ASHRAE Journal - September 2022 - 8

LETTERS
Revisiting the
1,000 ppm CO2
Limit
What author Robert E. Stumm P.E.,
writes in " Revisiting the 1,000 ppm
CO2 Limit " in the June 2022 ASHRAE
Journal, is both interesting and
stimulating.
It takes me back to the seven years
I spent with John Janssen, Chair,
Dave Grimsrud, Vice-Chair and
the other 62.1981R members writing
Standard 62-1989. Back then
outdoor CO2 ppm levels were in the
low 300s, whereas today they exceed
400. At the time, the maximum
1,000 ppm CO2 level guide set in the
62-1989 standard was based on an
outdoor level of just over 300 ppm
and an outdoor air ventilation rate
of 15 cfm/person. The need for
15 cfm/person minimum outdoor
air supply arose from the committee
members' experience. It is also my
experience since then.
Recall that the 1978 worldwide
energy embargo crisis led to building
ventilation systems' intake air
quantities being reduced to as little
as 5 cfm/person to save heating and
cooling energy. Some buildings even
boarded up their HVAC air intakes.
Then came the sick building syndrome
in buildings and new tighter
homes with indoor air-sourced VOC
and mold growth emission concentrations
now several times higher.
Outdoor ventilation rates in buildings
may be much higher than set
in the current ASHRAE ventilation
standard. For example, a 2004 survey
by Persily and Gorfain found
that the median outdoor air ventilation
rate in a survey of 100 randomly
selected office buildings was 49 L/s
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ASHRAE JOURNAL ashrae.o rg
per person (105 cfm per person),
which is much higher than required
by ASHRAE ventilation standards.
This survey also measured the recirculation
rate as 40% of the total air
supply, or 33 L/s per person.1
My perspective on CO2 was, and
still is, that carbon dioxide is not
an indoor air contaminant of concern
at levels much higher than
1,000 ppm, and a minimum of
15 cfm of outdoor air should be
maintained to address indoor-generated
gaseous contaminants. The
air in non-nuclear submarines, for
example, might contain 5,000 ppm
to 7,000 ppm of CO2.2 I have been
told as much as 20,000 ppm CO2 has
been measured.
Rather, for me CO2 is an easy-tomeasure
tracer for a) estimating per
person outdoor air ventilation rate,
although one must ensure lag time
to equilibrium is taken into account,
and also b) as a surrogate for occupant
bioeffluent concentration,
where human bioeffluent comprises
not only many gases besides carbon
dioxide.
In the case of a person ill with a
respiratory infection, it also includes
infectious aerosols from their normally
exhaled breath, and these
aerosols can float in the air for hours
and some of them can be inhaled by
others. Think COVID-19, the flu, the
common cold, measles, chicken pox
and tuberculosis.
So, we need our ventilation systems
not only to dilute contaminant
gases but also infectious aerosols.
That need not be only with outdoor
air or HEPA filters in the recirculation
air system.3 MERV 13 filters,
commonly used in buildings, will
remove 50% of infectious aerosols of
all sizes, while 1 in. pleated MERV 12
S E PTEM B E R 2022
electrostatic filters available for residences
will remove 20%.4,5
So, yes, ventilation with at least
15 cfm/person of outdoor air even
at higher levels of CO2 should continue.
However, if the higher levels
of outdoor CO2 are accompanied by
higher levels of SVOCs such as combustion
particulate aerosols, they
should be filtered out first.
References
1. Persily, A., J. Gorfain. 2008.
" Analysis of Ventilation Data from the U.S.
Environmental Protection Agency Building
Assessment Survey and Evaluation (BASE)
Study. " National Institute of Standards and
Technology.
2. Perrson, O., L. Wadso. " Indoor air
quality in submarines. " Indoor Air 2002
Proceedings 806- 811.
3. Walkinshaw, D. 2020. " COVID-19 and
beyond: a brief introduction to passenger
aircraft cabin air quality. " ASHRAE Journal
(10) https://tinyurl.com/6an9xasw
4. Owen, K., G. Kerr. 2020. " What you
can and can't do with ASHRAE Standard
52.2. " ASHRAE Journal (12).
5. Diez Maroto, M. 2011. " Filtration
Efficiency of Intermediate Ventilation
Air filters on Ultrafine and Submicron
Particles. " Master's Thesis. Department
of Energy and Environment, Division of
Building Services Engineering, Chalmers
University of Technology.
Douglas S. Walkinshaw, Ph.D., P.Eng.,
Fellow/Life Member ASHRAE, Ottawa, ON, Canada
The Author Responds
It is documented that submarine
crews on long deployments experience
hypercapnia. Upon returning
to shore, the hypercapnia abates.
What will happen when people
will have to contend with elevated
atmospheric CO2 levels life-long?
It is a major paradigm shift to think
the outdoor air we use to cleanse
our indoor air may one day become
unhealthy worldwide.
Robert Stumm
https://www.tinyurl.com/6an9xasw https://ashrae.org/
ASHRAE Journal - September 2022

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