What a room CO2 monitor can show
A rising graph can show that fresh air isn't keeping up with the people in the room. Test one safe ventilation change and see whether the level falls. A reading of 1,000 ppm doesn't prove that CO2 harmed your thinking.
Source
People usually drive indoor CO2 up
Breathing adds CO2. Occupancy, body size, and activity affect how quickly it accumulates.
Building
Fresh air and airflow shape the graph
Open doors or windows, HVAC controls, and the sensor position can all change the reading.
Monitor
Check placement and calibration
Place the sensor away from faces, windows, vents, and corners, and check whether its readings drift over time.
Limit
The monitor isn't a complete air-quality meter
It doesn't detect carbon monoxide, particles, VOCs, mold, viruses, or a person's oxygen.
Studies don't agree on whether common indoor levels directly harm thinking. A large 2026 trial found no clear short-term effect up to 4,200 ppm. Ventilation still matters because people also add heat, odors, and other pollutants to a room.
Save this test
Save the curve, not one alarming number
My Fog keeps the room, number of people, sensor position, and ventilation change with the graph.
Repeat with similar numbers of people when testing a building change. Use CO2 to judge rooms, not your medical recovery.
Ventilation and safety by age, sex, and health
The monitor reads the room, not your body. There are no separate cutoffs for children, adults, men, women, pregnancy, or older age. The number of people, their size and activity, and the fresh air entering the room affect the reading.
Infants and children
Judge a bedroom, classroom, or childcare room by how many children are there, what they're doing, and the ventilation rules for that space.
Teenagers and adults
Check study, work, exercise, sleep, and meeting rooms separately. More people and more movement make CO2 rise faster.
Women, pregnancy, and postpartum
Choose ventilation that doesn't bring in smoke, unsafe heat or cold, or create a security risk. During pregnancy or after birth, get medical help for lasting trouble breathing, chest pain, fainting, or confusion.
Men
Larger bodies and harder activity can add CO2 faster.
Older adults and people with health conditions
The graph can help check the room's ventilation. It can't measure a person's oxygen or lung function. New or severe symptoms still need medical care when the room reading looks ordinary.
How to place and check the monitor
Choose a monitor made to measure CO2. A true infrared (NDIR) sensor that saves its readings helps. A device that estimates CO2 from VOCs doesn't measure it.
Put it where people breathe and air can move around it. Keep it away from faces, windows, doors, vents, heaters, cooking, and direct sun. EPA suggests about 3 to 6 feet above the floor.
Let the monitor warm up and follow its calibration guide. Check whether the reading looks reasonable outdoors or in a room with plenty of fresh air.
Record the room, date, number of people, activity, doors, windows, HVAC, weather, temperature, humidity, and any cooking or fuel burning.
Measure the room in use
Measure the bedroom overnight, classroom during lessons, or work room during normal use. An empty-room reading doesn't show the room in use.
Test one ventilation change
Try one safe change, such as opening a door or window or changing the exhaust or HVAC setting. Compare the same room with the same number of people.
Read the curve
Look at the starting level, rise, peak, time above your chosen baseline, and recovery after people leave or ventilation changes.
Check the monitor
If the number makes no sense or doesn't change outdoors, check the warm-up, placement, calibration, sensor type, and possible drift before you act.
Check other hazards separately
Use a working carbon-monoxide alarm for fuel burning, PM2.5 monitoring for particles and smoke, and building checks for dampness or VOCs.
How to understand an occupied-room CO2 curve
Start with the outdoor reading and the monitor position. Then check the rise, peak, time above your baseline, and recovery. Check how many people were there and how the HVAC was running.
Near the chosen baseline
Near the room's occupied baseline or below a chosen ventilation target
Fresh air may have kept up with the people in the room during this test. Other air problems may still be present.
Repeated rise
Repeated rise above the chosen baseline
Check the number of people, their activity, the sensor position, calibration, and HVAC. CDC uses below 800 ppm as one possible baseline, not a rule for every room.
Persistent or extreme reading
Persistent or unexpectedly high readings with people present
Check whether the room gets enough fresh air for how it's used. Leave and get help for a possible gas release, fuel-burning problem, or severe symptoms.
800 and 1,000 ppm aren't universal health cutoffs
CDC offers below 800 ppm as one possible baseline. ASHRAE explains that 1,000 ppm was tied to ventilation per person and odor perception. Room type, people, activity, and air distribution change what either number means.
See research details
The notes below cover room targets, thinking studies, sensor limits, filters, and dangerous exposures.
Choose a baseline for the room's actual use, then compare readings taken with similar HVAC settings and numbers of people.
Do not label 999 healthy and 1,001 harmful. Use the value as a ventilation guide.
Lowering one room number doesn't guarantee a fixed gain in thinking. Better ventilation may still improve the wider indoor environment.
Note where the device sits, then repeat in another typical spot before deciding the whole room behaves the same way.
Match the action to the measured problem and check outdoor air before changing ventilation.
For unexpected extreme readings, sudden breathlessness, confusion, collapse, or a possible fuel-burning or confined-space problem, leave and get emergency help instead of testing at home.
What you can test in the room now
Change the room, then check the graph again. Keep the number of people and outdoor conditions as similar as you can.
Measure for two days first
Measure the same occupied periods for two days before changing anything. Save the start, peak, time above the chosen baseline, recovery, occupancy, doors, windows, HVAC, temperature, and humidity.
Test the smallest safe ventilation change
Try a door, safe window, exhaust, or HVAC outdoor-air setting that fits weather, smoke, noise, security, and comfort. Compare the same room and occupancy instead of aiming for one unusually low reading.
Use the right device for other hazards
Keep a working carbon-monoxide alarm near anything that burns fuel. Use PM2.5 data for smoke and particles.
Report a repeated rise
If the level keeps rising after safe changes, save the graph. Ask building management or an HVAC worker to check the fresh-air supply, controls, and airflow.
Do not buy oxygen, supplements, detox products, or medical treatment for a room reading. Never disable an alarm or stay in a dangerous place to collect more data. Leave and get emergency help for severe trouble breathing, collapse, chest pain, or sudden confusion. Act fast if fuel burning, dry ice, fermentation, or a gas release may be involved.
What to save from a CO2 monitor
Keep these together
- Room, date, monitor, sensor type, calibration, placement, and outdoor reading
- Start, peak, time above the chosen baseline, recovery, and graph or export
- Occupancy, activity, doors, windows, HVAC mode, exhaust, temperature, and humidity
- Outdoor smoke, AQI, weather, noise, security, and any reason a window wasn't safe
- Symptoms kept as a separate time record, the ventilation change tested, and the building follow-up
Question for the visit
“Does this repeated graph suggest too little fresh air or poor airflow for the people using the room? What building change should we test next?”
Sources for Indoor CO2 Monitor
CO2 monitoring uses, possible 800 ppm baseline, placement, calibration, occupancy, and limitations.
Meaning of 1,000 ppm, room and occupancy differences, direct-effect evidence, and sensor needs.
Careful use of indoor CO2 as ventilation information.
Consumer monitor uses and limits.
Breathing-zone placement, free airflow, documentation, and indoor siting cautions.
Outdoor-air, filtration, and building ventilation actions.
Critical review of indoor CO2 and cognitive function.
Randomized 398-adult trial with no systematic short-term cognitive effect up to 4,200 ppm.
Noncausal classroom exposure and cognitive-score associations in 54 university students across about 440 to 1,630 ppm.
Occupational 5,000 ppm TWA and 40,000 ppm IDLH context.
Occupational exposure limit context.
See each claim's sources
range
CDC presents below 800 ppm as one potential ventilation baseline and warns that one target does not fit every room or occupancy.limitation
ASHRAE says 1,000 ppm is a ventilation and odor indicator rather than a universal health cutoff.procedure
Placement, calibration, room mixing, occupancy, and HVAC airflow can materially change a monitor reading.limitation
Evidence for direct cognitive harm at common indoor CO2 concentrations is mixed, and a 2026 randomized trial found no systematic effect up to 4,200 ppm during short exposures.safety
The 5,000 ppm occupational eight-hour limit and 40,000 ppm IDLH value answer high-hazard questions and are not room ventilation targets.