 Measuring carbon dioxide inside
buildings
Workers and students spend
about half of their waking hours at work or school.
Therefore, maintaining adequate indoor air quality (IAQ) in
their buildings is becoming a top priority of facility
managers and building operators. To maintain adequate indoor
air quality it is essential to provide outside air to dilute
indoor air pollutants and exhaust these contaminants along
with moisture and odors.
Why measure carbon dioxide?
Most heating, ventilating, 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.
It's virtually impossible for occupants and building
operators to judge how much of the air coming out of an air
supply duct is simply re-circulated air and how much is
outside air. Current technology now allows easy and
relatively inexpensive measurement of carbon dioxide (CO2 )
as an "indicator" to help ensure that ventilation systems
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 the air on this planet. The amount of CO2 in a
given air sample is commonly expressed as parts-per-million
(ppm)—the number of molecules of carbon dioxide per million
molecules. The outdoor air in most locations contains about
350 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 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
continuously generated by the occupants, CO2 can accumulate.
How much CO2 is too much?
The concentrations of CO2
found in most schools and offices are usually well below the
5,000 ppm occupational safety standard (time-weighted
average for a 10-hour workday within a 40-hour workweek) for
an industrial workplace. While levels below 5,000 ppm are
considered to pose no serious health impacts, experience
indicates that individuals in schools and offices with
elevated CO2 concentrations tend to report drowsiness,
lethargy, and a general sense of stale air. Researchers are
looking for linkages between elevated CO2 concentrations and
reduced productivity and achievement.
What are the guidelines and standards
for ventilation?
Various codes and standards
define ventilation rates for schools and office spaces. The
most widely accepted standard is the American Society of
Heating, Refrigeration, and Air Conditioning
Engineers(ASHRAE) Standard 62–1989. Some state and local
codes have adopted the ASHRAE ventilation requirements.
According to ASHRAE Standard
62-1989, classrooms should be provided with 15 cubic feet
per minute (cfm) of 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 future rates may be different.
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.
The following table
illustrates the relationship between outside air ventilation
rates and the resultant indoor CO2 levels, assuming an
outdoor CO2 of 350 ppm.
Ventilation Rate and
Resultant CO2 Concentrations
(at 350 ppm outdoor concentration)
Indoor Carbon Dioxide
(ppm) |
Outside Air Ventilation
(cfm Per Person) |
CO2
Differential
(Inside – Outside) |
| 800 ppm
suggests about |
20 cfm
(or less) |
500 ppm |
| 1,000
ppm suggests about |
15 cfm
(or less) |
650 ppm |
| 1,400
ppm suggests about |
10 cfm
(or less) |
1,050
ppm |
| 2,400
ppm suggests about |
5 cfm
(or less) |
2,050
ppm |
The CO2 values in this table
are approximate, and are based on a constant number of
occupants (sedentary adults), a constant ventilation rate,
an outdoor air CO2 concentration of about 350 ppm, and good
mixing of the indoor air.
Is it that simple?
Unfortunately, the
interpretation of CO2 data is often a more significant
source of error than instrument accuracy. Meaningful
assumptions of ventilation rate based on CO2 values require
the building or zone to be occupied for a duration long
enough to allow 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 the steady-state or
equilibrium has been reached, low CO2 readings don't
necessarily mean adequate ventilation. For example, consider
a CO2 measurement taken in a school classroom during the
first class period of the day—it is unlikely that the CO2
concentration will have accumulated to the point where an
equilibrium condition has been reached. Therefore,
assumptions based on this CO2 measurement may lead to an
overestimation of the ventilation rate.
On the other hand, consider a
CO2 measurement taken in the same classroom during the last
class 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, see table above).
Sources of error in
interpreting CO2 data 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
-
Measurement errors
(instrument or calibration problems, measurement
location, and/or poor mixing of the air within the
space)
How can I calculate percent outside
air?
It can be difficult and
unreliable to directly measure the amount of outside air
entering large air handling units. An effective method is to
measure the concentration of carbon dioxide in the outside
air, return air, and mixed air streams. The values obtained
are used in the following formula to determine the
percentage of outside air for a particular air-handling
unit:
% Outside Air (OSA) = (Cr -
Cs / Cr - Co ) X 100
Where:
Co is the carbon dioxide
concentration (ppm) in the outside air
Cr is the carbon dioxide concentration (ppm) in the return
air
Cs is the carbon dioxide concentration (ppm) in the supply
air (or mixed air)
The total supply air volume is
required in order to calculate the approximate cfm of
outside air supplied to the building using the percentage:
Outside Air (cfm) = % Outside
Air X Total Supply Air (cfm)
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Services:
Carbon Dioxide
North Bay Office
1990 N. California Blvd., 8th Floor
Walnut Creek, California 94596
(800) 660-1808 Toll Free
(925) 937-0700 Phone
South Bay Office
6472 Camden Ave., Ste. 102B
San Jose, California 95120
(800) 660-1808 Toll Free
(408) 866-4141 Phone
"I began
working for a high-tech firm based in Santa Clara about two
years ago. Every day that I went to work I started to feel
unhealthy and sick. Major headaches and dizziness
accompanied me within the first hour I arrived at work and
sat in my cubicle. When I left work on Friday for the
weekend, I started to feel much better as my symptoms went
away. I started noticing this pattern almost ever single
weekend. I spoke with my manager and she said that she was
planning on investigating the building because other
employees had complained of the very same thing, and she was
concerned of their health and wellbeing.
ERT, Inc.
was called out to investigate the building and quickly
determined that high levels of CO2 were prevalent. The
result was stale air. Air that has been used and breathed
and not replaced, seen as a rising concentration of carbon
dioxide, not to mention humidity. These effects had a
detrimental influence on the building, promoting the growth
of mold as well ! This was not the only effect. My manager
noticed the level of days off for sickness rose, and the
quality of work sank, leading to something referred to as
Sick Building Syndrome, since it appeared to affect
everybody in the building. The causes were not known for a
long time, but now it is recognized that this is a result of
breathing stale air with a high concentration of carbon
dioxide over an extended period. Studies have shown that a
level of 1000 ppm carbon dioxide will reduce the ability to
concentrate by about 30 %, a significant drop by any means.
ERT, Inc.
immediately recommended improved ventilation and
installation of air-conditioning systems throughout the
entire building. I know that my co-workers and I are
sincerely grateful that ERT, Inc. was called out to perform
this investigation, they quickly determined the root of the
problem, and provided recommendations that improved the
indoor air quality. Now working in a comfortable environment
is something I will never take for granted again."
Linda Yen
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