The biggest source of carbon dioxide in most homes is the people living there. Every person exhales about 200 liters of CO2 per day at rest, and that number climbs with physical activity. But breathing isn’t the only contributor. Cooking, heating, poor ventilation, and even pets all add CO2 to your indoor air, and in a tightly sealed modern home, those levels can climb high enough to affect how you think and feel.
Your Breathing Is the Primary Source
Every breath you take pulls in air containing about 0.04% CO2 and pushes out air containing roughly 5% CO2. Over a full day, a single adult at complete rest produces around 785 grams of CO2. Add moderate activity for 16 waking hours and that figure rises to about 2.2 kilograms per person per day. In a household of four, that’s nearly 9 kilograms of CO2 released into your home’s air every 24 hours, just from breathing.
Pets contribute too. A medium-sized dog produces CO2 at roughly the rate of a small child, and even a cat adds a measurable amount. The more living, breathing occupants in a space, the faster CO2 accumulates.
Gas Stoves, Ovens, and Heating
Any appliance that burns fuel inside your home produces CO2 as a byproduct of combustion. Gas stoves and ovens are common culprits, releasing CO2 (along with nitrogen dioxide and water vapor) directly into your kitchen every time you cook. A gas burner running for 30 to 45 minutes in a small kitchen can push CO2 levels well above 1,000 ppm if the room isn’t well ventilated.
Unvented space heaters are particularly significant sources. Kerosene heaters, propane heaters, and natural gas space heaters that lack a flue or exhaust vent release all their combustion products straight into the room. Research on kerosene space heaters confirms that CO2 levels routinely exceed guideline values during operation, with concentrations climbing in direct proportion to how long the heater runs. Vented furnaces and boilers, by contrast, channel exhaust gases outside, so they contribute far less to indoor CO2.
Fireplaces and wood stoves fall somewhere in between. When drafting properly, most combustion gases go up the chimney. But a poorly sealed fireplace or one with a weak draft can leak CO2 and other pollutants back into the room.
How Tight Construction Traps CO2 Inside
The amount of CO2 in your home depends not only on how much is produced but on how quickly fresh outdoor air replaces it. This is measured in air changes per hour (ACH), the number of times per hour that a volume of air equal to the room’s total volume gets swapped with outside air.
Modern energy-efficient homes are built tight. A home with an air change rate of just 0.2 per hour (common in newer, well-insulated construction) will reach a steady-state CO2 concentration about 660 ppm above outdoor levels from normal occupancy alone. Since outdoor air already sits around 423 ppm as of 2024, that puts your indoor reading above 1,080 ppm with no combustion appliances running at all.
Bump the air change rate to 0.35 per hour, and the buildup above outdoor levels drops to about 377 ppm. At 0.5 air changes per hour, it drops further to 264 ppm above outdoor baseline. The practical takeaway: the tighter your home, the more important intentional ventilation becomes. Opening windows, running exhaust fans, or using a mechanical ventilation system all increase air exchange and keep CO2 from climbing.
Less Obvious Sources
Several smaller sources can add to the total, especially in enclosed spaces. Home brewing is one that surprises people. Fermenting a standard 5-gallon batch of beer produces roughly 400 to 570 liters of CO2 gas, released slowly over one to two weeks. In a basement or closet with little airflow, that’s enough to noticeably raise concentrations in the immediate area.
Indoor plants are sometimes cited as CO2 sources because they release CO2 through respiration at night when photosynthesis stops. This is technically true. Leaf respiration continues in darkness, though the rate drops by about 25% over an 8-hour night. In practical terms, however, the CO2 output from a few houseplants is negligible compared to a single sleeping person. You would need hundreds of large plants in a sealed room to meaningfully shift the needle.
Other minor contributors include candles, incense, and tobacco smoke, all of which involve combustion and release CO2 along with other pollutants.
What Indoor CO2 Levels Mean for You
Outdoor air averages about 423 ppm of CO2 globally. Inside a well-ventilated home, levels typically sit between 400 and 700 ppm. In bedrooms overnight, crowded living rooms, or homes with low ventilation, readings of 1,000 to 2,000 ppm are common, and poorly ventilated bedrooms with the door closed can exceed 2,500 ppm by morning.
These numbers matter more than most people realize. A controlled study from the Harvard T.H. Chan School of Public Health found that cognitive function scores dropped 15% when CO2 levels reached about 945 ppm and fell 50% at around 1,400 ppm compared to cleaner air conditions. Seven of the nine cognitive domains measured, including strategic thinking and information usage, declined at each higher concentration. On average, every 400 ppm increase in CO2 was associated with a 21% drop in cognitive performance.
Despite a widespread belief that 1,000 ppm is an official indoor limit, no current ASHRAE ventilation standard actually sets a CO2 concentration cap. That figure was removed from the standard in 1999. Still, the cognitive research suggests that keeping levels below 1,000 ppm is a reasonable target for comfort and mental sharpness.
Practical Ways to Reduce Indoor CO2
Since most indoor CO2 comes from breathing and combustion, the most effective strategies center on ventilation. Running a kitchen exhaust fan that vents outdoors (not a recirculating filter hood) while cooking with gas removes combustion byproducts at the source. Cracking a window in bedrooms at night, even slightly, prevents the steep overnight CO2 buildup that comes from sleeping in a sealed room.
If your home has a mechanical ventilation system, such as an HRV or ERV (heat or energy recovery ventilator), keeping it running continuously provides a steady supply of fresh air without wasting much heating or cooling energy. For homes without one, bathroom and kitchen exhaust fans create negative pressure that pulls fresh air in through gaps and intentional vents.
A portable CO2 monitor, available for around $100 to $200, lets you see exactly how levels fluctuate in different rooms and at different times of day. Many people are surprised to find that their bedroom hits 1,500 ppm or more by 6 a.m. with the door closed. Simply leaving the bedroom door open can cut that peak significantly by allowing air to mix with the rest of the house.