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Building Science

What Determines Indoor Carbon Dioxide Levels?

A number of factors can influence CO2 concentrations in a building. Knowing what they are and how ventilation requirements change depending on circumstances is a good first step toward healthy indoor air quailty.

Carbon dioxide (CO2) is often in the news as the leading contributor to human-induced climate change. Since the start of the Industrial Revolution, the burning of fossil fuels, land clearing, and other human activities have led to an increase in outdoor CO2 from ~280 parts per million (ppm) to 417 ppm. Humans and animals also exhale CO2 as a normal part of their metabolism, and CO2 levels in occupied buildings can be several times higher than outdoors. CO2 levels in homes frequently exceed 1000 ppm and can sometimes approach 3000 ppm.

For decades, the conventional wisdom among indoor air specialists was that at these levels, CO2 itself is not toxic. But because humans constantly exhale it, it could act as a proxy for “bioeffluents”—the odors, droplets, and particles that humans give off as they breathe and perspire. By ventilating to keep CO2 low, other, harder-to-measure bioeffluents could be diluted to acceptable levels.

The arrival of a new and particularly nasty bioeffluent—the COVID-19 virus—has spurred interest in indoor CO2. Since CO2 and the virus are both exhaled in human breath, increasing ventilation to lower CO2 can also reduce rates of COVID-19 transmission.

It’s long been known that, at very high concentrations, CO2 is an asphyxiant. Levels exceeding 40,000 ppm are an immediate life hazard; OSHA sets the 8-hour permissible exposure at 5000 ppm. In the last 15 years, studies have suggested that the CO2 levels routinely encountered in homes may themselves impact human health and performance. CO2 concentrations above 1000 ppm, the threshold long recommended for managing bioeffluents, can decrease performance on cognitive tests. Moderately elevated CO2 has also been linked to reduced sleep quality, fatigue, elevated blood pressure, and sick-building syndrome.

The need for an HRV

My first experience managing indoor CO2 came several years ago at my company office. We built the 1000-sq.-ft.…

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  1. Expert Member


    Another in a series of excellent blogs. Keep them c0ming!

    1. Jon_Harrod | | #3

      Thanks, Malcolm. It has been a while since I've done any kind of simulation modeling. It's a fun and eye-opening way to explore building science issues. Subject to field verification of course!

  2. tekjunkie28 | | #2

    I would like to add into this my own house and measurements I have taken.
    My house is a 1972 Brick ranch style with finished basement and garage. Occupancy is 2 adults, 2 kids, 2 labs and 4 cats. Measured ACH50 is 8.38 and that was taken in December, Location in Va. I have sealed up the house a bit more but I cant tell because my electric bills are still through the roof. CO2 measurements with people are home easily is around 1500ppm. At night with the door open and 1 dog the master bedroom CO2 levels go up to 2000ppm. After 7 hours of no humans at home CO2 level are still above 1000. I even normally keep a window cracked but it doesn't seem to help. Fans help A LOT but even my "quiet" bathroom fan is louder then I want to hear.

    I'd love to be able to make changes to improve these numbers but equipment cost and a 22 year old Heat pump are making moving ahead practically impossible.

    1. Jon_Harrod | | #4

      I noticed the same thing with cracking a window--it doesn't seem to help much unless there's also a fan driving the air movement. Have you looked into Panasonic bath fans? These are the quietest I'm aware of, especially ones in the 50-80 cfm range. Regarding the heat pump and electric bills, have you checked that your aux (resistance) heat is properly configured? I've read several accounts of the expensive aux heat coming on when the heat pump should be doing the work.

      1. tekjunkie28 | | #6

        Im pretty sure my electric strips are off. I average around $200 a month for electric and during january that cost goes up to $275-$300+. I could double check up I'm positive thats not the issue. I would think it would cost me for if that was the issue and run times would be shorter. I have foamed and caulked and foamed and caulked. I need to add more attic insulation but I need to get ductwork fixed before I do all that.

        I have just replaced the bathroom fan about 2 years and its semi quiet. Much quieter then the last one but its tied into the bathroom lights. I need to get up there and add a switch and dedicated wiring.

    2. Expert Member
      Michael Maines | | #5

      Part of the problem may be that CO2 is about 1.5 times as dense as air, so it really needs fans to push it around.

  3. fresnoboy | | #7

    I am dealing with high CO2 levels in our new home - up about 1500 if I keep all the doors closed, and all 6 of us and the dog are at home. The house is very tight, but does have an ERV. Even with the ERV running, CO2 levels vary in the house, and as I posted in a forum, I also see very high levels >5000, around the sewer ejector pump, though I don't smell any sewer gas.

    I am trying to get a technician with a lab grade instrument to measure it, but could be leaks from the ejector pump causing the dense CO2 to pool in the basement mechanical room. HAs anyone here seen anything like that causing CO2 levels to rise in a tight house?

    We do cook a lot on a gas range, but we do have a nice ventilation system and make up air for it. My wife and I are good about switching it on, but the teenagers usually forget to turn on the vent. I am working on using a CO2 sensor in the kitchen to turn that on automatically. :)

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