CO2 levels can be “high” in three very different contexts: the atmosphere, the air inside a building, or your bloodstream. Each has distinct causes, and the phrase means something different depending on which one you’re asking about. Here’s what drives CO2 up in each case.
What’s Raising CO2 in the Atmosphere
The atmosphere currently holds about 427 parts per million (ppm) of carbon dioxide, a record high measured at NOAA’s Mauna Loa Observatory in 2024. Before the Industrial Revolution, that number hovered around 280 ppm. The difference is almost entirely driven by human activity.
Burning fossil fuels for transportation, electricity, and industrial production is the dominant source. In the United States, transportation is the single largest emitter, with over 94% of its fuel coming from petroleum. Electricity generation is the second major contributor; in 2022, 60% of U.S. electricity still came from burning coal and natural gas. Industrial processes round out the top three, both from burning fuel for energy and from chemical reactions needed to manufacture materials like cement and steel.
Nature produces CO2 too. Respiration, decomposition, forest fires, and volcanic eruptions release roughly 190 billion tonnes of CO2 into the atmosphere each year. But natural systems also reabsorb most of that through photosynthesis and ocean uptake, keeping things roughly in balance. The problem is the additional CO2 from fossil fuels, which natural sinks can’t fully absorb. That surplus accumulates year after year.
What Makes Indoor CO2 Build Up
The most common reason indoor CO2 climbs is simple: people breathing in an enclosed space. Every exhaled breath contains about 4% CO2, and in a room with poor ventilation, that adds up quickly. The more people in a room and the less fresh air circulating, the faster levels rise.
Other indoor sources include cigarette smoke and fuel-burning appliances that aren’t properly vented, like gas stoves, furnaces, or space heaters. But in most homes and offices, occupancy and ventilation are the main factors.
You may have heard that indoor CO2 should stay below 1,000 ppm. That number is widely repeated, but ASHRAE (the organization that sets ventilation standards) has clarified that it never established 1,000 ppm as an air quality limit. It’s better understood as a rough indicator that a space is getting enough fresh outdoor air per person, not a health threshold. ASHRAE does not recommend any specific CO2 concentration as a reliable measure of air quality or ventilation adequacy.
When Indoor CO2 Becomes a Problem
For most people, concentrations up to about 10,000 ppm (1%) cause no obvious effects beyond possible drowsiness. That’s ten times the commonly cited 1,000 ppm guideline, which gives you a sense of how conservative that benchmark is. At 50,000 ppm (5%), people experience strong respiratory stimulation, dizziness, confusion, headaches, and shortness of breath. Reaching those levels in a normal home or office is extremely unlikely, but it can happen in confined industrial spaces or poorly ventilated areas with combustion sources.
Where indoor CO2 matters most practically is in crowded, stuffy rooms. Concentrations between 1,000 and 2,500 ppm are common in packed classrooms and meeting rooms, and some studies have linked that range to reduced cognitive performance and increased drowsiness. Opening windows, improving mechanical ventilation, or simply reducing the number of people in a space brings levels down.
What Causes High CO2 in Your Blood
In the body, CO2 is a normal byproduct of metabolism. Your cells produce it constantly as they burn fuel for energy, and your lungs exhale it with every breath. The healthy range for CO2 in arterial blood is 35 to 45 mmHg. When levels rise above that, the condition is called hypercapnia.
Hypercapnia happens when your lungs can’t clear CO2 as fast as your body produces it. There are two main mechanisms. The first is hypoventilation, which simply means you’re not breathing deeply or frequently enough to expel the CO2 your body generates. The second, and more common in clinical practice, is a mismatch between airflow and blood flow inside the lungs. Even if you’re breathing, some parts of the lung may not be exchanging gases efficiently, so CO2 builds up in the blood instead of being exhaled.
Conditions That Raise Blood CO2
Lung diseases are the most frequent culprits. Chronic obstructive pulmonary disease (COPD) damages the tiny air sacs where gas exchange happens, making them less effective at releasing CO2. Severe asthma, pneumonia, and other conditions that obstruct or inflame the airways can have a similar effect. Sleep apnea causes repeated pauses in breathing during sleep, which reduces ventilation and allows CO2 to accumulate, sometimes significantly over the course of a night.
Obesity can contribute by restricting how fully the lungs expand, a condition sometimes called obesity hypoventilation syndrome. Neuromuscular diseases that weaken the muscles of breathing, like ALS or muscular dystrophy, also impair the body’s ability to ventilate properly.
Not all causes are lung-related. Kidney failure can raise CO2 levels because the kidneys help regulate the body’s acid-base balance. When they aren’t functioning well, the balance shifts in ways that show up as elevated CO2 on a blood test. Cushing’s syndrome and metabolic alkalosis (which can result from prolonged vomiting or severe dehydration) also raise blood CO2 through different metabolic pathways.
How Your Body’s Fuel Affects CO2 Production
Your cells produce different amounts of CO2 depending on what they’re burning for energy. Carbohydrates generate the most CO2 relative to the oxygen consumed, with a respiratory quotient (RQ) of 1.0. Fats produce the least, with an RQ of 0.7, and protein falls in between at about 0.82. On a typical mixed diet, the body’s overall RQ is around 0.8.
For most healthy people, this difference is trivial. But for someone with severe lung disease who already struggles to exhale enough CO2, a diet very high in carbohydrates can tip the balance further. This is one reason nutritional support for critically ill patients sometimes favors higher-fat formulas, to reduce the CO2 load the lungs need to clear.
Signs of High CO2 in the Body
Mild hypercapnia often shows up as headaches, drowsiness, and difficulty concentrating. These symptoms overlap with dozens of other causes, which is why they’re easy to dismiss. As CO2 rises further, you may notice shortness of breath, a flushed face, elevated heart rate, or confusion. In severe cases, very high blood CO2 can cause muscle twitching, panic, and loss of consciousness.
If you’re experiencing persistent unexplained drowsiness, morning headaches (especially if you snore heavily), or worsening breathlessness, a simple blood test called an arterial blood gas can measure your CO2 level directly. The test is quick, and the results tell your doctor whether your lungs are keeping up with CO2 clearance.