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Indoor Air Quality: Carbon Monoxide and Carbon Dioxide


ISU Extension Pub # AEN-125
Author:  Tom Greiner, Agricultural & Biosystems Engineering
Department, Iowa State University.
Revised: December 1991
Content Reviewed:  May 1995
 
Carbon monoxide

Carbon monoxide (CO) and carbon dioxide (CO2) are produced 1) when
fossil fuels are burned, 2) from cigarette smoke, and 3) from human and animal respiration.  Carbon monoxide is odorless, colorless, and highly toxic.  It kills by reducing the oxygen supply in the body and is a deadly poison.  It adversely affects human health at only a few parts per million and causes death at 250 parts per million (250 ppm).

Properly installed and maintained heating appliances cause little threat from carbon monoxide. Poorly installed and maintained systems can be deadly. All fossil fuels contain carbon. During the combustion process the carbon in the fuel combines with oxygen in the air. With sufficient oxygen, sufficient turbulence, and at high ignition temperatures, nearly all of the carbon combines with two atoms of oxygen, producing the relatively innocuous carbon dioxide. Heating appliances are designed to provide excess oxygen, and a clean, properly installed and maintained system will produce primarily carbon dioxide (CO2), a large amount of water vapor, small amounts of carbon monoxide (CO), and a number of other pollutants.  These products of combustion from a properly maintained heating system will be vented outdoors through the chimney, and do not pose an undue risk to the building occupants.

When insufficient oxygen is available for complete combustion, one atom of carbon combines with one atom of oxygen and carbon monoxide is produced.  If the heating appliance or the venting system is defective, some or all of the carbon monoxide might be circulated into the building, posing an extremely hazardous health risk.  A warning sign of heating trouble might be sudden excessive levels of moisture in the home, since water vapor is also produced by combustion of fossil fuels.

Carbon dioxide

Carbon dioxide is a non-toxic gas. It has beneficial uses and is the "fizz" in carbonated beverages.  When frozen, it is "dry ice". At concentrations of from 2,500 ppm to 5,000 ppm carbon dioxide can cause headaches. At extremely high levels of 100,000 ppm (10 percent) people lose consciousness in ten minutes, and at 200,000 ppm (20 percent) CO2 causes partial or complete closure of the glottis.

Levels of 2,500 to 5,000 ppm do not normally occur in structures.  Use of any type of unvented fuel-burning space heater, such as a kerosene, natural gas, or propane heater will result in elevated levels. High levels also can occur when several people are in a poorly ventilated room.

Carbon dioxide is commonly used as an indicator of the adequacy of ventilation systems.  When the windows and doors are closed, all buildings need ventilation both summer and winter. In homes, this ventilation is typically provided by the normally occurring leaks and cracks around windows and doors. New, energy-efficient houses are now so tight that most leaks have been eliminated and some type of ventilation system may be needed. In commercial buildings the required ventilation is typically provided by a fresh air intake to the heating and cooling system.

Unfortunately, many firms have closed the fresh air intake to save energy.  Many other systems were installed without fresh air intakes.


The American Society of Heating Refrigerating and Air-Conditioning Engineers, Inc. publishes "ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air Quality."  This standard specifies that the minimum ventilation rate per person is 15 cubic feet per minute (cfm) of outdoor air.  Higher rates are in place for specified applications, i.e., the minimum rate is 60 cfm for a smoking lounge, 20 cfm for a school training shop, and 30 cfm for a hospital operating room.  Residential dwellings are covered by a special specification which is 0.35 air changes per hour, but not less than 15 cfm/person. (Note, additional special requirements are listed in the ASHRAE Standard and the complete standard should be consulted for specific recommendations.)

Since carbon dioxide is produced by human respiration, the amount of carbon dioxide can be easily used as an indicator of the adequacy of fresh air ventilation in occupied buildings. Outdoor levels are approximately 300 ppm.  The ASHRAE standard requires that sufficient fresh air be provided to keep the level below 1,000 ppm. The CO2 levels in buildings with sufficient ventilation will range between these two readings.  Buildings with insufficient ventilation will range from 1,000 ppm up.  Often the levels will be low in the morning and increase while the building is occupied. In buildings occupied during the day the reading should be taken in mid-afternoon, because this is when the CO2 reaches its highest level.

To determine ventilation rates, the carbon dioxide levels inside and outside the building must be measured in parts per million (ppm).  The ventilation rate is equal to the value 10,500 divided by the difference in indoor and outdoor CO2 concentration.  The ventilation rate will be in cubic feet per minute (cfm) per person.

For example, assume the peak afternoon indoor concentration is 1,000 ppm and the outdoor concentration is 300 ppm.  The difference between these two is 700 ppm. Dividing 10,500 by 700 ppm yields a ventilation rate of 15 cfm of outdoor air per person.

This equation only works when there is a relatively uniform density of people in the room and when steady-state conditions have been reached. It assumes that the people are engaged in light activities, and that there are no combustion sources in the space.

In conclusion, high levels of carbon dioxide often indicate inadequate ventilation.  Persons in buildings with high CO2 levels may complain of burning eyes, tiredness, and headaches.  These symptoms can be caused by a combination of carbon dioxide and the many other pollutants that occur in a poorly ventilated space. Complying with the requirements of ASHRAE 62-1989 is one step toward improving the indoor air quality of these buildings. 
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