Understanding Carbon Dioxide (CO2)

Carbon dioxide is a naturally occurring gas made of one carbon atom and two oxygen atoms. It's a normal, essential part of the atmosphere: plants use it for photosynthesis, and every person and animal produces it as a byproduct of breathing.

Outdoors, CO₂ is remarkably consistent. In 2026, the global average sits at roughly 425 to 430 parts per million (ppm), a figure that has been rising steadily by around 2 to 3 ppm each year as a result of human activity. Indoors, the picture changes quickly: CO₂ in an occupied room can climb well beyond outdoor levels within an hour, simply because people are breathing in a space that isn't being refreshed with outdoor air fast enough.

The biggest source of indoor CO₂, by far, is people. Every exhale releases CO₂, and the more people in a room, the faster levels climb. A single adult sitting quietly can raise CO₂ in a small, sealed bedroom noticeably within an hour; two people sleeping overnight with the door and window closed will often push levels several times higher than outdoor air by morning.

Combustion is the other major source. Gas stoves and hobs, fireplaces, wood burners and unflued gas heaters all release CO₂ as they burn fuel, alongside other combustion byproducts such as carbon monoxide and particulate matter. Attached garages with an idling vehicle and poorly ventilated basements can also see CO₂ climb, particularly where airflow between spaces is limited.

At the concentrations typically reached in homes and offices, CO₂ itself isn't usually a toxicity concern; the associated comfort and performance effects tend to show up long before CO₂ reaches genuinely hazardous levels. What elevated CO₂ reliably tells you is that a room isn't getting enough fresh air, and that other pollutants, such as VOCs, humidity and particles from people and activities in the room, are likely accumulating alongside it.

Research has repeatedly linked moderately elevated CO₂ with reduced cognitive performance. A widely cited 2012 study published in Environmental Health Perspectives placed participants in a controlled chamber at 600, 1,000 and 2,500 ppm CO₂ and measured decision-making performance across nine scales. Performance dropped significantly at 1,000 ppm, with larger declines at 2,500 ppm, particularly on tasks involving initiative and strategic thinking. Separate office-based research has linked lower CO₂ and better ventilation with higher cognitive function scores among workers. Together, this evidence supports treating CO₂ as a useful early-warning signal for spaces that need better airflow, even though it isn't a direct measure of toxicity.

CO₂ readings are easiest to interpret against a few commonly used reference bands.

CO₂ readings are easiest to interpret against a few commonly used reference bands.

Occupancy: More people in a room means more exhaled CO₂ and faster build-up.

Room size: Smaller rooms accumulate CO₂ faster than larger ones for the same number of occupants.

Ventilation rate: How quickly outdoor air replaces indoor air is the single biggest factor in how high CO₂ climbs.
Closed doors and windows: Overnight and during meetings, closed doors are one of the most common reasons CO₂ builds up.

HVAC design: Systems with a fresh-air intake or economiser mode dilute CO₂ far more effectively than those that only recirculate indoor air.

Combustion appliances: Gas stoves, fireplaces and unflued heaters add CO₂ directly to the room's air.

Because CO₂ is primarily a ventilation problem, the fixes are mostly ventilation-based:
1. Open a window, even briefly, to exchange indoor air with outdoor air.
2. Use trickle vents and keep them unobstructed so background ventilation continues even with windows closed.
3. Run extractor fans in kitchens and bathrooms during and after activities that add moisture and pollutants.
4. Consider mechanical ventilation with heat recovery (MVHR) in airtight, energy-efficient homes where opening windows isn't practical.
5. Reduce occupancy density in small rooms where possible, or ventilate more frequently when a room is fully occupied.
6. Don't rely on an air purifier to lower CO₂; most purifiers filter particles and some VOCs but do not bring in fresh outdoor air.

CO₂ is most accurately measured using non-dispersive infrared (NDIR) sensors, which work by shining infrared light through a sample of air and measuring how much of it CO₂ molecules absorb. NDIR sensors are considered the gold standard for consumer and professional CO₂ monitoring because they are stable, accurate and not easily fooled by other gases.

Cheaper devices sometimes use eCO2 (estimated CO₂) sensors, which actually measure VOCs and use an algorithm to estimate a corresponding CO₂ figure rather than measuring CO₂ directly. eCO2 readings can diverge significantly from true CO₂ levels, particularly when VOC sources like cleaning products or new furniture are present, so a genuine NDIR sensor is worth looking for in any monitor used to make ventilation decisions.

NDIR sensors benefit from periodic calibration, since their accuracy can drift slightly over time. Many consumer devices use automatic baseline correction (ABC), which assumes the lowest reading over a rolling period, often around a week, represents fresh outdoor air, and recalibrates against that assumption. This works well in most homes but can be less reliable in spaces that are rarely well-ventilated, such as some basements, where manual calibration may be more appropriate.