CO2 Breathing Emission Calculator

What Is a CO₂ Breathing Emission Calculator?

Humans exhale carbon dioxide with every breath. In a closed or poorly ventilated room, CO₂ from occupants can build up quickly and affect comfort, concentration, and health. This calculator estimates indoor CO₂ concentration based on room size, ventilation rate, number of people, their activity level, and how long they stay in the space.

Outdoor air typically contains about 0.04% CO₂ (400 ppm). Indoors, values above 1,000 ppm often signal insufficient ventilation. Levels above 5,000 ppm can cause drowsiness, and much higher concentrations become dangerous. Understanding how breathing emissions interact with ventilation helps you assess classrooms, offices, bedrooms, and meeting rooms.

How Indoor CO₂ Buildup Is Calculated

The model uses a well-mixed room mass balance. Each person generates CO₂ at a rate $G$ that depends on activity: about 0.003 L/s when sleeping, 0.005 L/s at rest, and 0.008 L/s during work or exercise. Ventilation removes CO₂ at a rate proportional to air changes per hour (ACH) and room volume.

At steady state, when generation equals removal:

$$C_{ss} = C_{out} + \frac{N \cdot G \cdot 10^6}{ACH \cdot V \cdot 1000 / 3600}$$

Where $C_{ss}$ is steady-state CO₂ in ppm, $C_{out}$ is outdoor CO₂ in ppm, $N$ is the number of people, $G$ is CO₂ generation in L/s per person, $ACH$ is air changes per hour, and $V$ is room volume in m³.

For a finite duration $t$ (hours), starting from initial concentration $C_0$:

$$C(t) = C_{ss} - (C_{ss} - C_0) \cdot e^{-ACH \cdot t}$$

An empty room starts at outdoor CO₂. An already-occupied room uses your entered current level as $C_0$. The calculator also converts ppm to percent CO₂ and maps the result to a health status table.

Room Types and Ventilation Rates

Different spaces have typical ACH values. Bedrooms and living rooms average about 0.5 ACH. Offices are closer to 1 ACH, while classrooms and conference rooms may reach 2 ACH. Kitchens run around 3 ACH, and smoking areas can exceed 10 ACH. Unventilated spaces may drop to 0.1 ACH. Select a preset or enter a custom ACH for your scenario.

Example: Two People in a Bedroom for 2 Hours

A 4 m × 3 m × 2.5 m bedroom (30 m³) with 0.5 ACH ventilation holds two resting adults for 2 hours. Outdoor CO₂ is 400 ppm. Steady-state CO₂ would reach roughly 2,800 ppm, but after only 2 hours the concentration rises to about 1,200 ppm, still within a comfortable range. The same scenario in an unventilated room (0.1 ACH) would climb much faster toward unsafe levels.

CO₂ Health Guidelines

Below 5,000 ppm: Generally safe for short-term occupancy.
5,000 to 15,000 ppm: Drowsiness and reduced alertness become possible.
15,000 to 30,000 ppm: Mild stimulation and breathing discomfort.
30,000 to 50,000 ppm: Moderate to dangerous symptoms.
Above 50,000 ppm: Severe, potentially life-threatening exposure.

For related environmental calculations, try the Atmospheric Dispersion Calculator for outdoor pollutant spread, the Relative Humidity Calculator for indoor moisture, or the PPM Calculator to convert concentration units.

Frequently Asked Questions

What is a safe indoor CO₂ level?

Many building guidelines target below 1,000 ppm for good air quality. Below 5,000 ppm is generally considered safe for short periods. Above 5,000 ppm, drowsiness and reduced cognitive performance become more likely. The calculator flags levels using standard occupational health bands.

How does room ventilation (ACH) affect CO₂?

ACH measures how many times per hour the room air is replaced. Higher ACH removes CO₂ faster and lowers steady-state concentration. A bedroom at 0.5 ACH accumulates CO₂ much faster than a classroom at 2 ACH with the same number of occupants.

Why does activity level change CO₂ production?

Metabolic rate rises with physical exertion. A sleeping person emits about 0.003 L/s of CO₂, someone sitting emits about 0.005 L/s, and an active worker can reach 0.008 L/s. More CO₂ per person means faster buildup when ventilation is limited.

What is the difference between transient and steady-state CO₂?

Steady state is the level reached after many hours when generation and ventilation balance. Transient concentration depends on how long people have been in the room. A short meeting may not reach steady state, which is why duration matters in the calculation.

Should I use outdoor or current CO₂ as the starting level?

Use outdoor CO₂ (about 400 ppm, or 0.04%) if the room was empty and aired out before occupancy. If people were already inside, enter the current measured or estimated CO₂ level for a more accurate projection.

How can I reduce indoor CO₂ buildup?

Open windows, run HVAC with fresh air intake, use exhaust fans, or reduce occupancy time. Increasing ACH is the most direct way to lower CO₂. In meeting rooms, periodic breaks with doors open also help flush accumulated CO₂.

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