Carbon dioxide affects your body, your thinking, the food you eat, and the planet’s climate. At today’s global average of 422.8 parts per million (ppm) in the atmosphere, CO2 is already reshaping ocean chemistry and crop nutrition. Indoors, concentrations routinely climb high enough to measurably impair cognitive performance. At extreme levels, CO2 is a potent poison that can kill in seconds.
How CO2 Affects Your Body
Your cells constantly produce carbon dioxide as a waste product of metabolism. Your blood carries it to your lungs, where you exhale it. This process is tightly linked to your blood’s acid-base balance: CO2 combines with water in the blood to form carbonic acid, which splits into bicarbonate and hydrogen ions. Your body uses this reaction as a buffer system, keeping blood pH in a narrow range. When you breathe in air with elevated CO2, this system gets pushed off balance, your blood becomes more acidic, and your breathing rate increases as your body tries to compensate.
At moderate indoor concentrations, most people won’t notice dramatic physical symptoms, but subtler effects are already underway. Research has linked normal indoor CO2 levels (which often exceed outdoor concentrations by several hundred ppm) to headaches, mucosal irritation, perceptions of stuffy or poor air quality, and increased sick days.
Cognitive Performance Drops at Common Indoor Levels
The most striking effect of moderately elevated CO2 is what it does to your ability to think. In a controlled study from Harvard’s T.H. Chan School of Public Health, cognitive function scores dropped 15% when CO2 was around 945 ppm and 50% at roughly 1,400 ppm, compared to well-ventilated conditions. On average, every 400 ppm increase in CO2 was associated with a 21% decrease in cognitive scores across all domains tested.
A separate study published in Environmental Health Perspectives tested decision-making at three CO2 levels. Compared to 600 ppm, performance at 1,000 ppm dropped 11 to 23% across most decision-making scales. At 2,500 ppm, the decline was severe: scores fell 44 to 94%, with some metrics reaching levels the researchers classified as “marginal or dysfunctional.” The one exception was focused activity, which actually increased at higher CO2 levels, suggesting that people narrow their attention as their broader strategic thinking deteriorates.
These concentrations are not unusual. Crowded classrooms, conference rooms, and bedrooms with closed windows frequently reach 1,000 to 2,500 ppm. Outdoor urban air can hit 500 ppm. The workplace safety limit set by both OSHA and NIOSH is 5,000 ppm as an eight-hour average, a threshold established decades ago based on acute toxicity rather than cognitive effects.
Acute CO2 Poisoning
At concentrations far above what you’d encounter in a normal building, CO2 becomes directly dangerous. The progression follows a rough pattern by concentration:
- Above 2% (20,000 ppm): Breathing deepens noticeably.
- Above 4% (40,000 ppm): Respiration increases markedly, and headaches, dizziness, and confusion set in.
- Above 10% (100,000 ppm): Visual disturbances, tremors, convulsions, and loss of consciousness can occur.
- Above 30% (300,000 ppm): Unconsciousness happens within seconds, breathing stops within about a minute, and cardiac arrest follows shortly after.
Fatal cases in the medical literature involve CO2 concentrations between roughly 14 and 26%, sometimes alongside reduced oxygen. These incidents typically happen in enclosed industrial settings like fermentation tanks, dry ice storage areas, or underground spaces where CO2 accumulates in low-lying pockets because it is heavier than air.
Effects on Plant Growth and Food Nutrition
Plants use CO2 for photosynthesis, so rising atmospheric levels do boost growth. This “fertilization effect” is real: most major crops use a photosynthetic pathway that is limited by available CO2, and higher concentrations increase their biomass and yield. On the surface, this sounds beneficial.
The tradeoff is nutritional quality. As plants grow faster under elevated CO2, their protein and micronutrient concentrations decline. Research across a wide range of crops shows that zinc drops the most, followed by iron and protein. Rice and wheat, the staple foods for more than half the world’s population, both show significant decreases in these essential nutrients. Chickpeas are particularly affected, with zinc concentrations falling by as much as 37.5% under elevated CO2 conditions. The result is crops that look the same or larger but deliver less nutrition per bite.
Ocean Acidification
The ocean absorbs roughly a quarter of the CO2 humans emit. When CO2 dissolves in seawater, it forms carbonic acid, the same reaction that happens in your blood. The acid releases hydrogen ions, making the water more acidic. Since the Industrial Revolution, the ocean’s surface pH has dropped by 0.1 units. Because the pH scale is logarithmic, that small number represents a 30% increase in acidity.
This shift is already harming marine life that builds shells and skeletons from calcium carbonate: corals, oysters, mussels, and many types of plankton. These organisms pull carbonate ions from the water to construct their structures, but rising acidity means more hydrogen ions bond with available carbonate, leaving less for shell-building. If acidity increases enough, existing shells and coral skeletons begin to dissolve. Since many of these calcifying organisms sit at the base of marine food webs, the effects ripple upward through ocean ecosystems.
Climate Feedback Loops
CO2’s most well-known environmental effect is trapping heat in the atmosphere. But the warming it causes also triggers secondary sources of CO2 and methane. Permafrost, the permanently frozen ground across Arctic regions, contains enormous stores of organic carbon accumulated over thousands of years. As temperatures rise, this ground thaws and microbes begin breaking down the organic material, releasing CO2 and methane into the atmosphere, which causes further warming, which thaws more permafrost. Current evidence suggests this will be a gradual, prolonged process rather than a sudden release, but the total amount of carbon locked in permafrost is large enough to meaningfully accelerate warming over decades.
Similar feedback loops exist in other systems. Warmer oceans absorb less CO2 (gases dissolve less readily in warmer water), leaving more in the atmosphere. Forest fires, which increase with heat and drought, release stored carbon while also removing trees that would otherwise absorb CO2. Each of these loops means that the effects of current emissions extend well beyond the CO2 itself.