Carbon dioxide is a normal part of the air you breathe and a natural byproduct of your own metabolism. At the concentrations found in outdoor air (around 430 ppm today), it’s completely harmless. But CO2 becomes a health concern at higher concentrations, and the thresholds where problems begin are lower than most people realize.
How Your Body Handles CO2
Every cell in your body produces carbon dioxide as a waste product of energy production. Your blood carries it to your lungs, where you exhale it with every breath. This process also plays a central role in regulating your blood’s pH: CO2 dissolves in blood to form a weak acid, and your body keeps that acid level in a tight range by adjusting how fast and deeply you breathe. Sensors in your brain and blood vessels constantly monitor CO2 levels and trigger faster breathing when levels rise.
This system works well under normal conditions. Problems start when the air around you contains so much CO2 that your lungs can’t get rid of it fast enough. When CO2 builds up in your blood, it shifts your blood chemistry toward being more acidic, which affects your heart, your blood vessels, and your nervous system.
Safe and Unsafe Concentration Levels
CO2 concentrations are measured in parts per million (ppm). Here’s how different levels affect you:
- 400–500 ppm: Normal outdoor air. No health effects.
- 600–1,000 ppm: Typical range in well-ventilated indoor spaces. The American Society of Heating, Refrigerating and Air-Conditioning Engineers recommends keeping indoor CO2 below 1,000 ppm as a marker of adequate ventilation.
- 1,000–2,500 ppm: Common in crowded or poorly ventilated rooms. This is where measurable effects begin. A controlled study exposing 22 people to these levels found moderate declines in decision-making ability at 1,000 ppm, with more substantial drops at 2,500 ppm across most cognitive measures tested.
- 2,500–5,000 ppm: Found in very stuffy rooms, packed conference halls, or buildings with inadequate HVAC systems. A 2023 meta-analysis confirmed that complex cognitive tasks suffer noticeably in this range, with the authors recommending a stricter workplace limit below 1,000 ppm for jobs requiring high-level thinking.
- 5,000 ppm (0.5%): The current OSHA and NIOSH workplace exposure limit for an 8-hour workday.
- 20,000 ppm (2%): Causes noticeably deeper breathing.
- 40,000 ppm (4%): Marked increase in breathing rate. This is also the NIOSH threshold for “immediately dangerous to life or health,” meaning you should leave the area right away.
- 100,000 ppm (10%): Visual disturbances, tremors, and loss of consciousness.
- 250,000 ppm (25%): Can cause death.
The gap between “stuffy conference room” and “life-threatening” is large. But the gap between “normal air” and “impaired thinking” is surprisingly small. You can be sitting in a meeting room at 2,500 ppm, making noticeably worse decisions, without feeling anything obviously wrong.
Why Indoor Air Quality Matters
Most people encounter elevated CO2 not in industrial settings but in everyday spaces: classrooms, offices, bedrooms, and cars with the windows up. When multiple people share a closed room with poor ventilation, CO2 from exhaled breath accumulates quickly. Schools and office buildings routinely reach 1,500 to 3,000 ppm during occupied hours, well into the range where cognitive performance drops.
The symptoms at these everyday levels are subtle. You won’t feel short of breath or dizzy. Instead, you might notice you’re drowsier, less focused, or slower at complex problem-solving. These effects have been associated with poor work and school performance in multiple studies. Opening a window, improving ventilation, or simply taking breaks in fresh air can bring concentrations back down. A CO2 monitor (widely available for under $100) can tell you exactly where your indoor spaces stand.
The Dry Ice and Enclosed Space Danger
The most dangerous everyday CO2 exposure comes from dry ice, which is frozen CO2 that continuously releases gas as it warms. A single kilogram of dry ice produces roughly 541 liters of CO2 gas. Because CO2 is about 1.5 times heavier than air, it sinks and pools near the floor, where it can build to dangerous concentrations even while the air at head height seems fine.
This makes enclosed spaces particularly risky. Dry ice should never be transported in the passenger cabin of a car. If it has been left in a closed vehicle, trunk, or small room for more than 30 minutes, you should open doors and windows for at least a minute before entering or unloading. Cellars, walk-in coolers, and shipping containers are especially hazardous because the heavy gas has nowhere to escape.
One counterintuitive danger: oxygen monitors alone won’t protect you. It’s possible to have an acceptable oxygen level of 18% while simultaneously having a CO2 concentration above 5%, which is already dangerous. The CO2 itself is the hazard, not just the displacement of oxygen.
Carbonated Drinks and Swallowed CO2
The carbon dioxide dissolved in sparkling water, soda, and beer is safe to drink. CO2 in carbonated beverages primarily affects mouthfeel (that fizzy, slightly acidic sensation) and plays only a minor role in tooth erosion. Research has found weak and contradictory associations between carbonated drinks and acid reflux. Stomach discomfort from carbonation generally only appears when you drink more than about 300 ml (roughly 10 ounces) of a carbonated beverage in a short period, and that discomfort is mechanical, from gas expanding in your stomach, not a toxic effect.
Any health concerns about soda come from the sugar, acid, or other ingredients, not the CO2 itself.
CO2 in the Atmosphere
Global atmospheric CO2 currently sits around 430 ppm, up from a pre-industrial baseline of about 280 ppm. This increase drives climate change, but the direct health risk from breathing outdoor air at 430 ppm is zero. Even if atmospheric CO2 doubled, the outdoor concentration would still be far below the threshold for any physiological effect. The environmental consequences of rising atmospheric CO2 (heat waves, air quality changes, ecosystem disruption) are a separate and serious issue, but they’re not caused by breathing the gas itself.
Who Faces the Greatest Risk
People with lung conditions like COPD or severe asthma are more vulnerable to elevated CO2 because their lungs are already less efficient at expelling it. Their blood CO2 levels may already run higher than normal, so additional exposure from poor ventilation tips them into symptoms faster. Workers in brewing, fermentation, welding, or any industry using dry ice also face elevated risk if ventilation is inadequate.
For most people, the practical takeaway is straightforward: outdoor air and reasonably ventilated indoor spaces pose no CO2 risk. Stuffy, crowded rooms can quietly impair your thinking. And enclosed spaces with CO2 sources like dry ice can become genuinely life-threatening within minutes.