smell the gas is about 0.01 ppm. The Department recommends hydrogen sulfide levels be kept at less than 0.01 ppm, since an odor indicates the presence of sewer gas inside a building. An investigation should be conducted immediately after noticing a hydrogen sulfide odor to identify and correct the source.


Ozone is a respiratory irritant produced by equipment that uses high voltage electricity. Photocopiers and ion generator air cleaners can release ozone into the indoor environment. THE DEPARTMENT recommends that ozone levels not exceed 0.08 ppm.


Particulates can be classified as either respirable (less than 5 microns in diameter) or nonrespirable. Respirable particles can penetrate into the lower lung and can cause damage. Nonrespirable particles are trapped in the upper respiratory system and can cause irritation. The Department recommends that particulates 2.5 micrometers to 10 micrometers in diameter (PM10) be maintained at less than the USEPA National Ambient Air Quality Standard (NAAQS) of 150 micrograms of particulates per cubic meter of air (ug/m3) and particulates 2.5 micrometers in diameter or less (PM2.5) be maintained at less than the NAAQS of 65 ug/m3 during a 24-hour time period.


Formaldehyde is a common constituent of adhesives used in particle board, carpeting and furniture. The use of formaldehyde has been modified in recent years to reduce its release from these products. Increased ventilation during and following renovation or new construction should reduce the level of formaldehyde. The Department recommends the level of formaldehyde not exceed 0.1 ppm for offices and 0.03 ppm for homes.

Nitrogen Dioxide

Nitrogen dioxide is produced during the incomplete combustion of natural gas or other fuels. The Department recommends that the level of nitrogen dioxide not exceed the NAAQS of 0.05 ppm over a 24-hour period.

Tobacco Smoke

The Department recommends smoking not be allowed indoors. If smoking is allowed indoors, it should be limited to a designated smoking area with adequate exhaust separate from the main heating, ventilation, and air conditioning (HVAC) system.


Radon is a colorless, odorless, radioactive gas naturally produced when uranium decays in the soil. Uranium and its decay products are commonly found in soil and rock in Illinois. Radon gas enters buildings through cracks, crawlspaces, basement drains and other openings in the foundation or concrete slab. The gas decays into radioactive particles that, when inhaled, cause cell damage and potentially lung cancer. Radon gas is the second leading cause of lung cancer in the United States. and the leading cause of lung cancer among non smokers.

The Department recommends that levels in homes be less than 4.0 picocuries per liter (pCi/L). If levels exceed 4.0 pCi/L, radon mitigation should be considered to reduce the risk of adverse health effects. The only way to determine the level of radon in your home is to have the air tested. The Illinois Emergency Management Agency can provide a list of businesses that conduct radon testing and laboratories that have test kits for sale. Visit or call 800-325-1245 for additional information.

Table 1 -- Summary of Guidelines

Humidity 20% - 60% 30% - 60% N/A N/A
Temperature 68° - 75° (winter) 68° - 75° (winter) N/A N/A
73° – 79° (summer) 73° - 79° (summer)
Carbon Dioxide 1,000 ppm 1,000 ppm 5,000 ppm 5,000 ppm
(<800 ppm preferred)
Carbon Monoxide 9 ppm 9 ppm 50 ppm 25 ppm
Hydrogen Sulfide 0.01 ppm N/A 20 ppm 10 ppm
Ozone 0.08 ppm N/A 0.1 ppm 0.05 ppm
Particulates 0.15 mg/m3 (PM 10 ) (150 μg/m3) 24-hr 0.065 mg/m3 (PM 2.5) (65 μg/m3) 24-hr N/A 15 mg/m3(total) 10 mg/m3(total)
5 mg/m3(resp.) 3 mg/m3(resp.)
Formaldehyde 0.1 ppm (office) N/A 0.75 ppm 0.3 ppm
0.03 ppm (home)
Nitrogen Dioxide 0.05 ppm N/A 5 ppm 3 ppm
Radon 4.0 pCi/L N/A 100 pCi/L 4 WLM/yr (working level months/year)

* Occupational Safety and Health Administration Permissible Exposure Limit — this level is a time-weighted average and is an enforceable standard that must not be exceeded during any eight-hour work shift of a 40-hour work week.

** American Conference of Governmental Industrial Hygienists Threshold Limit Value — this level is a recommended time-weighted average upper limit exposure concentration for a normal eight to 10-hour workday and a 40-hour work week.

N/A-Not Applicable or Not Established

(II) Recommendations for Achieving Acceptable Indoor Quality

ASHRAE 62-2001 sets guidelines for outdoor air requirements for ventilation and occupancy maxima for different facilities. Adhering to these guidelines should ensure that carbon dioxide levels are maintained less than 1,000 ppm, and that complaints are kept to a minimum. Proper thermostat location and a well-maintained heating, ventilation and air conditioning (HVAC) system should allow for temperatures to be maintained within the thermal comfort range. Operating a building under positive pressure will reduce the infiltration of outside contaminants into a building. Using local exhaust in areas such as copy rooms, chemical storage areas, and print shops can reduce other sources of indoor air contaminants.

Building owners also should consider source control, since it can have a major impact on indoor air quality. Construction, painting and roofing should be performed while areas are unoccupied. During renovation activities, ventilation systems should be operated 24 hours a day. Using increased ventilation during and after these activities will reduce the impact on indoor air. “Dry” furnishings and materials such as carpeting, tile, textiles and wood products, when appropriate, should be allowed to “air-out” for a minimum of three days before installation, or until there is no noticeable odor or irritation. “Wet” materials, such as adhesives, glazes, caulks and paints, should be used sparingly. “Dry” furnishing materials should not be installed until “wet” materials have been allowed to dry.

Following the Department guidelines should preclude the use of air cleaning devices. If it is determined that air cleaning devices need to be used in localized areas, refer to the USEPA document, Residential Air Cleaners.

(III) Educational Activity on Indoor Air Quality

Besides the Illinois Department of Public Health (see Department contact listing at the end of this document), several agencies and groups can provide educational materials and information concerning indoor air quality. Some of these organizations are:

American Lung Association, 800-LUNG-USA, or the local American Lung Association

National Institute for Occupational Safety and Health (NIOSH)
Clearinghouse for Occupational Safety and Health Information
Publications Office
4676 Columbia Parkway
Cincinnati, Ohio 45226
800-356-4674 (connects to TTY)

Illinois Department of Labor
1 West Old State Capitol Plaza, Room 300
Springfield, Illinois 62701

Occupational Safety and Health Administration (OSHA)
There are five division offices in Illinois. Please contact the one nearest you.

  • Fairview Heights 618-632-8612
  • Calumet City 708-891-3800
  • Des Plaines 847-803-4800
  • North Aurora 630-896-8700
  • Peoria 309-589-7033

USEPA Region V, Superfund

77 West Jackson Street

Chicago, Illinois 60604

312-886-3011 Fax 312-886-4071

(IV) Recommended Approach for Indoor Air Investigations

Investigation of indoor air quality problems is not an easy task. The Department approach to indoor air quality investigations follow the methods developed by the National Institute for Occupational Safety and Health (NIOSH) in their publication, Guidance for Indoor Air Quality Investigations. This NIOSH technique is best described as one of exclusion. The investigation tries to narrow the range of possible problem causes. The Department has found six major sources of indoor air quality problems:

  1. Inadequate Ventilation -- These problems involve lack of adequate fresh air and uneven distribution of fresh air within a building.
  2. Humidity and Temperature -- These problems involve levels of these parameters outside the normal comfort range.
  3. Inside Contamination -- Copy machines, office products, and chemicals stored indoors have been identified as significant sources of indoor air problems in some investigations.
  4. Outside Contamination -- This is caused by the re-entrainment of previously exhausted contaminants, generally caused by improper air intake placement or by periodic changes in the wind conditions. A common problem is vehicle exhaust fumes from parking garages or loading docks being drawn into a building ventilation system.
  5. Microbial Contamination -- This type of problem is usually associated with water leaks, water infiltration, elevated indoor humidity, humidifiers, and contaminated ventilation ductwork.
  6. New Building Materials -- This results from building materials (including carpeting) releasing gasses into the air during and shortly after the materials are first installed. Increased ventilation after installation will enhance the dissipation of these chemicals. These problems usually resolve with time.

Typical indoor air investigations will include a review of several items:

  • visual inspection and plan review of the HVAC system
  • observation of conditions in the area of concern
  • interviews with individuals experiencing problems
  • measurement of ventilation and comfort parameters (carbon dioxide, humidity, temperature, and carbon monoxide)

A general walk-through evaluation of the building should be performed noting specifically:

  • water damaged ceiling, wall, and floor finishes
  • dust streaking near air vents and diffusers
  • mold growth on building components
  • apparent building use changes
  • the presence of odors
  • condition and utility of outside air dampers
  • condition of the air filtration system