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Air quality

CO₂ Explained: What It Is, Why It Forms, and How to Keep Indoor Levels Healthy

CO₂ is a key parameter for indoor air quality. It affects our concentration, wellbeing, and sleep, and can even promote the spread of pathogens. At the same time, CO₂ is a natural part of our atmosphere and harmless as long as certain limit values aren't exceeded.

What effect does CO₂ have?
Emily

What is CO₂?

CO₂ is a colorless, odorless gas made up of one carbon atom and two oxygen atoms. It occurs naturally in the atmosphere and is an important part of biological and chemical processes. CO₂ is produced, among other things, by human respiration, combustion processes, and the biological breakdown of organic matter. It's essential for plants, which need it for photosynthesis. Outdoor CO₂ levels typically range between 400 and 450 ppm.

How does CO₂ build up indoors?

Indoor CO₂ concentrations can vary widely. Human breathing is the biggest source: each person produces roughly 15 to 20 liters of CO₂ per hour. In modern, well-sealed buildings, CO₂ accumulates especially quickly because natural air exchange is limited. Other sources include combustion processes such as candles, fireplaces, or gas stoves. Room size, the number of people present, and air circulation also affect CO₂ concentration: small, heavily used rooms with closed doors warm up faster, which can cause CO₂ to build up locally.

Normal indoor CO₂ levels

CO₂ is measured in ppm (parts per million). The following values serve as a guide:

CO₂ Levels and Their Meaning
CO₂ level (ppm) Quality Meaning
400–800 ppm very good ideal, fresh air quality
800–1000 ppm good harmless, slight tiredness possible
1000–1400 ppm critical concentration and performance decline
1400–2000 ppm poor fatigue, headaches, poor sleep
above 2000 ppm very poor significant strain, impaired cognitive function

Workplace guidelines and health organizations recommend staying permanently below 1000 ppm.

Effects of excessive CO₂ levels

Elevated CO₂ concentrations have a direct effect on the body. Many people feel tired, have trouble concentrating, get headaches, or sense “heavy air” long before they know the cause. Although CO₂ doesn't displace oxygen, the body reacts sensitively to changes. Studies show that very high levels can reduce performance by up to 50 percent.

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CO₂ as an indicator of air quality

CO₂ itself is not toxic, but it serves as a reliable indicator of indoor air quality. High levels indicate that the air is stale and often that there's an elevated aerosol load as well. Rooms with high CO₂ concentrations tend to have less adequate fresh-air supply, which can increase the risk of infection — especially in schools, offices, or public transport.

5 tips for healthy CO₂ levels

  1. Use burst ventilation, not tilted windows: windows should be opened fully rather than just tilted
  2. Ventilate before and after your meeting: rooms where many people gather at the same time climb into critical ranges the fastest.
  3. Let in fresh air while cooking or heating: combustion processes raise CO₂ levels significantly, so a brief burst of ventilation in between is essential for healthy indoor air.
  4. Leave doors open where possible: this helps CO₂ disperse and reach the nearest ventilation route faster.
  5. Use an air quality monitor: a device like the air-Q doesn't just show the CO₂ level, it also warns you when values tip into concerning ranges.

Summary

What is CO₂, and why does it matter indoors?
CO₂ is a naturally occurring, colorless and odorless gas produced by respiration, combustion, and biological processes. Indoors, CO₂ is a key indicator of air quality, because high levels point to stale air and insufficient fresh-air supply.
What CO₂ levels are considered normal indoors?
Levels of 400 to 800 ppm are considered very good, and values up to 1000 ppm are unproblematic. From around 1000 ppm, concentration and performance noticeably decline. From 1400 ppm, headaches, fatigue, and poor sleep can occur. The goal should be to permanently stay below 1000 ppm.
Why does the CO₂ level in rooms rise so quickly?
Because people exhale CO₂ and modern buildings are extremely airtight. Without adequate ventilation, with several people in the room, or with additional sources such as candles, gas stoves, or fireplaces, the level rises very quickly.
What are the consequences of excessive CO₂ levels?
Elevated CO₂ concentrations lead to fatigue, headaches, difficulty concentrating, and poor sleep quality. Studies show that strongly elevated levels can reduce mental performance by up to 50 percent.
Why is CO₂ considered an important indicator of good air quality?
CO₂ shows how much stale air is present in a room. High CO₂ levels often go hand in hand with a high aerosol load, increased infection risk, and insufficient ventilation. It's one of the most reliable parameters for quickly assessing air quality.
How can you permanently keep indoor CO₂ concentrations low?
Ventilate regularly with brief bursts of fresh air, let in fresh air before and after meetings, ventilate during combustion processes, leave doors open, and use an air quality device like the air-Q that warns you before levels become critical.