Carbon dioxide (CO₂) is a naturally occurring, colourless, odourless, and non-flammable gas present in the Earth’s atmosphere. It is produced through natural processes such as respiration and decomposition, as well as human activities including the burning of fossil fuels.
In outdoor environments, CO₂ levels are typically around 400–420 ppm (parts per million), although they can be higher in urban areas due to traffic and human activity, sometimes reaching 600–900 ppm. Outdoors, CO₂ is generally not considered a direct health risk at typical concentrations, but it is an important indicator of background air conditions, emission sources, and broader environmental and climate-related trends.
Indoors, CO₂ is primarily generated by human respiration, meaning its concentration can increase quickly in occupied spaces, especially where ventilation is limited. For this reason, CO₂ is widely used as an indicator of indoor air quality and ventilation effectiveness.
According to commonly used European guidelines, indoor air quality is generally considered good when CO₂ levels remain below 800 ppm. Levels between 800 and 1400 ppm indicate moderate air quality, while concentrations above 1400 ppm suggest insufficient ventilation. In practice, many standards recommend maintaining CO₂ levels as close as possible to outdoor conditions (around 400–500 ppm), with values below 1000 ppm commonly used as a general target for well-ventilated spaces.
Elevated CO₂ levels can affect comfort and cognitive performance. Concentrations above approximately 1500 ppm have been associated with reduced concentration, headaches, and drowsiness, while lower levels are linked to better productivity and cognitive function. At extremely high concentrations (above 100,000 ppm), CO₂ can cause oxygen deprivation and serious health effects; however, such levels are not typically encountered under normal conditions.
CO₂ levels indoors can be effectively managed by ensuring an adequate supply of fresh air. This can be achieved through mechanical ventilation systems or by opening windows. Monitoring indoor air quality can help optimise ventilation strategies, especially when balancing fresh air intake with outdoor pollution levels. Additional measures, such as avoiding indoor smoking, using kitchen ventilation (e.g. range hoods), and managing occupancy, can also help maintain lower CO₂ concentrations. While plants may have a small positive effect, proper ventilation remains the most effective solution.