The most effective way to reduce indoor CO2 is to bring in more outdoor air, either by opening windows or improving your ventilation system. Every person in a room exhales about 200 liters of CO2 per day at rest, and in a poorly ventilated space, concentrations can climb past 1,000 ppm within an hour or two. The good news: most fixes are simple, cheap, and immediately effective.
Why Indoor CO2 Builds Up
Outdoor air typically sits between 380 and 500 ppm of CO2. Indoors, the primary source is you. A resting adult breathes roughly 6 liters of air per minute, and about 5% of every exhaled breath is CO2. That works out to roughly 785 grams of CO2 per person per day. During light activity, breathing rate nearly triples, pushing CO2 output up proportionally.
In a sealed bedroom or office with a few people, concentrations can reach 1,500 to 2,500 ppm in well under an hour. Cooking with gas adds to the load. The only thing that brings levels back down is replacing stale indoor air with fresher outdoor air.
What High CO2 Does to Your Brain
The standard ventilation guideline from ASHRAE (the main engineering body for building systems) sets 1,000 ppm as the maximum for adequate ventilation. That number isn’t arbitrary. In controlled experiments, researchers found moderate declines in decision-making performance at 1,000 ppm and more substantial impairment at 2,500 ppm, affecting 7 out of 9 cognitive measures tested.
A Harvard study on office workers found that for every 500 ppm increase in CO2, response times slowed by 1.4 to 1.8%, and overall work throughput dropped by 2.1 to 2.4%. The researchers found no lower threshold where the effect disappeared, meaning even modest increases in CO2 had a measurable cost. If you’ve ever felt foggy or sluggish in a packed conference room, this is likely part of the reason.
Open Windows Strategically
Opening windows is the fastest, simplest intervention. In one study, researchers measured a roughly 350 ppm drop in CO2 when windows were opened in a diagonal cross-ventilation setup with seven occupants in the room. Most of that reduction happened in the first 15 minutes, after which levels stabilized. Wider openings cleared CO2 faster: a 25 cm opening outperformed a 5 cm opening significantly.
For the best results, open windows on opposite or diagonal sides of a room to create cross-ventilation. Even cracking a window on one side helps, but airflow is much stronger when air has both an entry and exit path. In cooler weather, short bursts of wide-open windows (10 to 15 minutes) can flush a room without losing too much heat.
Improve Your Mechanical Ventilation
If opening windows isn’t practical because of noise, weather, pollution, or building design, your HVAC system is the next lever. The key variable is how much outdoor air it pulls in versus how much indoor air it simply recirculates.
Most central HVAC systems have an outdoor air intake damper that controls this mix. In many homes, the damper is set conservatively to save energy, or it may be stuck closed. Having an HVAC technician verify that your outdoor air damper is functional and properly sized is one of the highest-impact steps you can take. Motorized dampers can be set on a timer to open at intervals, pulling in fresh air when the system runs without staying open continuously.
For homes without ducted systems, standalone energy recovery ventilators (ERVs) or heat recovery ventilators (HRVs) are purpose-built for this problem. They continuously exchange indoor air for outdoor air while transferring heat between the two streams, so you get fresh air without a big energy penalty. These units typically mount in a wall or ceiling and can ventilate a single room or a whole house depending on capacity.
Exhaust Fans and Spot Ventilation
Bathroom and kitchen exhaust fans pull air out of your home, which forces fresh air in through any available gap or intake. Running your kitchen range hood while cooking (especially with a gas stove, which produces CO2 directly) makes a noticeable difference. Keeping bathroom exhaust fans running for 15 to 20 minutes after a shower also helps cycle air through the house.
If your home is tightly sealed and you run exhaust fans frequently, you’ll want a dedicated fresh air intake somewhere in the building to avoid creating negative pressure, which can cause backdrafting from combustion appliances like furnaces or water heaters.
Manage Occupancy and Room Size
CO2 is a per-person problem, so the number of people in a space relative to its volume matters enormously. A small bedroom with two adults and a closed door will hit 1,500 ppm far faster than a living room with the same two people. Sleeping with the bedroom door open, or at least cracked, lets CO2 disperse into the rest of the house rather than concentrating around you overnight.
In offices or shared spaces, spreading people across more rooms or staggering meeting times in conference rooms reduces peak CO2 buildup and gives ventilation systems a chance to keep up.
Houseplants Won’t Solve This
This is one of the most common suggestions online, and it doesn’t hold up. A study published in PLOS One that measured the effect of adding plants to office buildings found no significant association between the number of indoor plants and CO2 concentration. Plants do absorb CO2 during photosynthesis, but the rate is far too slow to offset what even a single person exhales. You would need an impractical density of plants under grow lights to make a dent. Houseplants have real benefits for humidity and aesthetics, but CO2 reduction isn’t one of them.
Monitor Your Levels With a CO2 Meter
You can’t feel the difference between 600 ppm and 1,200 ppm, so a CO2 monitor is the only way to know where you stand. Consumer-grade monitors range from about $80 to $250 and sit on a desk or shelf, displaying real-time readings.
Look for a monitor that uses an NDIR (non-dispersive infrared) sensor rather than a solid electrolyte type. NDIR sensors offer better long-term stability, accuracy, and lower power consumption. Most reputable consumer monitors (Aranet4, CO2.click, and similar devices) use NDIR sensors and are accurate to within about 50 ppm.
Once you have a monitor, the numbers will tell you exactly which interventions work in your specific space. You’ll likely notice that CO2 climbs predictably overnight in bedrooms and during cooking, and that it drops rapidly when you open a window or two. Aim to keep levels below 800 ppm for optimal cognitive performance, and treat anything above 1,000 ppm as a signal that your space needs more fresh air.