Workers and students spend about half their waking hours at work or school. Therefore, maintaining adequate indoor air quality (IAQ) in schools and the workplace is becoming a top priority for facility managers and building operating engineers. An essential element for maintaining adequate indoor air quality is outside air to dilute indoor air pollutants and exhaust these contaminants along with moisture and odors from our buildings.
Editor's note: This article was accurate at the time of writing and is still applicable in principle for many existing buildings. Some of the information has been superseded by ASHRAE 62.1 2007 or 2010. Please consult ASHRAE 62.1 2007 or 2010 for specifics that may apply to your situation.
Why measure carbon dioxide?
Most heating, ventilation and air conditioning systems (HVAC) re-circulate a significant portion of the indoor air to maintain comfort and reduce energy costs associated with heating or cooling outside air. When occupants and building operators sense air coming out of an air supply duct, it’s virtually impossible to judge how much of this air is simply re-circulated air and how much is outside air. Current technology allows easy and relatively inexpensive measurement of carbon dioxide (CO2) as an indicator to help ensure ventilation systems (for high density occupancy zones) are delivering the recommended minimum quantities of outside air to the building’s occupants.
What is carbon dioxide?
Carbon dioxide is a natural component of air. The amount of CO2 in a given air sample is commonly expressed as parts per million (ppm). The outdoor air in most locations contains down to about 380 ppm carbon dioxide. Higher outdoor CO2 concentrations can be found near vehicle traffic areas, industry and sources of combustion.
Where indoor concentrations are elevated (compared to the outside air) the source is usually due to the building’s occupants. People exhale carbon dioxide—the average adult’s breath contains about 35,000 to 50,000 ppm of CO2 (100 times higher than outdoor air). Without adequate ventilation to dilute and remove the CO2 being continuously generated by the occupants, CO2 can accumulate.
How much carbon dioxide is too much?
The concentrations of CO2 found in most schools and offices are well below the 5,000 ppm occupational safety standard (time weighted average for an eight-hour workday within a 40-hour work week) for an industrial workplace. While levels below 5,000 ppm are considered to pose no serious health threat, experience indicates that individuals in schools and offices with elevated CO2 concentrations tend to report drowsiness, lethargy and a general sense that the air is stale. Researchers are looking for links between elevated CO2 concentrations and reduced productivity and achievement.
What are the guidelines and standards for ventilation?
Ventilation rates for schools and office spaces are defined by various codes and standards. The most widely accepted standard is the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) Standard 62. Some state and local codes have adopted the ASHRAE Standard 62 ventilation requirements.
According to ASHRAE Standard 62, classrooms should be provided with 15 cubic feet per minute (cfm) outside air per person, and offices with 20 cfm outside air per person. Ventilation rates for other indoor spaces are also specified. Standard 62 is currently being revised, so these rates may change. Using CO2 as an indicator of ventilation, ASHRAE has recommended indoor CO2 concentrations be maintained at—or below—1,000 ppm in schools and 800 ppm in offices. Clearly the outdoor CO2 concentration directly impacts the indoor concentration. Therefore, it is critical to measure outdoor CO2 levels when assessing indoor concentrations. ASHRAE recommends indoor CO2 levels not exceed the outdoor concentration by more than about 600 ppm.
Unfortunately, the interpretation of the CO2 data is often a more significant source of error than instrument accuracy. Meaningful assumptions about ventilation rate based on
CO2 values require the building or zone to be occupied long enough to allow the CO2 levels to reach a balance with the ventilation rate. This balance is known variously as equilibrium, unity or steady-state. In an occupied building with a very low ventilation rate the CO2 levels will likely continue to increase throughout the day, never reaching a steady-state concentration. On the other hand, buildings with an aggressive ventilation rate and good mixing of the outside air may prevent CO2 from accumulating much beyond outdoor levels—resulting in low CO2 concentrations throughout the day.
Unless this steady-state or equilibrium has been reached, the building ventilation rate can be overestimated. For example, consider a CO2 measurement taken in a school classroom during first period. It is unlikely that the CO2 concentration will have accumulated to the point where equilibrium has been reached. Therefore, assumptions of outside air ventilation rates based on this CO2 measurement will lead to an overestimation of the ventilation rate. Thus low CO2 readings don’t necessarily mean adequate ventilation.
On the other hand, consider a CO2 measurement taken in the same classroom during the last period of the day. Assuming the ventilation rate and occupancy of the classroom have remained fairly consistent throughout the day, it is reasonable to assume that a CO2 concentration below about 1,000 ppm indicates 15 cfm per person, assuming also that the outside air CO2 is in the 350 ppm range (650 ppm differential inside minus outside).
Sources of error include ventilation systems that modulate the amount of outside air allowed into the building over the course of a day, occupancy rates that fluctuate significantly and measurement errors (instrument or calibration problems, measurement location, and/or poor mixing of the air within the space).
Using CO2 to calculate percent outside air
Direct measurement of the amount of outside air entering large air handling units can be both difficult and unreliable. Measuring the concentration of carbon dioxide in the outside air, return air and mixed air streams is often much easier than other methods. The values obtained are used in a formula to determine the percentage of outside air for a particular air-handling unit.
Today’s technology and IAQ
Today, the measurement of carbon dioxide is an important tool to help ensure adequate outside air ventilation while simultaneously saving energy by reducing the number of over-ventilated buildings. Technological breakthroughs have made it possible to use relatively inexpensive CO2 sensors to continuously monitor the CO2 in buildings. These CO2 values can be used by the heating, ventilation and air-conditioning (HVAC) control system to automatically modulate the volume of outside air to maintain indoor CO2 at or below a preset target concentration. This strategy is known as demand controlled ventilation (DCV). DCV systems are especially useful for those spaces or zones that experience variable occupancy rates: The ventilation rate responds proportionally to changes in the occupancy density.
What do these instruments cost ... and how accurate are they?
The accuracy of most of the CO2 measurement instruments available today is more than adequate for use as an indicator of ventilation in offices and schools. The accuracy of these devices requires occasional checks. Some of these instruments measure only CO2 while others will simultaneously measure temperature, relative humidity and other gases such as carbon monoxide. Instruments that either record internally or have output capability to an external data logger are valuable for trending and troubleshooting. The cost of these instruments ranges from about $500 to more than $5,000 depending on features. Installed costs of complete DCV systems have a much broader price range.
What about all those other indoor pollutants?
Clearly, elevated indoor CO2 levels suggest inadequate outside ventilation air—and it follows that inadequate ventilation permits other potentially harmful air pollutants to build up and create health, productivity and comfort problems.
It is essential to keep in mind that many indoor air pollutants are generated independent of occupancy (surface off-gassing, stored materials, air entry from contaminated utility tunnels, etc.). Where pollutant sources are independent of occupancy, CO2 may not be a good yardstick to evaluate the quality of the indoor air.
Unusually strong sources of indoor air pollutants can easily overwhelm the ventilation rates suggested by the ASHRAE Standard 62. A general rule of thumb is that a pollutant concentration is reduced by about 50 percent for each doubling of the ventilation rate (for those sources that have a fairly constant generation rate).
However, high ventilation rates can have huge impacts on energy costs and comfort. Therefore many indoor air pollutant sources are best controlled with methods other than basic dilution with outside air. Not surprisingly the energy efficient approach is also the wise approach: Ventilation only reduces exposure, while removing the source can completely eliminate exposure.
Therefore, methods of source control include keeping pollutant sources out of the building through wise choice of furnishings and finish materials, using exhaust fans to capture and remove pollutants, and controlling pressures between zones to keep pollutants from migrating to populated or sensitive areas. Good practice suggests that we exclude, remove and otherwise minimize pollutants suspected to have the potential to cause health problems or affect performance and comfort.
1. American Society of Heating Refrigerating, and Air Conditioning Engineers
(ASHRAE). 1992. ASHRAE Standard 62: Ventilation for Acceptable Indoor Air
Quality. Atlanta, GA.
2. ASTM Standard D-6245 – 98 Using Indoor Carbon Dioxide Concentrations to
Evaluate Indoor Air Quality and Ventilation
3. IAQ Diagnostics Reference Manual: Hands-On Assessment of Building
Ventilation and Pollutant Transport. University of Tulsa, College of Engineering
and Applied Sciences, Department of Chemical Engineering
4. National Institute for Standards and Technology. 1994 Manual for Ventilation
Assessment in Mechanically Ventilated Buildings. NISTR #5329-1994