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What a Carbon Dioxide Monitor Tells You

Measuring The Indoor Carbon Dioxide Provides Important Information About The Indoor Air Quality

OK, it’s time to spell this out in black and white because there seems to be some confusion.  Monitoring carbon dioxide (CO2), as I’ve been doing with my Aranet4 portable CO2 monitor* for the past couple of years, is a good thing.  It’s not a perfect measure of indoor air quality.  It’s more like a “Check engine” light on your car.  So here’s the basic stuff you need to know to use carbon dioxide monitoring results wisely.

CO2 monitoring fundamentals

Carbon dioxide in indoor spaces comes mainly from people breathing or from combustion.  If there’s no unvented combustion in a space, the carbon dioxide level is primarily from the exhaled breath of people.  (Or sometimes animals.  A contractor friend of mine once went to a house where the homeowner kept a donkey indoors.  Really!  And as weird as that sounds, it was NOT the worst part of the story.)

There’s a bottom end to the range of CO2 you’re likely to see when you carry a CO2 monitor with you to a restaurant, airport, or trade show.  Indoor CO2 won’t be lower than the outdoor CO2.  Since the Industrial Revolution, the outdoor number has been rising steadily because we burn a lot of fossil fuels.  We’re now at about 420 parts per million (ppm) in the outdoor air.  It does fluctuate some, but you won’t ever get an accurate measurement of, say, 300 ppm.

The upper end of the CO2 level you’re likely to see will be in the thousands of parts per million.  I regularly see it in the 1,500 to 3,000 ppm range.  The highest I’ve seen is over 7,000 ppm.  (More about that below.)

Consumer-grade devices like the Aranet4* (which costs ~$250) are surprisingly accurate.  The Aranet4 specifications claim an accuracy of ±30 ppm, but for the kind of monitoring I’m talking about here, ±100 ppm (or even higher) is good enough.

Finally, carbon dioxide, CO2, is NOT carbon monoxide, CO.  We exhale CO2, a colorless, odorless gas that’s harmless at normal concentrations.  Incomplete combustion produces CO, a colorless, odorless gas that can be deadly at relatively low concentrations.  You need a different kind of monitor to detect carbon monoxide.

It’s not the CO2

This week I went to the AHR Expo, a giant trade show for the HVAC industry.  I brought my CO2 monitor with me and posted a couple of photos on LinkedIn showing the CO2 level in the exhibit hall.  Some people thought the problem was the high level of carbon dioxide itself.  But was it?

The carbon dioxide level in the exhibit hall of the AHR Expo was high.
The carbon dioxide level in the exhibit hall of the AHR Expo was high.

There’s been some research on the topic of the effect of elevated CO2 levels on people.  Some of those studies have found that CO2 does affect our cognitive abilities.  Others have found little to no effect.  The ASHRAE position document on indoor carbon dioxide (pdf) says this:

Existing evidence for direct impacts of CO2 on health, well-being, learning outcomes, and work performance at commonly observed indoor concentrations is inconsistent, and therefore does not currently justify changes to ventilation and IAQ standards, regulations, or guidelines.

Using a CO2 monitor indoors isn’t about the CO2.  It’s about what the CO2 level tells you.  But before we get to that, there’s another thing it doesn’t tell you.

It’s not a measure of indoor air quality

The indoor CO2 level does not tell you how good or bad the indoor quality is.  It doesn’t tell you which indoor air pollutants you’re being subjected to or what your risk level is.  That’s because a CO2 reading tells you only one thing:  the concentration of CO2 in the air being sampled by the monitor.  But it hints at something else, and that’s where its usefulness lies.

With a high CO2 reading, the indoor air quality actually could still be pretty good.  Why?  Because good indoor quality results from a layered approach.  Those layers include source control, airtightness, moisture control, filtration, and ventilation.  The CO2 level gives you a hint about only one of those layers.

It’s a useful indicator

So what does it hint at?  That would be the ventilation rate relative to the CO2 sources.  Why is this important?  Because ventilation is one of the layers mentioned above.  It’s a dilution method that replaces indoor air with outdoor air.  As long as the outdoor air is cleaner than the indoor air, the indoor air improves.  Or at least it doesn’t get as bad as it would be without the ventilation.

The main concern that has driven people to start carrying CO2 monitors is the airborne transmission of infectious diseases.  Scientists learned early in the COVID-19 pandemic that the coronavirus responsible for it spread mainly through the air.  The particles expelled into the air by an infected person can carry a lot of the virus.

Ventilation is one way to reduce your risk of getting an airborne infectious disease.  Since they’re particles, though, you can also get rid of them with filtration.  The Corsi-Rosenthal box fan air cleaner is one way to do that.

CO2 levels tell a story

One nice thing about the Aranet4* is that it has an app that shows you a graph of the CO2 level over time.  Below you can see my graph for Monday of this week, when I went to the AHR Expo in downtown Atlanta. Let’s look at the story being told here.

  • Just before 10 am:  The spike on the left is when I was driving to the office.
  • 10 am to nearly 2 pm:  In the office.  Our CO2 usually hangs out in the 700 to 900 ppm range, but it was running a little higher that day.
  • 2 to 3 pm:  500 to 1,000 ppm when I was outdoors at the train station, then on the train, and then outdoors between the train station and the Georgia World Congress Center.
  • 3 to ~3:20 pm:  In the lobby and registration area.  Not a high density of people in that area, and the CO2 level was between 1,000 and 1,500 ppm.
  • 3:20 to 5:30 pm.  In the exhibit hall, the CO2 spiked up to a high of nearly 2,600 ppm.  The dip in there between 4:30 and 5 pm was when I went with Michael Ridler and Robert Bean to do an interview in the lobby area.  Even there, though, the CO2 level was still above 1,000 ppm.
  • 5:30 to end of day:  Train back to office, in the office for a bit, car trip home, and then home for the evening.
This graph of the carbon dioxide level throughout the day tells a story.
This graph of the carbon dioxide level throughout the day tells a story.

The important takeaway here is what the general CO2 levels were in different places.  Above 1,000 ppm means the ventilation isn’t keeping up the CO2 output very well.  The indoor air quality may still be OK, but if anyone there is sick with something that transmits through the air, your risk of getting sick increases.  When it’s above 2,000 ppm, your risk is even higher.

One way to think about this is in terms of rebreathed air.  The higher the CO2 level, the more air you inhale that recently came out of someone else’s lungs.  I wrote an article showing engineer David Elfstrom’s spreadsheet correlating CO2 level with percent of rebreathed air.  At 2,500 ppm, for example, that model predicts that 5.5% of the air you inhale is rebreathed air.  Don’t take that number too literally, though.  Instead, think of it as an indicator.

Context matters

Another important factor in all of this is the setting.  The graph above shows really high CO2 levels at a crowded indoor event.  If you’re alone at home with high CO2 levels, you probably don’t need to worry as much.  Yeah, you could use more ventilation if your CO2 is hanging out above 1,000 ppm, but you’re less likely to get COVID or the flu.

Here’s another CO2 graph, this one showing the highest CO2 level I’ve measured.  After a conference last summer, four of us drove to the airport in a rental car for about an hour.  We sat there talking for a while when I remembered my Aranet4 and looked at it.  I was shocked to see we were over 7,000 ppm.  The air conditioner somehow got switched to recirculate so there was a whole of rebreathing going on there for a while.

This is what happens when you put four people in a car with the air conditioner in recirculate mode
This is what happens when you put four people in a car with the air conditioner in recirculate mode

Another place where the carbon dioxide level goes and stays above 1,000 ppm is on commercial flights.  You’re packed into a long metal tube with a bunch of other people, breathing recirculated air with little fresh air added to it.  But filtration on a plane is pretty good.  Unless you’re sitting close to a sick person, you’re probably OK without a mask.

The sum and substance is simple:  Watching indoor carbon dioxide levels is like keeping an eye on the “Check engine” light on your car.  It doesn’t always mean something is wrong, but it does mean you probably ought to do something.


Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a popular book on building science.  He also writes the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


Related Articles

A Layered Approach to Indoor Air Quality

Carbon Dioxide and the Air You Rebreathe

6 Options for Ventilating Homes in Humid Climates


* This is an Amazon Associate link. You pay the same price you would pay normally, but Energy Vanguard may make a small commission if you buy after using the link.


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This Post Has 36 Comments

  1. I have 2 of them. The specs for the Aranet4 HOME Indoor Air Quality Monitor are Accuracy: 0-2000ppm, ±50ppm ± 3% reading: 2001-9999ppm ±10% of reading. Not ±30ppm, this is a high grade sensor not found in units at this price. (source:
    I find they average 50ppm to 100ppm on the high side
    I also have several others including the Portable CO2 Detector and Alarm which is ±30ppm. (
    This is the site to evaluate and purchase a unit.
    hope this helps, also people stop breathing on your device! (great resource
    Joe Medosch

  2. I have been using an Awair device for the past year to measure indoor air quality. Like you, I see a measured increase in CO2 during parties and dinner gatherings. You can imagine the measurements I got for VOCs and PM2.5 when I repainted my fireplace cover even with doors and windows open.

    One issue with Awair’s device is how it grades humidity here in the SE US. Anything above 50% puts you in a warning area. That’s hard to do during our sticky summers even with HVAC dehumidification. I just wish I could tweak the threshold to a more manageable level, say 55%. 😉

    1. Norman: Yeah, I complained to them about that yellow instead of green for relative humidity over 50%. I hope they’ll change it to 60%.

  3. Really good piece Allison. It should help more folks better deal with IAQ issues – a great translation of a complex issue. Oh yea – it also shows people that there are valid reasons to still wear N95 masks in crowded public spaces – you set a good example.

    1. Thanks, Roy. I did wear the mask when I was walking around the trade show by myself but took it off when I was other people because talking in a loud space with a mask on isn’t so easy.

  4. Your little tester would seem helpful at answering a confounding question ….. How safe is the air on a plane from the perspective of re-breathing the air expelled from other passengers, especially when Covid is spreading.

    1. Barry: Excellent question. I should have addressed that in the article, but it was already pretty long. Planes generally have pretty good filtration so that’s one of those cases where context matters. If you’re sitting right next to an infected person, the plane’s filtration system may not help you. That’s where a mask would be beneficial. But if the infected person is a few rows away, your risk should be a lot lower.

  5. I built a double stud home forty years ago. But in an air exchanger (deschamp) presently using WAVE monitor for VOC , temperature, humidity CO 2, and radon for the past year have been using Panasonic 200 erv. CO 2 levels are kept ~500 to 600. Use the boost when friends are over CO2 monitor always in sight I agree a (tight) house needs to have some sort of Indoor air quantity monitor. Live in heating zone three.

  6. No wonder I was disoriented while wandering the exhibits at AHR!

    Maybe we should look at this from the angle of what is in the air we exhale?
    A quick check of a couple sources: “nitrogen [78.04%], oxygen [16%], CO2 [4%-5%], hydrogen [5%], inert gases [0.9%], water vapor, inorganic VOCs: nitric oxide (10–50 ppb), nitrous oxide (1–20 ppb), ammonia (0.5–2 ppm), carbon monoxide (0–6 ppm), hydrogen sulphide (0–1.3 ppm) etc. and organic VOCs such as acetone (0.3–1 ppm), ethanol, isoprene (∼105 ppb), ethane (0–10 ppb), methane (2–10 ppm), pentane[0–10 ppb] etc. ” []

    Btw, you gotta love the “etc’s” in the above quote! One thing to remember: our lungs, with a respiratory area of about 100 sqm are the fastest delivery system. 7000ppm of CO2 is probably the least of your problems in a car with 4 people with recirculate on.

  7. I’m kind of surprised the CO2 levels are that high in the exhibit halls with that much air volume. Does air quality equalize or stratify within a space? For example, would CO2 readings be more/less/same at 20 feet above the floor?

    1. Phil: I was surprised, too, and for the same reason. I did feel air movement as I was walking around, so there should have been good mixing in the space. I wish someone had run a string of CO2 monitors from the floor to the ceiling to answer your questions.

    2. AHR exhibit hall was 1,120,428 sqft at 30ft height = 33,612,840 cubic ft.
      Assuming 2 people per 100sqft, 500ml exhaled air volume, CO2 is 4% of exhaled air, 20 breaths per minute [walking/talking], 500ppm (volume) starting CO2: we reach over 1000ppm after 1 hour, over 1600 after 2 hours, 2200 after 3 hours.

      (To simplify, I also assumed constant CO2 content in exhaled air. Also: 500ml exhaled air volume may be a bit low for people walking/talking.)

      If we bump up the exhaled volume to 700ml, we get to 2900ppm after 3 hours.

      The above is without ventilation. Allison’s readings fall in the same ballpark, so assuming the building was being supplied fresh air, maybe there is stratification of the heavier CO2 happening (poor mixing by the HVAC system)?

      1. Great conversation here. Looks like many of you were chewing on this overnight as well. Too many unknowns. What were the CO2 levels when the doors opened for the day? Were there any other sources of CO2 in the hall (unvented combustion appliances, etc.)? Possible CO2 introduction from outdoor air (vehicle exhaust, etc.). Stratification seems a likely cause. Many more questions to be answered. All of this could be tested with proper instrumentation and experimental procedure – and should be. I have been telling people to continue wearing N95 masks, and that they are likely in less danger of picking up airborne pathogens whilst in large spaces such as big box stores, grocery stores and such – am I wrong? Looks like a job for building science! All because Dr. Bailes brings his CO2 monitor to the AHR hall. Chief troublemaker indeed!

  8. Great discussion!! I’m definitely going to try to get a vertical string of CO2 monitors hung somewhere in the hall during Efficiency Vermont’s Better Buildings By Design conference in April…

  9. Allison, I was also at the AHR Expo and disappointed that I didn’t run into you. Did you happen to wander through Hall A? There were food trucks operating in it. I could smell and see the exhaust from the cooking appliances. I am not sure if the engines or generators were operating.

    1. Roy: Yeah, I’m sorry I missed you, too. I didn’t spend any time at all over on the ASHRAE side. And yes, I did go in the room with the food trucks and the smoky haze. The CO2 level wasn’t bad in there, but I’m sure the particles and VOCs would have been off the charts. Maybe even some carbon monoxide. Someone told me they had electrical hookups so they didn’t need generators. I think the smoke was from all the cooking on propane grills.

  10. On a related note, an addendum to ASHRAE Std. 62.2 to not allow unvented combustion devices (heaters, gas logs, etc.) finally passed the Board of Directors and all appeals were denied this time. It took over 10 years to make this happen. Unless it is blocked by ANSI, it should be published in the near future. Now if we could just get more jurisdictions to adopt 62.2 . . . .

    1. Well done! How anyone with even a basic “101” level understanding of gas combustion could think un-vented gas appliances in occupied spaces is OK is beyond belief. Clearly, those in favour lack that basic understanding, have great faith that nothing can go wrong, or just don’t care about occupant safety – maybe all of the above. An addendum should not be necessary, the whole concept should be expunged, especially given the recent hysteria over gas ranges – most of which at least have overhead exhaust hoods.

  11. In a former position at a major university, I used to check IAQ with a CO2 sensor. I had a student collect CO2 concentration data in the basketball arena with 18,000 people present. The CO2 levels rose linearly during the game indicating that there was no outdoor air ventilation. Discussions with the building maintenance personnel confirmed that.

    I then monitored CO2 levels in my classroom (a large lecture hall) which was recently renovated with the addition of central air conditioning. Once again I saw this linear rise of CO2 indicating that there was no outdoor air ventilation. I checked into this and found out that the air conditioning contractor did not install the outdoor air supply duct that was specified in the drawings. He got called back.

    There is an OSHA workplace limit for CO2 of 5000 ppm for an 8-hour average.

  12. My Awair always showed higher CO2 in the bed room, even higher than the kitchen with its gas pilots. I replaced the gas water heater on outside of bedroom wall (it finally died) with a HPWH (120v plugged, no resistance heating from Rheem) and CO2 went right down. Guess my walls and basement were not as well sealed as I hoped. Have to look into that. Also my gas bill went down 50% even after the deeply suspicious surge in prices.

    1. John, I am sure you’ll never look back, and it wasn’t just CO2 that went down!

      Going for the 15A Rheem HPWH is somewhat similar to choosing a right-sized heat pump for your home without the resistance emergency heat strips. The difference is, when you replace a WH, you replace the whole system (well, still got hot water delivery issues that Allison talked about in previous posts). When you replace a heating/cooling unit for your home, you’re only replacing the heat exchanger part of the system. The rest of this system, as John noticed, is the building envelope, just like the insulation surrounding the tank on a water heater.

  13. I believe they are using the Sensirion CO2 sensors.

    The SCD3x series have same electrical connection roughly same foot print but slightly different specs.

    The SCD4x are newer smaller sensors, resolution changes depending on the unit specified. The price changes a little.

    The big difference over the last 4 years as been the availability do to supply stream issues. E.g. they may have altered the spec and shipped with a difference sensor SCD41 instead of SCD40, or SCD30.

    I’ve used four of the different models, the boards I laid out – allow me to interchange SCD3x and SCD4x.

    For building CO2 analysis – they’re all pretty darn accurate… and low power.

  14. “Another place where the carbon dioxide level goes and stays above 1,000 ppm is on commercial flights. You’re packed into a long metal tube with a bunch of other people, breathing recirculated air with little fresh air added to it. ”

    Interesting. According to the WHO:
    “Most modern aircraft have recirculation systems, which recycle up to 50% of cabin air. Ventilation provides a total change of air 20–30 times per hour.”

  15. Really informative article as usual. It was helpful to read ASHRAE’s position on indoor carbon dioxide. That aside, do you personally feel that CO2 levels above 1000 ppm affect you negatively?

    1. The impact of CO2 on individuals when the levels climb above 1000ppm has insufficient data to prove immediate harm. For individuals the ability to perceive an impact to their ability to think or perform is often confused by the suggestion that the IAQ is impacting them.

      It’s not a hard red line that if you cross over it – you can feel or detect harm. In fact most people can’t detect it themselves – observers and researchers testing may be able to measure, then analyze the data and report the results days or weeks afterwards.

      The reason that the impact is seldom noticeable – or even an impact at all, is because our bodies over the 1000’s of years has adapted to regulate the CO2 balance that impacts our bodies blood chemistry. We are actually very good at responding to slow changes that return back to a normal healthy level. And that response depends a lot on the individual their current health (age, gender, prior existing health conditions, e.g. a person with COPD may respond quite adversely to higher CO2, just as a person with congestive heart failure).

      More importantly at this time is the education of the public that we want to manage the indoor CO2 levels to remain below 1000ppm if possible – in that the individuals at risk – are not harmed.

      Keeping it below 1000ppm also means that we’ve a better means to address the body odors — which was in part — some of the original research done by Pettenkofer back in the 1850 -1860’s (German public health).

  16. I also have been carrying my Aranet4 everywhere I go and have been getting similar readings at home, in restaurants, in theatres, Dr offices, and over 5,000 ppm CO2 in a SUV with 6 people and the heater/AC set to recirculation. Even by myself in the car the readings will jump to over 2,000 in a couple minutes when the fan is not on. Not surprising given that human breath is 35,000 to 50,000 parts per million (ppm) of CO2. The SUV readings dropped to about 1,100 in a couple minutes with the recirculation off and the fan on, at least in the front seats, not sure about the third row of seats in the back.
    We have a 10-day road trip coming up, the fan will be on ALL the time, and maybe a window opened once in a while. 😀
    I was impressed with readings of less than 1,000 in a packed auditorium at my granddaughters school. Hopefully their classrooms have similar ventilation.
    Maybe I should put the Aranet4 in their school backpacks for a couple days? 😉 Probably not, the girls are very curious and I am sure the school teacher would notice…
    Thanks for a great article.

  17. After having tested several other monitors in the $100 to $250 price range, I settled on the Aranet 4. I learned a lot about calibration…

    First, all the monitors I tested featured auto-calibrate. Some, including the Ararnet 4, can also be manually calibrated. I recommend avoiding monitors that cannot be manually calibrated unless the monitor is regularly exposed outside air. Then the question is how regularly? Good luck with that! If the monitor will be used primarily in one location, auto-calibrate should be avoided as it will periodically adjust its baseline to the lowest reading over some typically undefined time period. Some companies actually warn not to use the auto-calibrate feature, presumably because they’ve received so many complaints.

    Second, atmospheric pressure has a proportional impact on NDIR sensor readings. This requires some explanation… The impact pressure is significant if you use a CO2 monitor at higher elevations (or in an airplane where cabin pressure is typically set to about 8000 ft). Temperature also affects readings but much less so than pressure. Here’s a primer about that.

    Based on my testing, the Aranet 4 is the only CO2 monitor in the $100 to $250 price range that auto-compensates for temperature and pressure. I live at 4,400′ MSL so this was easy for me to test. One product has a user-defined altitude setting that helps, but it baffles me that none of the other monitors I tested implement the compensations described in the above link, even though most have both temperature and pressure sensors!

    If a monitor can be manually calibrated, that will adjust the baseline to the current elevation, but this just applies an offset. However, the pressure relationship is not linear… the error at higher CO2 levels can be substantial. I posted about this in the Aranet forum, and confirmed the results of my test of the Aranet 4.

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