Carbon Dioxide (CO2)

CO2 is a natural product of aerobic respiration — every person exhales roughly 200 mL per minute — but atmospheric concentrations remained stable at roughly 280 ppm for the 10,000 years before industrialization [1]. The burning of coal, oil, and natural gas that powered the Industrial Revolution from the 18th century onward released carbon from geological storage at unprecedented rates. In 2023, atmospheric CO2 reached 422 ppm — the highest in at least 3 million years [2]. Indoors, CO2 builds rapidly when many people share poorly ventilated spaces: classrooms and conference rooms commonly exceed 1,000 ppm (the threshold for detectable cognitive impairment) and can reach 2,000-3,000 ppm by end of day [1]. Industrial sources include combustion, fermentation (breweries, bakeries), dry ice sublimation, and fire suppression systems [2].

Atmospheric CO2 at current outdoor concentrations (~422 ppm) poses no acute health risk [1]. Indoor CO2 accumulation is the relevant personal exposure: classrooms, offices, and bedrooms with closed windows accumulate CO2 from occupant respiration [2]. Low-ventilation buildings in cold climates (where windows are closed for energy conservation) are worst affected. Sleeping in a bedroom with a closed door and one or more occupants can raise CO2 to 1,500-2,500 ppm by morning [1]. Industrial settings with incomplete combustion (poorly tuned boilers, internal combustion engines in enclosed spaces) can produce CO2 concentrations dangerous to human life [2].

At 1,000 ppm, multiple double-blind studies show measurable impairment in decision-making, concentration, and working memory — even in young, healthy adults [1]. At 2,500 ppm, cognitive scores can drop 50% in some domains. These concentrations are regularly exceeded in occupied classrooms, meeting rooms, and sleeping areas [2]. At very high concentrations (above 5,000 ppm, possible in industrial accidents), CO2 causes headache, dizziness, and at extreme levels (above 10%) unconsciousness and death by asphyxiation [1]. The climate connection is the larger-scale concern: CO2's greenhouse warming effect is already causing measurable increases in extreme heat events, sea level rise, and weather pattern disruption that create cascading public health impacts [2].

Students and office workers in poorly ventilated buildings experience chronic cognitive impairment that reduces productivity and learning without recognizing the environmental cause [1]. Children in underinvested school buildings with old HVAC systems are particularly affected; ventilation standards are frequently not met in aging school infrastructure [2]. Infants and people with respiratory conditions are more sensitive to CO2 accumulation during sleep. Workers in confined spaces (sewers, tanks, mines) face life-threatening CO2 buildup from biological decomposition or displacement of oxygen [1]. Everyone is at risk from climate-change impacts driven by atmospheric CO2 — heat illness, worsened air quality, and infectious disease spread [2].

1. Open windows or increase mechanical ventilation, especially during occupied hours — aim to keep indoor CO2 below 800 ppm using a low-cost CO2 monitor ($50-150) [1]. 2. In sleeping areas, crack a window slightly or run an air exchanger to prevent overnight CO2 buildup that impairs sleep quality and morning cognition [2]. 3. Place CO2 monitors in classrooms and office spaces; if readings consistently exceed 1,000 ppm, advocate with building management for upgraded ventilation. 4. At home, run kitchen range hoods when cooking (gas stoves produce CO2 and other combustion gases) [1]. 5. Support building energy codes that require adequate ventilation rates — many older buildings were built to energy-saving standards that sacrificed fresh air exchange. 6. Reduce your household fossil fuel consumption through efficiency improvements and clean energy choices to address the climate dimension [2].