ASHRAE Journal - June 2022 - 15

TECHNICAL FEATURE
proportion to whatever other by-products are produced,
depending on the specifi c reaction. CO2 is considered
not to be a health hazard at low concentrations, but it can
be measured easily. Thus, although it is not foolproof or
absolute in any sense, as many deviations do exist, measuring
CO2 concentration offers an easy, albeit indirect,
way to gage the concentration of the other pollutants.
The use of CO2 as a surrogate air quality and ventilation
measure was proposed in 1858.5 Over the years, it
has come to be regarded by many as a fair indicator of
both indoor air quality and the adequacy of the ventilation
in a space. Also in 1858, the CO2 concentration of
1,000 ppm or less was introduced as a rule of thumb
for helping to decide when air is " clean. " The value was
determined from anecdotal observations of when people
became uncomfortable in various indoor spaces.5
In more recent times, the suggestion has been made
that CO2 monitoring be used to help control the spread
of coronavirus.6 As human exhalation is usually the
primary source of CO2 in an occupied space, an elevated
CO2 level could imply that one may be breathing others'
exhalation, possibly laden with infectious particles.
Thus, additional ventilation is required. A tuberculosis
outbreak at a university in Taipei was stopped when air
ventilation was increased and CO2 levels were brought
down to 600 ppm above the outdoor ambient CO2 level.7
These examples show the utility of managing CO2
concentrations relative to outdoor air levels, as specifi ed
in Standard 62-1999's guidance for ventilation rates to
achieve CO2 levels below 700 ppm above outdoor levels
for controlling human bioeffl uent concentrations in
sedentary settings.2
One should be to derive an absolute indoor CO2 level
limit for satisfactory air from a relative level by taking
the 700 ppm relative value from Standard 62-1999 and
adding to it the outdoor CO2 concentration, which was
just under 350 ppm in 1989.8 Thus:
700 ppm + 350 ppm = 1,050 ppm
(1)
This results in an absolute CO2 limit of about
1,000 ppm, which is the value cited in Standard 62-1989.
Outdoor CO2 Levels
There is much that is erroneous about the computation
presented in Equation 1. Most concerning should be
the value for outdoor CO2 concentrations. It is no longer
TABLE 1 Nominal daily maximum CO2 levels (ppm) at select urban sites compared
with Mauna Loa values.
Phoenix, Ariz.
Baltimore
Evanston, Ill.
Los Angeles
SITE DATA YEAR MAUNA LOA
2000
2006
2011
2015
369
382
392
400
URBAN
57512
48813
44014
62215
DIFFERENCE
206
106
48
222
Mauna Loa Data from Reference 8. Data current as of July 2021.
1989, and outdoor CO2 levels have been, and are still,
rising. As of this writing, the CO2 level at the Mauna
Loa Observatory in Hawaii is 417 ppm.8 Climate models
that assume intensive use of hydrocarbon fuels into the
future predict that CO2 levels will possibly reach near
1,000 ppm by 2100.9,10 At continued rates of per capita
fuel consumption, population growth and deforestation,
CO2 levels of 600 ppm may be seen within the next three
decades.9
In addition, urban centers are heavy sources of CO2
emissions, and they exhibit notably higher CO2 levels
than those found on the Hawaiian mountaintop. These
centers are often encased by " CO2 domes " in which
CO2 concentrations are markedly higher than in their
adjacent rural areas.11 Urban CO2 levels exhibit seasonal
cycles and daily variances. A sampling of CO2 levels at
urban sites is presented in Table 1.
With constantly rising ambient CO2 levels and local
outdoor CO2 levels in variance, the merit of an absolute
CO2 level for guiding building ventilation practice is
questionable-unless, of course, there is an absolute CO2
level that really is proportionate to some certain airborne
pollutant concentration, whether it be indoors or
outdoors.
Aggregation of Indoor and Outdoor Air Pollutants
Many outdoor air pollutants can exacerbate the same
human body response mechanisms and stress the same
body organs as indoor air pollutants. Combined, their
effects may be compounding. It thus makes sense for
ventilation practitioners to concern themselves with
the aggregation of pollutants from outdoor and indoor
air. Numerous indoor air quality studies suggest that
while there is no clear CO2 concentration threshold
below which " good " air quality is perceived, some
people begin to note diminishing air quality indoors
when ventilation rates are brought down to where CO2
values are around 500 to 600 ppm.16 One international
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ASHRAE Journal - June 2022

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