Better Air Quality Sensing With CO2

Measuring air quality, as anyone who has tried to tackle this problem can attest, is not as straightforward as it might seem. Even once the nebulous term “quality” is defined, most sensors use something as a proxy for overall air health. One common method is to use volatile organic compounds (VOCs) as this proxy but as [Larry Bank] found out, using these inside a home with a functional kitchen leads to a lot of inaccurate readings. In the search for a more reliable sensor, he built this project which uses CO2 to help gauge air quality.

Most of the reason that CO2 sensors aren’t used as air quality sensors is cost. They are much more expensive than VOC sensors, but [Larry] recently found one that was more affordable and decided to build this project around it. The prototype used an Arduino communicating over I2C to the sensor and an OLED screen, which he eventually put in a 3D printed case to carry around to sample CO2 concentration in various real-world locations. The final project uses a clever way of interfacing with the e-paper display that we featured earlier.

While CO2 concentration doesn’t tell the full story of air quality in a specific place, it does play a major role. [Larry] found concentrations as high as 3000 ppm in his home, which can cause a drop in cognitive function. He’s made some lifestyle changes as a result which he reports has had a beneficial impact. For human-occupied indoor spaces, CO2 can easily be the main contributor to poor air quality, and we’ve seen at least one other project to address this concern directly.

19 thoughts on “Better Air Quality Sensing With CO2

    1. VOC sensors are not necessary from user standpoint and absolutely necessary from design perspective because cheap “eCO2” sensors substitute actual CO2 detection with temperature corrected VOC to CO2 lookup table. This is by the way explained in TFA.

    2. Yes, outdoor level of CO2 is normally around 400ppm. Indoors it can quickly rise to 800ppm and beyond which has been shown to cause a reduction in productivity of thinking-based tasks by 15%. Schools have been found to have in some cases CO2 levels exceeding 2500ppm (0.25%)! Deadly levels are around 2%. For reference, air normally contains around 20% oxygen. In a completely sealed room, CO2 levels will rise to lethal levels long before oxygen becomes scarce.

      In the days before extreme concern about power usage, people used to open up windows when they found the rooms became stuffy. Not any more. Classrooms and work buildings have been built with windows that normally can’t be opened.

  1. Most of the reason that CO2 sensors aren’t used as air quality sensors is because CO2 has little to do with the primary causes of poor air quality. You’d be better of getting a few potted plants if you are irrationally fearful of CO2 due to the programming you received at school.

    1. VOCs and CO2 are two separate issues. VOCs can be hazardous in the absence of elevated CO2, and elevated CO2 can be hazardous in the absence of VOCs.

      And if you think schools ‘program’ people, you may wish to consider that many people possess the capability of critical thinking, beyond the blind belief you evidently consider the norm.

      1. CO2 is very rarely hazardous and you will get a perceivable physiological response to high levels long before you reach disabling levels. Low O2 is another matter, same with CO levels.

  2. CO2 is not a air contaminant. I don’t see any of the mask zealots concerned about it either, and I’m sure wearing a rag over your face hole leads to a ton of rebreathing of CO2. And any medical professional (surgeons etc) have been wearing masks for hours on end and no untoward CO2 effects. And also permissive hypercapnia is a real and useful ventilation strategy in the ICU. But that is just medicine and science or whatever.

    1. As you know, CO2 goes right through masks, and surgeons don’t routinely collapse from CO2 poisoning or other imaginary effects of masks, though CO2 in much higher quantities can cause problems.

      What indoor CO2 levels really tell you is how good the air circulation is; if the CO2’s accumulating, so is everything else, including airborne Covid viruses if people are around.

  3. Yes and no. CO2 in “household” quantities (not submarine, or closed rebreather systems) is not a health risk. But because CO2 sensors are cheap, CO2 can be measured as a proxy for other pollutants: If your CO2 levels are up, you haven’t aired out the house in a while and a fan can kicked in.
    But that’s rough air quality, more people in the living room doesn’t mean your new furniture emits more formaldehyde. And HVAC systems are better at filtering, so the CO2 to house dust correlation may also be weak.
    But CO2 becomes interesting again with airborne illnesses. There’s a MIT Covid-estimator that calculates transmission probability depending on people per volume and fresh air intake. Probably holds true for the seasonal sniffelsflucold as well. A CO2 sensor could moderate a room’s air intake depending on occupancy.

  4. Of all the modern heaters in use, a gas furnace is the only one that routinely sucks in fresh air for newer installations. Gas recirculating hot water (e.g. radiators, which is what I have), electric (coil), and heat pump heating, and cooling systems don’t “need” fresh air except in commercial environments where there are LOTS of people exhaling. I have a 100+ yo house with lots of drafts, and not many occupants, so I don’t seen my CO2 climb much above the 400 ppm ambient level. Modern homes are so well-sealed that CO2 builds up quickly compared to my drafty house. I have an “all in one” air quality sensor and discovered that it alarmed when I sprayed either a little “green” bug spray (chrysanthemum based) or an (anti-stink) air “freshener”, since they now use CO2 propellant instead of a Freon to reduce climate change (don’t snicker).

  5. For those claiming low CO2 concentrations are not an issue merely because they are below fatal levels: the LD50 of airborne Sulphur and Selenium compounds is well, well above the level required for you to experience a second taste of your breakfast if you inhale them. Elevated CO2 levels well below toxic have a demonstrated effect on cognitive performance ( and mood.

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