ASHRAE Journal - May 2022 - 36

TECHNICAL FEATURE
have unvented space heaters are likely to have other
combustion appliances including cooking equipment,
decorative appliances and space or water heating. Even
when installed and operated correctly, these are going
to produce more of the contaminants of concern than
homes without these appliances. Appropriate safety factors
also need to be considered.
Data for acceptable concentrations of combustionrelated
contaminants for health and building durability
impacts only exists for four contaminants of concern:
carbon dioxide, water vapor, nitrogen dioxide and carbon
monoxide.
Carbon Dioxide
CO2 is around us all the time, and outdoor concentrations
are normally around 400 ppm. At high enough
concentrations (3% or 30,000 ppm) it can be toxic. Note:
the x-axis in Figure 1 refers to increment above outdoor
concentration. OSHA sets the eight-hour workplace
limit at 5,000 ppm,7 but that is for workplace safety-a
much less stringent criterion than acceptable IAQ. The
ASHRAE 62 Standards, which are focused on acceptable
IAQ, typically produce values of about 1,000 ppm without
consideration of unvented appliances.
Persily8 examined carbon dioxide levels around this
level (1,000 ppm) and concluded there are no acceptability
or health issues. Thus, in the normal course of
ventilation system design, CO2 is not a contaminant of
concern but can be used as a ventilation metric. At concentrations
between 1,000 ppm and 5,000 ppm, carbon
dioxide becomes an acceptability and health concern,
and it becomes a contaminant. No current consensus
exists on where that limit should be in this range, but
ASHRAE has a new Position Document on indoor carbon
dioxide9 that is consistent with the information in this
section.
We must look at what is currently done and what the
science tells us to pick a reasonable value in that range.
We can look to extant ventilation and IAQ standards
around the world, which are often based on or include
CO2 limits. Standards like EN-1525110 set various levels
of allowed carbon dioxide depending on the quality of
air desired, and 1,600 ppm is typically the largest value
allowed by these standards. One of most recent reviews
was done by Sekhar, et al.11 A value of 1,600 ppm might
be used as the acceptable limit for our concerns here,
but the science may allow a higher value.
36
ASHRAE JOURNAL ashrae.o rg M AY 2022
FIGURE 1 Percentage of people dissatisfied as a function of carbon dioxide
increment from outside. Note that the x-axis used in the original plot differs from
the scale used herein. (From Zhang.12)
100
60
50
40
30
20
10
Zhang, et al13
Zhang, et al14
Regression model based on Zhang, et al (R2 = 0.87)
Fanger
500
1,000
1,500
2,000
2,500
Measured CO2 Concentration Above Outdoor (ppm)
Acceptability for any pollutant includes both perception
(e.g., odor, stuffiness, etc.) as well as health effects
including irritation. A special concern for CO2 is the
effect it may have on cognitive performance. Some
studies on cognitive performance degradations due to
CO2 (which is a measure of acceptability) have shown
impacts below 1,200 ppm, but others have not. More
studies show cognitive performance impacts above
2,000 ppm, but no clear consensus exists. There is the
common experience of sleepiness when CO2 levels rise,
such as on an airplane.
Acceptability is easier to determine than cognitive
performance. Studies have shown acceptability issues in
the range of 1,600 to 3,000 ppm. For example, Figure 1,
reproduced from Zhang,12 shows acceptability as a function
of increment above outdoors. Depending on what
one's definition of substantial majority is, this plot suggests
that a maximum total carbon dioxide concentration
should be set somewhere in the 1,600 ppm to 2,500
ppm range.
A strong component of acceptability is odor perception.
CO2 itself does not have an odor at these levels, but
many of the processes that produce it do. Zhang,12 for
example, has shown a synergistic effect on perceived
acceptability when both bioeffluents and CO2 are present.
While the acceptability of bioeffluents may primarily
be odor based, the direct acceptability of CO2 is physiologically
based, but may be acerbated by bioeffluents.
At increased levels carbon dioxide has a clear health
impact:
3,000
Percentage of Dissatisfied With Air Quality
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