ASHRAE Journal - June 2022 - 19

TECHNICAL FEATURE
CO2 levels are generally proportionate to outdoor CO2
levels, and (4) the effects of outdoor pollutants on human
wellness appear to be similar to symptoms of sick building
syndrome. It all stands to reason because CO2 and the
subject air pollutants are inherently coupled by the combustion
and biological processes that produce them.
Suggested here is that some statistical correlation
may exist between the concentration of CO2 and that of
the aggregate of other by-product pollutants affecting
human comfort and wellness. As there is likely a threshold
for the aggregate of subject air pollutants beyond
which human physiology no longer tolerates them, it
then follows that there might also exist some value for
CO2 concentration above which air quality is likely no
longer acceptable-maybe 1,000 ppm?
While variables in CO2 measurement,16 and variables
causing changes in ratios between CO2 and other pollutants
are many, further study of these hypotheses may
still be worth pursuing. However, the debate may be
sidestepped should CO2 itself-until recently thought
of as the benign surrogate-turn out to be a principal
source of discomfort and illness.
Direct CO2 Effects
On the International Space Station, the atmosphere
is tightly controlled. CO2 levels are kept notably higher
than in most earthbound indoor spaces because CO2
is diffi cult to remove; CO2 removal equipment size
becomes overwhelming for spacecraft of limited
size. For long missions, the maximum allowable CO2
concentration was originally 5,000 ppm, and higher
concentrations were permitted for shorter periods.
However, space station astronauts experienced physical
symptoms, most notably headaches, at CO2 levels twothirds
lower than was expected.29 CO2 levels have been
brought down a bit, but enduring headaches and other
maladies of exposure to the CO2 levels experienced on
spacecraft for prolonged periods of time is now a recognized
occupational hazard for space crews.30
Studies are emerging suggesting that prolonged exposure
to CO2 may pose health risks at levels presently
accepted for comfort and wellness. Studies summarizing
the studies also exist.31,32 While most studies show CO2
causes few acute overt health symptoms at levels below
5,000 ppm,31 several studies do state the likelihood of
several ailments from chronic CO2 exposure at levels
well below 5,000 ppm, including:32
* Chronic low-grade infl ammation exciting respiratory
and cardiovascular illnesses;
* Appetitive behaviors leading to obesity;
* Bone demineralization;
* Kidney calcifi cation;
* Chronic low-grade metabolic acidosis increasing
the risk of kidney and liver disease, type 2 diabetes, loss
of skeletal muscle, and osteoporosis;
* Oxidative stress leading to carcinogenic effects and
accelerated aging; and
* Mild hypercapnia (excess CO2 in bloodstream)
having symptoms that include fl ushed skin, drowsiness,
headaches, shortness of breath, increased heart rate and
being abnormally tired or exhausted.
Of particular interest are several studies providing
substantial evidence of acute exposure to CO2 at levels
as low as 1,000 ppm inducing signifi cant reductions in
cognition and decision-making abilities.31,32
As described by the medical community, breathing
under normal conditions is primarily regulated by the
CO2 concentration in the bloodstream via blood acidity
(pH) level, which in turn is proportional to the concentration
of CO2 in the air breathed. If breathing air CO2
concentration goes up, breathing becomes deeper and
more rapid. Recent studies show " clear linear physiological
changes in circulatory, cardiovascular and
autonomic systems, including increased heart rate, and
increased sympathetic stimulation at CO2 exposures in
the range of 500 to 5,000 ppm. " 33
Although more study is needed, the effects of lifetime
chronic exposure to moderately elevated levels of CO2
on human health would be diffi cult to prove. A real-life
environment in which to test a statistically sampled
population over its lifetime has yet to come about, and
hopefully never will. However, a preponderance of existing
literature already suggests a suffi cient likelihood
that CO2 levels have a direct effect on human wellness.
A few studies posit that the risks of elevated CO2 can be
serious.34,35 In this author's opinion, considering the
consequences if one mistakenly assumes that elevated
CO2 levels do not pose a direct risk, the assumption
should be made that they do. Liabilities for discounting
" suffi cient likelihood " can be high.
An absolute indoor CO2 limit would be useful not only
to help guide air quality assessments, but also to guide
environmental policies. Considering the recent studies
showing CO2 directly impacting human health, in
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ASHRAE Journal - June 2022

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