A good CO2 level in your home is under about 800 ppm (parts per million), and the daily average should stay below 1,000 ppm. That’s the limit Health Canada set in its 2021 guideline on CO2 levels in homes. Outdoor air is about 430 ppm, so that’s as low as a house can get.
Most of the time, CO2 isn’t dangerous at the levels a home reaches. What it tells you is how well the air in a room is being replaced. A high reading means the room needs fresh air. This guide explains what each level means, what causes high CO2 levels in houses, and how to bring them down.
The Short Answer
Here’s what each CO2 level means at home, and what to do about it.
| CO2 level | What it means | What to do |
|---|---|---|
| About 430 ppm | Outdoor air | Nothing: it’s as low as a home can get |
| Under 800 ppm | Good ventilation | Nothing |
| 800-1,000 ppm | Acceptable | Keep the daily average under 1,000 ppm |
| 1,000-1,500 ppm | The air isn’t being replaced fast enough | Open a window or run the ventilation |
| 1,500-2,500 ppm | Poor ventilation, common in closed bedrooms | Ventilate the room now |
| 5,000 ppm | US workplace limit for an 8-hour shift | Far above normal for a home: check your fuel-burning appliances |
What CO2 Levels in Homes Actually Tell You
Carbon dioxide (CO2) is the gas you breathe out. Indoors, most of it comes from the people in the room. Smoking and fuel-burning appliances that aren’t vented properly add more, according to Health Canada. The more people there are, and the longer they stay in a closed room, the higher the level goes.
At the same time, ventilation brings in outdoor air at about 430 ppm. So the reading on a CO2 monitor shows how well those two things are balanced: how much CO2 people add, compared with how much fresh air comes in. When the air isn’t being replaced, everything else people and the house give off builds up too, such as odors, moisture and chemicals from cleaning products and furniture. That’s why CO2 is mostly used as a sign of ventilation, not as a pollutant on its own.
ASHRAE, which writes the main US ventilation standards, is clear about one common mistake. In a brief updated in June 2026, it says that claims its Standard 62.1 requires indoor CO2 below 1,000 ppm are incorrect. Its ventilation standards for buildings and homes don’t set any CO2 limit for health or comfort. It also points out that a low reading doesn’t prove the air is clean, but a reading far above the normal range likely means the ventilation isn’t working.
If you’re choosing a device to watch these numbers, our guide to the best indoor air quality monitors compares options with accurate CO2 sensors. For radon, a gas a CO2 monitor can’t detect, see our guide to the best radon detectors. To check your home’s level first, read our guide on how to test for radon.
What Are Safe CO2 Levels in Homes?
Health Canada gives the clearest answer for homes. Its guidance on carbon dioxide in your home sets a long-term exposure limit of 1,000 ppm, averaged over 24 hours. Japan, Korea, France, Germany, Norway and Portugal use standards or guidelines between 600 and 1,000 ppm. The US has no federal CO2 limit for homes.
Note that 1,000 ppm is a daily average, not an alarm level. A bedroom can pass 1,000 ppm for a few hours overnight and still have a lower average across the day.
Above that level, Health Canada lists tiredness, headaches, eye irritation, a dry throat, dizziness and trouble concentrating. In the studies it reviewed, headaches and tiredness were reported more often above 1,500 ppm. However, Health Canada adds that these effects may not come from CO2 itself, but from poor indoor air quality in general.
The research on concentration is mixed. A 2012 study by Lawrence Berkeley National Laboratory found lower decision-making scores at 1,000 ppm than at 600 ppm, but it had only 22 participants. A 2017 Danish study added pure CO2 up to 3,000 ppm and found no effect, while the other things people give off did affect them. ASHRAE describes the evidence at normal indoor levels as inconsistent. In practice, this doesn’t change much: keeping CO2 low means you’re ventilating, and that helps either way.
Truly dangerous levels are much higher. The US workplace limit (OSHA) is 5,000 ppm averaged over an 8-hour shift, and NIOSH considers 40,000 ppm immediately dangerous to life. Homes don’t reach those levels in normal use.
Don’t confuse CO2 with carbon monoxide (CO). Carbon monoxide comes from fuel that doesn’t burn completely, it has no smell, and it’s dangerous at much lower levels. A CO2 monitor doesn’t detect it, so you still need a CO alarm. Our guide to smart smoke and carbon monoxide detectors covers options that send alerts to your phone.
What Causes High CO2 Levels in Houses
High CO2 levels in houses are almost always caused by people and too little fresh air. These are the usual causes:
- A closed bedroom overnight. Two people breathing for eight hours in a closed room add a lot of CO2. Researchers at the Technical University of Denmark (DTU) found that closed bedrooms without a ventilation system are typically above 1,150 ppm. In their research, sleep wasn’t affected below about 750 ppm, and above 2,600 ppm both sleep and the next day’s performance were affected.
- A tight, energy-efficient house. Good air sealing saves energy, but it also keeps stale air in. Without mechanical ventilation, a tight house has few ways to replace its air.
- Winter. With the windows closed, CO2 rises. In Health Canada’s studies of Canadian homes, the median level in winter was about 80 to 160 ppm higher than in summer.
- Guests and crowded rooms. A dinner party in a closed living room raises CO2 quickly, and so does a small apartment with several people in it.
- Fuel-burning appliances. A gas or oil appliance that isn’t vented properly, or an unvented space heater, adds CO2 (and other combustion gases) to the room.
How to Lower the CO2 Level in Your House
To lower CO2, you need to replace indoor air with outdoor air. Here’s what works:
- Open a window. It’s the fastest fix. In the DTU research, opening a window ventilated a bedroom better than opening the door. If the outdoor air is poor (wildfire smoke, for example), Health Canada suggests limiting outdoor air instead.
- Use mechanical ventilation. If your home has a ventilation system, run it on a higher setting or for longer, and have it checked. A heat recovery ventilator (HRV) or energy recovery ventilator (ERV) brings in outdoor air and removes the same amount of stale air, using the outgoing air to warm or cool the incoming air. An ERV also transfers moisture, which helps keep the humidity in your house at a comfortable level.
- Run the exhaust fans. Use the kitchen range hood (vented outside) when you cook and the bathroom fan when you shower. When they pull air out, outdoor air comes in to replace it.
- Keep fuel-burning appliances vented. Have furnaces, water heaters and gas appliances serviced, and don’t use unvented fuel-burning space heaters indoors.
Running the furnace fan, or leaving interior doors open, spreads the air between rooms. That can bring down the level in a crowded bedroom, but it doesn’t bring in outdoor air, so it doesn’t lower the total.
Two common ideas don’t help. Air purifiers don’t remove CO2: ASHRAE notes that CO2 isn’t affected by filtration and most air cleaning. A purifier is still useful for dust, pollen and smoke (see our guide to HomeKit air purifiers), but not for CO2. Houseplants don’t help either: research from the University of Reading found that the CO2 plants absorb at normal indoor light levels is very small.
How to Measure CO2 at Home
To get a real number, you need a monitor with an NDIR (non-dispersive infrared) sensor. It measures CO2 directly by checking how much infrared light the air absorbs.
Many cheaper air quality monitors, and some thermostats, show “eCO2” instead. That’s an estimate calculated from a VOC (chemical) sensor, not a CO2 measurement, so it can be wrong when chemicals and CO2 don’t rise together. The ecobee Premium thermostat, for example, shows estimated CO2 along with its air quality reading. We compare it with the other models in ecobee Essential vs Enhanced vs Premium.
To get readings you can trust:
- Place the monitor where people breathe. A shelf or nightstand at about head height works well, but not right next to someone’s face, a window or a vent.
- Check it outdoors. A good monitor should read close to outdoor air, about 430 ppm. If it reads far higher, it may need to be calibrated.
- Give it fresh air now and then. Many NDIR monitors calibrate themselves against fresh air. Aranet, for example, says the Aranet4 needs about 8 hours of fresh air (around 420 ppm) in every 30-day period, so open the window regularly in the room where it sits.
A smart monitor can also do something about the level. Some work with Apple Home, Alexa or Google Home, so an automation can turn on a fan or remind you to open a window when CO2 passes 1,000 ppm. Our guide to HomeKit temperature sensors includes an air monitor that shows CO2 in the Apple Home app.
Conclusion
A good CO2 level in your home is under 800 ppm, and safe CO2 levels in homes mean a daily average below 1,000 ppm. Outdoor air, at about 430 ppm, is the lowest you can reach. Readings between 1,000 and 2,500 ppm aren’t dangerous, but they tell you the room needs fresh air, and closed bedrooms at night are where that happens most. Open a window, run the ventilation or the exhaust fans, and measure with an NDIR monitor so you know the numbers are real.
What is a good CO2 level for a bedroom?
Under about 800 ppm. Researchers at the Technical University of Denmark found that sleep wasn’t affected below about 750 ppm, while closed bedrooms without ventilation typically go above 1,150 ppm overnight. Opening a window is the fastest way to keep a bedroom in the good range.
Is 1,000 ppm of CO2 bad?
No, but it’s a sign the room needs more fresh air. Health Canada sets 1,000 ppm as the long-term limit for homes, averaged over 24 hours, so a short time above it isn’t a problem. If the level stays above 1,000 ppm for most of the day, improve the ventilation.
Is 2,000 ppm of CO2 dangerous?
Not in the short term. The US workplace limit is 5,000 ppm over an 8-hour shift. At 2,000 ppm, though, the room is poorly ventilated, and headaches and tiredness are reported more often above 1,500 ppm. Open a window or run the ventilation.
What level of CO2 is dangerous?
OSHA’s workplace limit is 5,000 ppm averaged over 8 hours, and NIOSH considers 40,000 ppm immediately dangerous to life. Homes don’t reach those levels in normal use. If readings stay high when nobody’s home, have your fuel-burning appliances checked.
Do air purifiers lower CO2?
No. Air purifiers filter particles such as dust, pollen and smoke, but CO2 isn’t affected by filtration. To lower CO2, you need to bring in outdoor air: open a window, run a ventilation system such as an HRV or ERV, or use the exhaust fans.
Does a carbon monoxide detector detect CO2?
No. Carbon monoxide (CO) and carbon dioxide (CO2) are different gases, and each one needs its own sensor. A CO alarm protects you from carbon monoxide, which is dangerous at much lower levels, and a CO2 monitor tells you when a room needs fresh air. Any home with fuel-burning appliances or an attached garage needs CO alarms.
Written by Evan Mercer
Founder & Lead Researcher, Smart Home Matrix
Evan digs through spec sheets, firmware changelogs, and thousands of owner reviews so you don’t have to. Every guide starts with a compatibility check and ends with picks he would install in his own home. Read more about Evan and this site, or see exactly how we choose products.