The Hidden Health Risks of CO₂: Rethinking Acceptable Exposure Limits

Discussions about Carbon Dioxide (CO₂) and climate change typically focus on its role as a greenhouse gas driving global warming. However, a growing body of evidence warns us about the direct physiological threat that rising CO₂ levels pose to human health, particularly in indoor environments where concentrations can far exceed outdoor levels.

CO₂ is an invisible odorless gas in the air around us. Generally, the air we breathe is a mixture of gases predominantly Nitrogen (78%) and Oxygen (21%) with CO₂ only accounting for approximately 0.04%.

Ambient CO₂ levels generally range from 400 to 450 parts per million (ppm) with the typical average being at 425 ppm - already at a 140% increase from pre-industrial times. Whilst at these levels there is no significant health effects to humans, typical indoor CO₂ levels far exceed these levels reaching concentrations of 1000 to 2000 ppm and more due to poor ventilation.

Indoor CO₂ Levels

Humans are known to be the primary sources of CO₂ indoors as CO₂ is naturally produced in the human body and exhaled into air via breathing. As such, CO₂ can accumulate in enclosed spaces due to inadequate ventilation. Consequently, CO₂ has been traditionally considered to be simply an indicator of ventilation effectiveness. Yet growing research highlights CO₂ as a direct physiological stressor with measurable health impacts.

Fun fact: when we speak of going outside to get some“fresh” air, many people think of a higher amount of oxygen in the outdoor air. But this stays pretty much constant, no matter how poorly ventilated the room is. It’s the increasing amount of CO₂ that makes indoor air feel stuffy and causes you to get sleepy!

The Health Impacts of CO₂

Scientific evidence from key studies (see references) suggests that even moderate increases in CO₂ can have physiological and cognitive effects on humans.

• Above 500 ppm (0.05%)
  • Increased heart rate, increased blood pressure and peripheral blood circulation
  • At exposures above 700 ppm Sick Building Syndrome Symptoms appear such as headaches and tiredness and eye, nose and throat symptoms
• Above 1000 ppm (0.1%)
  • Noticeable Cognitive Performance decline, including reduced decision making capabilities and increased fatigue
  • Respiratory Symptoms in children
  • Extended exposure above 1400 ppm is linked to increased oxidative stress leading to cellular damage.
  • Long-term CO₂ exposure at greater than 2000 ppm may contribute to metabolic dysregulation, inflammation and increased strain on the heart muscles which may lead to chronic high blood pressure heart attacks, stroke and heat failure.
• Above 2500 ppm (0.25%)
  • Severe Cognitive Impairment with effects on strategic thinking and information utilization
• Above 10,000 ppm (1%)
  • Levels above 10,000 ppm are unlikely to be found in typical indoor environments. But for completeness, we’ll mention them here as well.
  • Increased respiratory rate resulting in an increase in the amount breathed in and out in one minute, with metabolic stress and increased brain blood flow
• Above 30,000 ppm (3%)
  • Decreased exercise tolerance with resultant difficulty breathing
• Above 50,000 ppm (5%)
  • Dizziness, headaches, confusion and shortness of breath
• Above 80,000 ppm (8%)
  • Dimmed vision, sweating, tremor unconsciousness and possible death

Of note, vulnerable population groups may be even more susceptible to these symptoms. Children are more sensitive due to higher respiratory rates and their developing brains whilst the elderly have a greater susceptibility to metabolic and cardiovascular effects. Additionally, individuals with respiratory or cardiovascular diseases may experience exacerbated symptoms at lower thresholds.

A CO₂ Scale for the Spaces We Inhabit

Given these findings, there is a need to shift how we assess CO₂ levels indoors. Instead of using CO₂ as a marker of ventilation efficiency, it should be recognised as a direct factor influencing human health and productivity. While CO₂ monitoring is increasingly accessible, there remains a critical question: At what levels should alarms be set to safeguard health and performance? A standardized CO₂ Exposure Scale similar to the Air Quality Index (AQI), could help guide public awareness and regulatory standards.

The Need for Optimized Standards

Primary research conducted on indoor CO2 levels within the range of 500ppm to 1000 ppm provides limited evidence to support claims due to the quality of the studies, small sample size and the fact that other indoor air pollutants may have been contributing to symptomatology within that CO2 range. This highlights the need for further studies to be conducted on this research theme. Yet the evidence is clear - indoor CO₂ levels directly impact human cognition, decision making and overall health.

As humans, we spend almost 90% of our lives indoors, from residences, to schools and workplaces. Frequent exposure to CO₂ levels above healthy limits, can lead to sick building syndrome, persistent fatigue, long-term health risks and decreased job satisfaction in adults. For children the stakes are even higher as elevated CO₂ levels can lead to subpar academic performance and increased respiratory symptoms.

The culminating effects of unhealthy CO₂ levels compounds, resulting in reduced productivity, increased absenteeism and compromised work and learning environments, ultimately leading to increased health, economic and societal costs. As increasing studies are finding more health implications of CO₂ levels, questions are arising around the traditionally acceptable industry target of below 1000 ppm. While carbon dioxide monitoring can be easily implemented in our buildings, at what levels should our alarms be set to protect our occupants’ well-being?

References:

Azuma K, Kagi N, Yanagi U, Osawa H. Effects of low-level inhalation exposure to carbon dioxide in indoor environments: A short review on human health and psychomotor performance. Environ Int. 2018 Dec;121(Pt 1):51-56. doi: 10.1016/j.envint.2018.08.059. Epub 2018 Aug 30. PMID: 30172928.

Lowther SD, Dimitroulopoulou S, Foxall K, Shrubsole C, Cheek E, Gadeberg B, Sepai O. Low level carbon dioxide indoors—A pollution indicator or a pollutant? A health-based perspective. Environments. 2021;8(11):125. doi:10.3390/environments8110125.

Stumm RE. Carbon dioxide’s direct impact on down-regulating the human species. Sci Total Environ. 2023 Dec 20;905:167198. doi: 10.1016/j.scitotenv.2023.167198. Epub 2023 Sep 19. PMID: 37734619.

Satish U, Mendell MJ, Shekhar K, Hotchi T, Sullivan D, Streufert S, Fisk WJ. Is CO2 an indoor pollutant? Direct effects of low-to-moderate CO2 concentrations on human decision-making performance. Environ Health Perspect. 2012 Dec;120(12):1671-7. doi: 10.1289/ehp.1104789. Epub 2012 Sep 20. PMID: 23008272; PMCID: PMC3548274.