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CO₂ Breathing Emission Calculator

Indoor air quality depends critically on how much carbon dioxide people generate and how effectively ventilation removes it. By entering room dimensions, occupancy, activity type, and ventilation rate, you can predict CO₂ buildup over time. This matters for offices, classrooms, and homes, where high CO₂ impairs cognition and comfort. The calculator uses exponential decay models to account for continuous emission and removal.

Last updated:

Creators Joanna Michałowska, PhD Plant Biology
4,837 people find this calculator helpful

How much CO₂ do humans exhale?

Atmospheric air is roughly 78% nitrogen, 21% oxygen, and 1% other gases—including just 0.04% carbon dioxide (400 parts per million). Your lungs inhale this lean mixture every moment.

Exhaled breath is strikingly different. It contains approximately 4% carbon dioxide by volume, along with nitrogen, oxygen, water vapour, and thousands of trace compounds. This 100-fold increase happens because your metabolism continuously produces CO₂ as cells burn fuel. Red blood cells ferry CO₂ from tissues to the lungs, where it diffuses into the alveoli and is expelled with each breath.

The rate of CO₂ production varies with physical activity. Sitting quietly generates around 200 mL of CO₂ per minute, while moderate exercise can triple that output. A room filled with people doing office work will see CO₂ accumulate unless ventilation is adequate.

CO₂ concentration model

Indoor CO₂ concentration grows as occupants exhale and decays as fresh air replaces stale air. The equilibrium point depends on emission rate, room volume, and air exchange rate. The formula below calculates concentration at any time t:

c = ((E × N) / (ACH × V)) × (1 − e−ACH×t) + (cout − c0) × e−ACH×t + c0

PPM = c × 1,000,000

  • c — CO₂ concentration as a decimal fraction (0.0004 = 400 ppm)
  • E — CO₂ emission rate per person in mL/min
  • N — Number of occupants
  • ACH — Air changes per hour—how many times the room's volume is replaced per hour
  • V — Room volume in cubic metres
  • t — Time elapsed in hours
  • c₀ — Initial CO₂ concentration in the room
  • c_out — Outdoor CO₂ concentration (typically 400 ppm or 0.0004)

Why indoor CO₂ levels matter

Cognitive performance declines measurably at elevated CO₂. Studies document reduced decision-making ability, slower processing speed, and worse problem-solving in spaces where CO₂ exceeds 1000 ppm—levels easily reached in poorly ventilated offices or classrooms within a few hours.

Beyond cognition, excessive CO₂ triggers physical symptoms: headaches, drowsiness, elevated heart rate, and rapid breathing. When concentration surpasses 3%, most people experience acute shortness of breath and cardiovascular stress. Industrial safety guidelines set 5000 ppm as an eight-hour exposure limit, though comfort and productivity suffer well before that threshold.

The mechanism is partly chemical: CO₂ dissolves in blood to form carbonic acid, lowering pH. Conversely, very low CO₂ raises pH and causes alkalosis, which is also harmful. Maintaining a balance around 400–800 ppm mimics outdoor conditions and supports both health and cognition.

Practical considerations for indoor air quality

Account for these factors when interpreting CO₂ projections and managing indoor environments.

  1. Room type and ACH vary widely — A typical office might have 3–5 air changes per hour, while a bedroom or storage room may have less than 1. Hospitals and clean rooms demand 10–15 or more. Always confirm your room's actual ventilation spec—a miscalculation here dominates the result.
  2. Activity level shifts emission by 3–4× — Sleeping or light office work generates ~200 mL CO₂/min per person. Moderate exercise jumps to 600–1000 mL/min. If you miscategorise activity (e.g., treating a conference room full of standing, talking people as if they're sitting quietly), your forecast will underestimate CO₂ rise significantly.
  3. Outdoor air quality isn't always 400 ppm — Urban areas near traffic or industrial zones may have 450–500 ppm outdoor CO₂. If your ventilation system draws from such a location, indoor steady-state levels will be higher. Check local air quality data if you're in a metropolitan area.
  4. Thermal stratification and dead zones — CO₂ doesn't mix uniformly in every room. Stagnant corners, spaces above suspended ceilings, and areas far from air returns can harbour pockets of much higher concentration than the calculator predicts. Ensure sensors are placed centrally and that air distribution is genuinely effective.

Reducing indoor CO₂

The most direct lever is ventilation: increase ACH by upgrading your HVAC system, upgrading filters and dampers, or using portable air cleaners with HEPA and activated carbon. Even manual opening of windows for 10–15 minutes can halve CO₂ concentration in small rooms.

Occupancy control is another approach. Stagger meeting times, limit the number of people in enclosed spaces, or schedule breaks to allow outdoor air circulation. In schools and offices, these steps improve both air quality and occupant wellbeing.

Plants absorb CO₂ through photosynthesis, but their contribution indoors is modest—typically a few percent reduction in a large room. Don't rely on plants alone as a mitigation strategy, though they offer psychological and aesthetic benefits.

Carbon dioxide removal technologies (like those using sorbent materials) exist but are expensive and rarely practical for routine indoor spaces. Engineering solutions—better seals, higher-performance dampers, and demand-controlled ventilation—are usually more cost-effective.

Frequently Asked Questions

At what CO₂ level do cognitive effects appear?

Research consistently shows measurable drops in problem-solving, decision speed, and strategic thinking around 1000 ppm. Performance decline becomes pronounced above 1400 ppm. In studies comparing office spaces at 600 ppm versus 1000 ppm, higher concentrations correlated with slower responses and poorer task completion rates. This doesn't mean instant incapacity—it's a gradual degradation—but it underscores why offices and classrooms should target levels below 1000 ppm for sustained productivity.

How long does it take CO₂ to reach dangerous levels in a sealed room?

In a small, sealed bedroom with two sleeping adults (low emission ~200 mL/min each) and zero ventilation, CO₂ can reach 2000 ppm in 4–6 hours and 3000 ppm by morning. In a conference room with 20 people talking (moderate activity ~500 mL/min each), dangerous levels (3000+ ppm) arrive in 30–45 minutes. Time-to-danger scales inversely with occupancy, activity intensity, and room volume. Even modest ventilation—1 ACH—stretches these timescales significantly.

Can plants meaningfully reduce room CO₂?

A mature, leafy houseplant absorbs perhaps 1–3 grams of CO₂ per day during daylight hours. In a 100 m³ room accumulating 50 kg of CO₂ annually from occupants, plants offset perhaps 0.2–1% of the load. They're beneficial for air quality in other ways (removing volatile organics, releasing humidity) and improve mood and aesthetics, but they're not a substitute for adequate ventilation. To meaningfully lower CO₂, prioritise mechanical or passive air exchange.

Why is outdoor CO₂ concentration rising, and does it affect indoor predictions?

Atmospheric CO₂ has climbed from 315 ppm in 1958 to over 420 ppm today due to fossil fuel combustion and land-use change. Most ventilation systems draw outdoor air, so indoor baseline CO₂ is now higher than historical assumptions of 350–400 ppm. If you live in a city or near busy roads, outdoor concentrations may be 450–500 ppm. This shifts your indoor steady-state upward by roughly the same amount—an important factor if you're designing spaces for precision work or sensitive occupants.

How does humidity interact with CO₂ and indoor air quality?

CO₂ and humidity operate independently from a chemical standpoint, but they share ventilation pathways. Increasing ACH removes both excess CO₂ and excess moisture. Relative humidity should stay between 30–60% for comfort and to suppress mould and dust mites. In winter, mechanical ventilation may dry air excessively; in summer, it may struggle with humidity ingress. Manage them as linked aspects of indoor climate control, using ventilation rates and supplementary dehumidification or humidification as needed.

What's the relationship between CO₂ and oxygen in a typical indoor space?

In normal indoor conditions below 5000 ppm CO₂, oxygen depletion is not the primary concern—CO₂ itself becomes problematic first. Oxygen remains near 21% by volume in well-ventilated spaces. The danger at very high CO₂ (above 10%) is primarily asphyxiation (CO₂ displaces O₂ and can trigger panic), not oxygen lack per se. For practical indoor environments, focus on CO₂ as the indicator; if CO₂ is controlled via good ventilation, oxygen is rarely a problem.

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