The most widely cited benchmark for acceptable indoor CO2 is 1,000 parts per million (ppm), but recent evidence suggests that levels well below that threshold already affect how you think and feel. Outdoor air currently averages about 423 ppm, so any occupied indoor space will be higher. The real question is how much higher is too high, and that depends on the type of building, how long people spend inside, and what they’re doing there.
Where the 1,000 ppm Number Comes From
Almost everyone in the building industry treats 1,000 ppm as the line between good and poor air quality, and most cite ASHRAE Standard 62.1 as the source. But ASHRAE itself hasn’t actually contained an indoor CO2 limit for almost 30 years. The number traces back to a 1989 version of the standard, which noted that delivering 15 cubic feet per minute of outdoor air per person would produce a steady-state CO2 concentration of about 1,000 ppm. That ventilation rate was chosen because it satisfies roughly 80% of visitors’ perception of body odors, not because 1,000 ppm was identified as a health threshold.
The earlier 1981 version of the standard actually set the limit much higher, at about 2,500 ppm, based on concerns about headaches and impaired judgment at 5,000 ppm with a safety factor of two built in. The jump down to 1,000 ppm in 1989 was never formally explained, and current editions of the standard don’t include any statement treating 1,000 ppm as a guideline or target.
What Different Guidelines Recommend
Despite the confusion around ASHRAE, several organizations have published their own benchmarks. The World Health Organization has recommended keeping indoor CO2 below 1,000 ppm as a general indicator of adequate ventilation. Workplace safety agencies set much higher legal limits: OSHA’s permissible exposure limit is 5,000 ppm averaged over an eight-hour shift, with a short-term ceiling of 30,000 ppm. NIOSH mirrors those same numbers. These occupational limits exist to prevent acute toxicity, not to optimize comfort or cognitive performance.
For schools and classrooms, guidance has tightened considerably. Spain, for example, now recommends CO2 stay below 700 ppm in classrooms and 550 ppm in hallways. A risk framework developed for educational settings categorizes levels up to 700 ppm as low risk, 700 to 800 ppm as moderate risk, 800 to 1,000 ppm as high risk, and anything above 1,000 ppm as very high risk. Those thresholds include background outdoor CO2 of about 400 ppm, so they’re measuring total concentration, not just the CO2 added by occupants.
How CO2 Affects Your Body and Brain
The health effects of indoor CO2 begin well before you hit occupational safety limits. Epidemiological studies have found associations between building-related symptoms and CO2 concentrations starting as low as 700 ppm. By 1,000 ppm, measurable changes in cognitive performance appear, along with respiratory symptoms in children. Linear changes in cardiovascular and circulatory function have been documented across the entire 500 to 5,000 ppm range.
A controlled study from Harvard’s T.H. Chan School of Public Health put specific numbers to the cognitive effects. Office workers exposed to about 945 ppm scored 15% lower on cognitive tests compared to their performance at 550 ppm. At roughly 1,400 ppm, scores dropped by 50%. Across all nine cognitive domains tested, including strategic thinking and complex decision-making, every 400 ppm increase in CO2 was associated with a 21% decrease in cognitive scores. These weren’t extreme industrial exposures. They were concentrations routinely found in conference rooms, classrooms, and open-plan offices.
CO2 as an Indicator of Infection Risk
Since the COVID-19 pandemic, CO2 monitoring has taken on a second role: estimating the risk of airborne disease transmission. Because people exhale both CO2 and respiratory aerosols, the CO2 concentration in a room serves as a rough proxy for how much rebreathed air you’re inhaling. Research modeling airborne transmission risk found that in a typical school classroom with 20 to 25 occupants over a six- to eight-hour day, CO2 should stay below 700 ppm to keep infection risk low. In a restaurant with 50 to 100 people and shorter exposure times of two to three hours, the threshold rises to about 900 ppm.
These numbers are lower than the traditional 1,000 ppm benchmark, which makes sense. The old number was designed around odor control, while infection risk modeling accounts for how long shared air circulates and how many people are breathing it.
Practical Targets by Building Type
Given what the research shows, here’s how the numbers break down in practice:
- Offices and workplaces: Aim for 600 to 800 ppm during occupied hours. Conference rooms frequently spike above 1,500 ppm within 30 to 60 minutes of a full meeting, so they need dedicated ventilation or shorter occupancy.
- Schools and classrooms: Below 700 ppm is ideal. Above 1,000 ppm, both learning ability and respiratory health are affected, particularly in younger children.
- Restaurants and retail: Below 900 ppm, especially in spaces with high occupant density and limited ventilation.
- Homes: Bedrooms often reach 1,000 to 2,500 ppm overnight with doors and windows closed. Opening a window or running a ventilation system can cut those levels significantly.
How To Monitor Indoor CO2
Consumer-grade CO2 monitors use infrared sensors (often labeled NDIR) that measure how much infrared light CO2 molecules absorb. These sensors vary in quality. Manufacturer-stated accuracy ranges from plus or minus 30 ppm on better units to plus or minus 75 ppm on cheaper ones. When individually calibrated and corrected for temperature and humidity, the practical error of a good sensor drops below 5 ppm, or about 1% of the reading. For building management purposes, even a sensor accurate to within 50 ppm gives you useful information about ventilation trends.
Placement matters. Mount the sensor at breathing height, away from windows, doors, and HVAC supply vents, which can give misleadingly low readings. Avoid placing sensors near where people exhale directly, like right next to a desk chair. The goal is to capture the room’s average concentration, not the CO2 plume from one person’s breath. Most monitors display readings in real time, so you can quickly see how CO2 changes when you open windows, adjust ventilation, or add more people to a room.
For anyone managing a building, trending CO2 over days and weeks reveals patterns that a single snapshot misses. You may find that mornings are fine but afternoons consistently exceed 1,200 ppm, or that one floor runs 300 ppm higher than another because of an HVAC imbalance. Those patterns point directly to ventilation problems you can fix.