15/05/2025

When I started discovering the impact of indoor CO₂ on our bodies, I was in shock. I was asking myself:

Why isn’t this problem mainstream?!

Hopefully, after reading this post, you’ll share my thoughts on this.

Some points before I start:
I want to provide high-quality information (with sources!) so this post can be shared without hesitation with anyone interested.
I’ll be relying on Canadian government agency report “Residential indoor air quality guidelines: Carbon dioxide” which summarizes the current state of science in regards of indoor CO₂. It also includes research about indoor CO₂ levels in Canadian homes and schools. In my previous post, I presented my anecdotal experience on how widespread elevated indoor CO₂ levels can be. But having some high-level, large-scale research brings more credibility. Also, please remember that each year we add around 2.5 ppm of CO₂ to the atmosphere (which results in higher indoor CO₂ levels as well). Some studies from 10–20 years ago might, therefore, even be underestimating the current indoor CO₂ exposure and its potential effects.

Ok, let’s get started!

Neurological Effects

A growing body of research has investigated the impact of elevated indoor carbon dioxide (CO₂) concentrations on human neurological performance, cognitive function, and symptoms such as headaches, fatigue, and difficulty concentrating. While many of these studies do not fully isolate CO₂ from co-pollutants (e.g., volatile organic compounds or bioeffluents), controlled chamber studies provide important insights into CO₂’s potential role.

Cognitive Performance and Mental Effort

Controlled exposure experiments show that even moderate increases in CO₂ concentrations can impair cognitive function:

  • Wargocki et al. (2000) demonstrated improved task performance (e.g., typing speed) with increased ventilation and lower CO₂ levels, with exposures ranging from 195 to 1,266 ppm above outdoor concentrations.
  • Satish et al. (2012) found reduced decision-making performance at 1,000 ppm compared to 600 ppm, using the Strategic Management Simulation (SMS) test.
  • Kajtar and Herczeg (2012) reported diminished task performance and increased mental effort at 3,000 ppm CO₂ compared to 600 ppm.
  • Allen et al. (2016) observed that a 400 ppm increase in CO₂ led to a 21% reduction in cognitive test scores, suggesting a dose-dependent decline in performance. This study also found an 18% improvement in scores with every 20 cfm increase in ventilation.

Symptom Reporting and Neurophysiological Discomfort

Several controlled exposure studies show that high CO₂ levels can increase neurological discomfort:

  • Zhang et al. (2017) found that exposure to 3,000 ppm CO₂ with bioeffluents significantly increased headache, fatigue, sleepiness, and difficulty thinking, along with worsened task performance. Notably, CO₂ alone (without bioeffluents) did not produce statistically significant effects.
  • Other studies (e.g., Maula et al. 2017; Vehviläinen et al. 2016; Maddalena et al. 2015) reported symptoms like headache and sleepiness under low-ventilation conditions, but could not distinguish whether the effects were from CO₂ or other pollutants.

Neurological Effects of CO₂ in Canadian Homes, Schools, and Daycares

According to data collected by Health Canada, indoor CO₂ concentrations in Canadian environments—including homes, schools, and daycare centres—can reach levels associated with neurological symptoms such as:

  • Headache
  • Dizziness
  • Fatigue or tiredness
  • Difficulty concentrating
  • Reduced cognitive performance (e.g., test scores)

Typical Indoor Levels in Canadian Homes:

Comparison of CO2 concentrations in Canadian homes to CO2 concentrations associated with health effects:

* Some associations were observed in epidemiological studies (in offices and schools) and controlled exposure studies (generally 1- to 5-hour exposures). However, many study limitations were noted, and causality was not linked to CO2. Carbon dioxide levels closer to the bottom (green) represent the lowest potential risk of health effects.
(source: Canada Health)

Typical Indoor Levels in Canadian Schools and Daycare Centres:

Comparison of CO2 concentrations in a limited number of Canadian schools and daycare centres to CO2 concentrations associated with health effects:

Some associations were observed in epidemiological studies (in offices and schools) and controlled exposure studies (generally 1- to 5-hour exposure). However, many study limitations were noted, and causality was not linked to CO2. Carbon dioxide levels closer to the bottom (green) represent the lowest potential risk of health effects.
(source: Canada Health)

The above Health Canada study (from 2014) shows that elevated CO₂ can indeed be quite a common problem. Since 2014, outdoor CO₂ levels have increased by about 30 ppm, so the importance of CO₂ control continues to grow each year.