Carbon dioxide is a normal byproduct of your own metabolism, and at typical outdoor concentrations (around 425 ppm as of late 2024), it poses no direct health risk. But CO2 can absolutely affect your health at elevated levels, and those levels are easier to reach indoors than most people realize. The real question isn’t whether CO2 is inherently dangerous, but how much you’re breathing and for how long.
Your Body Produces CO2 Constantly
Every cell in your body generates carbon dioxide as a waste product of turning food into energy. In total, your body produces roughly 13,000 millimoles of CO2 per day. That CO2 dissolves in your blood, where it combines with water to form a weak acid called carbonic acid. This reaction is central to how your body regulates blood pH, which stays in a tight range of 7.35 to 7.45.
Your lungs handle the balance. When CO2 levels in your blood rise, you breathe faster to exhale the excess. When levels drop, your breathing slows. This system works seamlessly in normal conditions. The problems start when the air you’re breathing already has too much CO2 in it, making it harder for your lungs to offload the excess efficiently.
Indoor Air Is Where Most Exposure Happens
Outdoor air currently sits around 425 ppm of CO2. That’s harmless. But indoor spaces tell a different story. In a closed bedroom, office, or classroom, CO2 from people’s breath accumulates quickly. Poorly ventilated rooms regularly reach 1,000 to 2,500 ppm, and bedrooms with closed windows can climb to 3,000 ppm or higher overnight.
The widely cited 1,000 ppm threshold for “good indoor air quality” is often attributed to ASHRAE Standard 62.1, the main U.S. ventilation guideline. But that standard hasn’t actually contained an indoor CO2 limit for nearly 30 years. The 1,000 ppm figure originated as a proxy for adequate ventilation rates, not as a health-based safety limit. In practice, actual indoor CO2 concentrations vary enormously depending on room size, number of occupants, and ventilation, ranging anywhere from 700 to 5,000 ppm.
Cognitive Effects Start Lower Than You’d Expect
A landmark study published in Environmental Health Perspectives tested 22 people in an office-like chamber at three CO2 levels: 600, 1,000, and 2,500 ppm. At 1,000 ppm, a level common in many offices and classrooms, decision-making performance dropped 11 to 23% across six of nine measured categories. At 2,500 ppm, performance plummeted 44 to 94%, with some scores falling into ranges classified as “dysfunctional.”
The decline was consistent and dose-dependent. Seven of nine performance scales (including initiative, information usage, and strategic thinking) showed a steady, monotonic decrease as CO2 rose. This wasn’t a subtle effect buried in statistical noise. The reductions were large enough to shift people from “average” performance to “marginal” or worse.
If you’ve ever felt foggy or sluggish in a packed meeting room and then sharper after stepping outside, elevated CO2 is a plausible explanation.
CO2 Affects Your Sleep Too
Bedroom CO2 levels matter more than most people realize. An experimental study found that sleep quality decreased significantly as CO2 concentrations rose. At 3,000 ppm, participants’ subjective sleep quality scores were only about 81% of what they reported at 800 ppm. The effects were measurable in brain activity as well: higher CO2 correlated with longer time to fall asleep and less deep sleep, the restorative phase your body needs most.
Bedrooms are particularly vulnerable because they’re often small, doors are closed, and one or two people breathe all night without fresh air exchange. Simply cracking a window or running a ventilation fan can make a meaningful difference.
At High Concentrations, CO2 Becomes Toxic
Workplace safety limits, set by both OSHA and NIOSH, cap the 8-hour exposure at 5,000 ppm, with a short-term ceiling of 30,000 ppm. These levels are mostly relevant for industrial settings like breweries, welding operations, or confined spaces where CO2 can accumulate rapidly.
The progression of symptoms with rising CO2 follows a predictable pattern. Mild elevations cause headaches, drowsiness, and slightly labored breathing. As levels climb further, confusion sets in, breathing becomes more rapid, and alertness drops. At very high concentrations, the body’s breathing reflex actually reverses: instead of breathing faster, the respiratory drive slows. This can lead to a dangerous state called CO2 narcosis, where a person becomes progressively less conscious and may stop breathing altogether. These extreme scenarios are rare outside industrial accidents or confined-space emergencies, but they illustrate why CO2 is taken seriously as a workplace hazard.
Long-Term Low-Level Exposure Is Less Understood
Most CO2 research focuses on acute exposure over hours or days. Chronic, low-level exposure is a newer area of concern, partly because rising outdoor CO2 from climate change means baseline indoor levels are creeping upward too. Preliminary animal research from The Kids Research Institute Australia found that lifetime exposure to just 900 ppm, only about double the current outdoor concentration, produced measurable behavioral and physiological impairments. Researchers have flagged potential effects on the lungs, kidneys, and bones, though human evidence at these levels remains limited.
This is worth paying attention to because 900 ppm is well within the range of a typical occupied indoor space. If you spend most of your day inside, your average CO2 exposure is almost certainly higher than outdoor air.
How to Monitor and Reduce Your Exposure
CO2 is colorless and odorless, so you can’t detect elevated levels without a sensor. Home CO2 monitors using infrared detection technology are widely available for $50 to $150. These sensors are reasonably accurate for practical purposes, typically within 20 to 60 ppm of the true reading. They won’t match a $3,500 lab-grade instrument, but they’re more than sufficient to tell you whether your bedroom is at 600 ppm or 2,000 ppm.
The most effective way to lower indoor CO2 is straightforward: increase ventilation. Open windows, use exhaust fans, or ensure your HVAC system is bringing in outside air rather than just recirculating. In bedrooms, even a slightly open window can prevent CO2 from climbing to levels that disrupt sleep. In offices and classrooms, CO2 monitors can serve as a real-time indicator of whether the space needs more fresh air.
For context, keeping indoor CO2 below about 700 ppm above the outdoor level (roughly 1,100 ppm total at current outdoor concentrations) corresponds to ventilation rates that most people find comfortable. Below that level, you’re unlikely to experience measurable cognitive effects. Above 1,500 ppm, the research suggests your thinking and sleep quality are taking a hit whether you notice it or not.