Low CO2 levels in your blood typically result from either breathing too fast (which blows off carbon dioxide) or a metabolic problem that depletes bicarbonate, the main form CO2 takes in your bloodstream. Normal arterial CO2 sits between 35 and 45 mmHg, while bicarbonate (the “total CO2” on a standard blood panel) ranges from 22 to 26 mEq/L. Falling below either threshold points to different underlying causes, and understanding which type of CO2 your test measured is the first step in figuring out what’s going on.
Two Types of “Low CO2” on Blood Tests
When doctors talk about low CO2, they could mean one of two things. An arterial blood gas test measures the actual partial pressure of carbon dioxide in your arteries. A basic metabolic panel, the far more common blood test, measures bicarbonate, which is labeled “CO2” or “total CO2” on your results. Both reflect carbon dioxide in the body, but they drop for different reasons.
Low arterial CO2 (below 35 mmHg) almost always means you’re breathing out more carbon dioxide than your body produces. Low bicarbonate (below 22 mEq/L) usually means acid is building up in your blood and consuming bicarbonate to neutralize it. Sometimes both are low at once, which happens when one problem triggers a compensatory response from the other system.
Breathing-Related Causes
The most straightforward way CO2 drops is hyperventilation: breathing faster or deeper than your body needs. Every exhale removes carbon dioxide, so when ventilation outpaces CO2 production, levels fall quickly. This shifts your blood toward the alkaline side, a state called respiratory alkalosis.
Anxiety and panic attacks are among the most common triggers. Your brain’s cortex can override the normal automatic breathing controls, pushing you to breathe rapidly even when oxygen levels are fine. The low CO2 that results actually causes many of the symptoms people associate with panic, including dizziness, tingling in the hands and face, and lightheadedness. That’s because carbon dioxide helps regulate blood vessel diameter in the brain. When CO2 drops, those vessels constrict, reducing blood flow and starving brain tissue of its normal supply.
Pain is another powerful driver. Injury, surgery, or any significant pain stimulus can accelerate breathing enough to lower CO2. Lung conditions like asthma, pulmonary fibrosis, and pneumonia activate stretch and irritant receptors in the airways that ramp up breathing rate beyond what’s needed for gas exchange. Even fever and infections can increase ventilation enough to push CO2 below normal.
Low oxygen is the single most common physiological trigger for hyperventilation. When oxygen levels in the blood fall for any reason, sensors in the neck’s major arteries detect the drop and force breathing rate up. The body prioritizes getting more oxygen in, and CO2 gets expelled as a side effect.
High Altitude and Low CO2
Climbing to high elevation is a textbook example of this oxygen-driven response. At around 5,000 meters (roughly 16,400 feet), resting breathing rate increases by about 3 breaths per minute, and arterial CO2 drops by approximately 16 mmHg compared to sea level. That’s a significant decline, enough to shift blood pH and trigger the body’s acid-base compensation systems. Even at more moderate elevations, like 2,500 meters, the effect is measurable.
Your body adapts over days to weeks by having the kidneys excrete more bicarbonate, gradually bringing blood pH closer to normal. But in the short term, the rapid CO2 loss contributes to the headaches, nausea, and mental fogginess of altitude sickness.
Metabolic Causes
When your blood panel shows low total CO2, the problem is often metabolic rather than respiratory. Here, bicarbonate gets used up buffering excess acid in the blood, and CO2 readings fall as a consequence.
Uncontrolled diabetes is a leading cause. When the body can’t use glucose properly, it breaks down fat for fuel and produces acidic byproducts called ketone bodies. These accumulate rapidly and overwhelm the blood’s buffering capacity, dragging bicarbonate levels down. This condition, diabetic ketoacidosis, can become life-threatening within hours.
Kidney disease and kidney failure are the other major culprits. Healthy kidneys constantly filter acids out of the blood and reclaim bicarbonate. When kidney function declines, acids accumulate and bicarbonate drops. A specific form called renal tubular acidosis occurs when the kidneys lose the ability to move acids into urine efficiently, even if overall kidney function seems relatively preserved.
Severe or prolonged diarrhea directly flushes bicarbonate out of the body through the intestines. The gut normally secretes bicarbonate-rich fluid during digestion, and most of it gets reabsorbed. With persistent diarrhea, that reabsorption fails, and bicarbonate levels plummet. This is one of the reasons severe gastroenteritis can become dangerous, especially in children and older adults.
Sepsis (blood poisoning) and lactic acidosis from poor circulation or intense exercise also consume bicarbonate as the body tries to neutralize the acid load.
Medications That Lower CO2
Certain drugs intentionally or accidentally reduce CO2 levels. Carbonic anhydrase inhibitors, prescribed for glaucoma and altitude sickness, work by blocking an enzyme the kidneys use to reclaim bicarbonate. The result is deliberate bicarbonate loss through urine, which lowers total CO2 on blood tests.
Aspirin overdose creates a complex picture. At toxic doses, salicylates directly stimulate the brain’s breathing center, causing hyperventilation and dropping arterial CO2. Simultaneously, they disrupt metabolism and generate excess acid, pulling bicarbonate down from the metabolic side too. This double hit is one reason salicylate poisoning is particularly dangerous.
What Low CO2 Feels Like
The symptoms depend on how quickly CO2 drops and how low it goes. When the cause is hyperventilation, you’ll typically notice lightheadedness, a sense of breathlessness (even though you’re actually over-breathing), tingling or numbness around the mouth and in the fingertips, and sometimes muscle cramps or spasms. These happen because low CO2 causes blood vessels in the brain to constrict, reducing blood flow, and also shifts calcium balance in the blood, making nerves more excitable.
When the cause is metabolic acidosis, the symptoms are different. You may feel fatigued, confused, or nauseated. Breathing often becomes noticeably deep and rapid as the body tries to compensate by blowing off extra CO2 through the lungs. This characteristic breathing pattern, sometimes described as “air hunger,” is a sign the body is working hard to correct its acid-base balance.
A bicarbonate level below 22 mEq/L signals that blood is too acidic. As acidosis worsens, breathing that was initially fast and deep can become slower, deeper, and labored. At that point, the body’s compensatory mechanisms are failing, and the situation can become life-threatening.
Temporary vs. Chronic Low CO2
A single episode of anxiety-driven hyperventilation will lower CO2 temporarily, but levels bounce back within minutes once breathing normalizes. This is uncomfortable but not dangerous. Breathing into cupped hands (not a paper bag, which can dangerously lower oxygen) and focusing on slow exhalation helps restore CO2 faster.
Chronic low CO2 is a different story. People with ongoing anxiety disorders, chronic pain, or habitual over-breathing can maintain mildly low CO2 levels for weeks or months. The kidneys compensate by excreting bicarbonate, which normalizes blood pH but leaves the body with less buffering capacity. This means any additional stress or illness can tip the balance more easily.
Persistently low bicarbonate from kidney disease tends to worsen gradually. Because the decline is slow, symptoms may be subtle until levels drop significantly. Regular blood panels catch this trend, which is why bicarbonate is included in routine metabolic testing. A single low reading might reflect dehydration or a temporary illness, but repeated low values point toward an underlying condition that needs attention.