Acceptable CO2 levels

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Hi all,
This question is not directly related to the group but i wanted to throw it
in as many of you might know the answer.

What is the acceptable CO2 level for a healthcare facility? Can i find the
number in any ASHRAE standards?


Amit Bhansali, M.S. , EIT

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Dear Amit,

Keep in mind that ASHRAE's "acceptable" level of CO2 has almost nothing to
do with health. You could describe it as more of a "body odor" or
"pleasantness" index. When the ambient levels of CO2 exceed their
recommendation (~1000 ppm), people start to feel that the indoor air is less
pleasant or "stuffy".

Health does not start to be affected until CO2 exceeds 5000 ppm. (per US EPA
and ACGIH)

James V. Dirkes II, P.E., LEED AP

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Thanks for the information. I think i was looking for the about the same
kind of reference/explanation as you mentioned in your email.


Amit Bhansali, M.S. , EIT

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When it comes to a 'healthcare' facility it is vital to understand what
happens in the facility as patient health can be affected in ways not taken
into consideration by EPA or ACGIH when calculating 'health' effects. You
may want to have a charette with all the involved parties. I suspect it is
often overlooked as everyone believes that the controls established for
operating theatres and so on addresses everything but go to a pulmonologist
and a cardiologist and ask them about average visitor/patient sensitivities
and concerns regarding things such as CO2 levels. As an example, ask them
how CO2 levels impact folks with emphysema or an enlarged heart. Those are
just two that come to mind and I am sure there are many others.

Seldom looked at but it should be all of the time with healthcare. At the
least, I feel that good work requires you meet the occupant representatives
and ask about concerns AND let them know they do have some options.

My 2 cents.


Andy Hoover

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Thanks to Mr. Dirkes.

Can anyone direct us to a study that says high CO2 in and of itself does
not cause health issues especially drowsiness? We have asked for such a
study from those providing filtration prior to considering such
filtration on our projects.

I think CO2 is definitely related to energy use/modeling because you can
reduce levels of outside air if allowing higher CO2 levels.



Bonafe, Wes's picture
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James Lo's picture
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Dear Bldg-Sim community,

A few more comments for your consideration:

ASHRAE Standard 62.1 is the principal reference used for acceptable indoor
air quality. It hasn't been mentioned in this discussion, so I want to make
sure you consult it. It's Appendix A says,

"CO2 is a bioeffluent generated by people at a rate determined by their
size, age, fitness, and activity level. At the same time people are
generating CO2, they are also producing odorous bioeffluents. These odorous
bioeffluents are generated proportionally to the rate of CO2 production,
although diet and personal hygiene also play a role. Nevertheless, CO2
concentration is a fairly dependable indicator of the concentration of the
odorous bioeffluents that the occupant component of the breathing zone
ventilation rate attempts to control. Hence, we can use CO2 concentration to
dynamically adjust the occupant component of the ventilation rate to reduce
outdoor air intake rates when zones are not occupied at their design

The key concept misunderstood in most discussions about CO2 is that
maintaining a specific level of CO2 in an occupied space is done NOT because
the CO2 is unhealthful, but because it's a convenient indicator of occupant
comfort. Comfort in this case is measured by "odorous bioeffluents",
otherwise known as "body odor".

While you can have harmful health effects from high levels of CO2, those
levels are (essentially) never encountered in a building. The only "real"
situation I am aware of where CO2 concentrations exceed 2000ppm (which is
less than 50% of OSHA's TLV) is in a submarine; not too much fresh air
available there!

I've paid very close attention to information about CO2 over the last 20
years or so because for many of those years, I worked for a manufacturer of
direct-fired heating equipment. That type of heating is commonly used in
industrial occupancies due to its exceptional efficiency (92%) and a few
other factors. One perceived downside of that equipment was that all of the
combustion products, including CO2, are released into the occupied space,
causing CO2 levels to rise, on occasion, to 2000ppm.

I also sat for several years on the ANSI Technical Advisory Group which
wrote the current national standards for that equipment. The member group
of a couple dozen code officials, testing agency and manufacturing
representatives conducted an extensive review of literature and found no
evidence either in the literature or their collective experience that CO2
was a concern under 5000ppm.

In summary, my personal and fairly informed opinion is that the CO2 level in
buildings is not even close to a health issue. Follow ASHRAE Standard 62.1
and all will be well. A recent post on this list referenced an article at by Joe Lstiburek which (I think rightly) points out the
huge energy implications of introducing more air than indicated by Std 62.1;
that's a bigger issue, I think.

p.s., I could not find any literature in my archives regarding a correlation
between CO2 and drowsiness. I don't think there's any literature indicating
a strong correlation, certainly not at normal building concentrations.

James V. Dirkes II, P.E., LEED AP

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Me again:

To me Mr. Dirkes' seems well informed on the issue of CO2 concentration.
Our concern has been the public's perception that one becomes sleepy, or
drowsy when exposed to PPM levels of CO2 in excess of 1000 PPM. As they
say perception is reality and I can rarely come out on top of a
discussion involving a concerned parent with "my opinion". Therefore
the need for a study or printed information to reinforce "my opinion".
We would considers a reference to the OSHA TLV, coupled with the 2000
PPM levels reached in a submarine. Has the U.S. Navy printed anything?
Can I backup my opinion that high CO2 alone is a non-issue. Mr. Dirkes'
comment was limited to CO2 and did not address all the other
contaminants that can build up in a space. There are products purported
to eliminate these however they cannot eliminate CO2. This requires
that CO2 be allowed to increase in concentration above 1000 PPM within
the space. This, by the way, is easy (relatively cheap) to measure. It
appears there is no direct evidence "other than OSHA's 5000 PPM TLV"
that up to 5000 PPM CO2 causes no harm. True?

As a disclaimer I have not read ASHRAE Standard 62.1.

Thanks for your assistance:

Wes Bonafe, P.E.

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Dear Wes,

I think you are asking "How do I tell a non-technical client / customer that
CO2 is not worth worrying about."

First, it's pretty hard to argue against popular opinion and the mainstream
media's general inclination to promote gloom and doom. After all, if ABC
or MSNBC or the local newspaper or (fill in the blank) says it, it MUST be

My approach is something like:

. Do my homework and become confident that I have a reasonable
understanding of the basis for whatever position I take.

. Be open to the (normal) reality that not everything is known about
almost any topic.

. Stand up for the truth and discourage worry or fear when it's not

. Present the best available information in a manner that helps
non-technical people understand it. In the case of CO2 or any indoor
contaminant, it would be something like "The premier experts in the field of
indoor air quality have condensed the requirements for acceptable indoor air
quality into a single standard that is recognized throughout the world and
updated regularly. That standard is called ASHRAE Standard 62.1. At
Moseley Architects, we follow that Standard as well as other related
standards and are confident that it represents the most current and
comprehensive way to assure your health in an indoor environment. Any

That still may not carry the day for certain of your clients, but what the
heck! Science is supposed to be a rational topic and you cannot argue an
irrational objection; don't try!

p.s., Since this is a building simulation forum, I should probably stop
here. Feel free to call or e-mail directly at
jim at

James V. Dirkes II, P.E., LEED AP

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I thoroughly agree with Mr. Dirkes and have confronted the same situation Mr. Bonafe has described at contentious board meetings and public hearings over the years.

I would just like to add a couple points to the conversation:

1. Under normal everyday levels and circumstances, CO2 is not a "contaminant" per we all know, it is around us all the time and we breathe and exhale it constantly.....although, it doesn't help that the EPA has recently added to the confusion by classifying CO2 as a public/pollution hazard in order to try to regulate it from a climatological point of view. Anyway, perhaps steering clear of using the word "contaminant" when discussing CO2 could help keep conversations with clients/the public productive.

2. The other thing that does not seem to have been pointed out specifically, is that ASHRAE 62 recommendations are based on a differential level of CO2 with the outdoors. It is the differential level between the indoor and outdoor levels of CO2 that is correlated with indoor air quality, not absolute levels of CO2 in the space. Since most areas in the U.S. have an outdoor CO2 level of around 380 ppm, and a differential level of 400-500 ppm has been correlated with human perceptions of comfort for a given a given level of human activity and (in more recent editions of ASHRAE 62) the use of certain building materials, the recommended indoor levels are around 800 ppm. So, the maximum levels of around 1000 ppm are recommended not because CO2 is unhealthly above that level but because the greater differential with ambient levels of CO2 would correlate to lower amounts of outdoor air being brought into the building. Studies have linked lower levels of
outdoor air in a space to a reduction in human performance and perceptions of comfort. So, perhaps focusing on the differential as the key to indoor air quality can reduce the fixation on a given level CO2 in a space.

Julia Beabout's picture
Joined: 2011-10-01
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I need to jump in perhaps a little off topic. I have been doing field measurement and verification of DCV for the past year. I found very few systems installed correctly. The biggest issue is that our area (until this week) still has ASHRAE 62-1989 as code!! Designers were not specific ling the minimum and maximum outside air flow levels. Specs and balance reports need to be checked, talked to the balance contractor up front, do they even know what DCV is? (many I talked to did not!). Commissioning must be done on these systems!! (as it should be on 100% of the building but that's another issue).

Brad Acker

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Healthcare facility ventilation requirements are often determined using the AIA/FGI Guidelines for Design and Construction of Health Care Facilities (full APA reference below), Table 2.1-2 (attached); not ASHRAE Standards. This table sets forth strict minimum outdoor air change per hour requirements for critical spaces (operating rooms, patient rooms, etc.). These outdoor air rates cannot be modified based on the CO2 levels in the zone.

AIA. (2006). Guidelines for Design and Construction of Health Care Facilities. Washington D.C.: American Institute of Architects.

Best regards,

Eric Bonnema

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