Carbon dioxide (CO2) is a naturally occurring gas that acts as a minor atmospheric component and a major biological regulator. Defining “below average” for CO2 depends entirely on the frame of reference, whether discussing the long-term history of Earth’s climate or the immediate air quality inside a building. This gas is a fundamental part of the carbon cycle, affecting planetary temperature globally and personal well-being locally.
The Global Context of Historical CO2 Levels
The global atmospheric concentration of CO2, measured in parts per million (ppm), provides the primary scientific baseline. Current concentrations have surpassed 420 ppm, reflecting a significant increase driven by human activity since the Industrial Revolution. This is significantly higher than the established historical reference: the pre-industrial baseline, which hovered consistently around 280 ppm.
Compared to this 280 ppm benchmark, current levels are approximately 50% higher, pushing the atmosphere into territory not seen for millions of years. In the context of Earth’s climate history, a “below average” CO2 level would be one lower than this pre-industrial benchmark. Concentrations below 280 ppm represent levels that existed during previous ice ages or before the rise of human civilization.
Analyzing air bubbles trapped in ice cores shows that atmospheric CO2 rarely exceeded 300 ppm over the last million years. Levels significantly below this range, such as those near 180 ppm, are associated with periods of deep glaciation. For scientists tracking climate change, “below average” measures how far the atmosphere has deviated from its stable, pre-industrial state.
Measuring Low CO2 in Indoor Air
When considering localized environments, such as homes or offices, the definition of “average” and “low” changes completely. The typical outdoor air concentration (400 to 420 ppm) becomes the practical minimum baseline for non-industrial settings, as this is the freshest air available for ventilation.
Indoor air quality (IAQ) monitoring uses CO2 as a proxy for ventilation effectiveness because humans exhale the gas. In occupied spaces, typical CO2 levels range between 400 and 1,000 ppm, depending on occupancy and air exchange rate. Levels consistently above 1,000 ppm indicate poor ventilation, which can lead to drowsiness and reduced cognitive function.
In this indoor context, a reading considered “below average” is anything approaching or slightly less than the outdoor baseline of 400 ppm. A measurement of 380 ppm, for instance, signals extremely high air exchange rather than a true deficit of CO2. Such low readings are desirable for human health, indicating a well-ventilated space free from stale air and airborne pollutants. Maintaining levels below 800 ppm during occupancy is often a goal for optimal comfort and performance in modern buildings.
Implications of CO2 Scarcity on Life
While high CO2 is a well-known environmental concern, truly low levels introduce significant biological challenges, particularly for plant life. Since CO2 is a substrate for photosynthesis, a severe drop in atmospheric concentrations would cause global ecosystems to suffer from carbon starvation.
The critical threshold for most plants is the CO2 compensation point, where carbon uptake equals carbon release during respiration. For C3 plants, this point is typically between 40 and 100 ppm. If global CO2 concentrations dropped below approximately 150 ppm, the vast majority of plant life would struggle to survive, initiating a rapid decline in biomass and severely impacting the food chain.
For human physiology, low environmental CO2 is not a concern because breathing is regulated by the body’s internal CO2 levels. The condition of having low CO2 in the blood is called hypocapnia, a physiological state caused by hyperventilation. This rapid breathing forces the excessive expulsion of CO2 from the lungs, leading to symptoms like tingling, dizziness, and muscle cramps. Hypocapnia is an internal, temporary response to over-breathing, not a result of low CO2 in the air we breathe.