Toxicity of Carbon dioxide

Carbon dioxide content in fresh air varies between 0.03% (300 ppm) and 0.06% (600 ppm), depending on the location (see graphical map of CO2 in real-time).

A person's exhaled breath is approximately 4.5% carbon dioxide by volume.

Adaptation to increased levels of CO2 occurs in humans. Continuous inhalation of CO2 can be tolerated at three percent inspired concentrations for at least one month and four percent inspired concentrations for over a week. It was suggested that 2.0 percent inspired concentrations could be used for closed air spaces (ex. Submarine) since the adaptation is physiological and reversible. Decrement in performance or in normal physical activity does not happen at this level.

It is dangerous when inhaled in high concentrations (greater than 5% by volume, or 50,000 ppm). The current threshold limit value (TLV) or maximum level that is considered safe for healthy adults for an eight-hour work day is 0.5% (5,000 ppm). The maximum safe level for infants, children, the elderly and individuals with cardio-pulmonary health issues is significantly less.

These figures are valid for pure carbon dioxide. In indoor spaces occupied by people the carbon dioxide concentration will reach higher levels than in pure outdoor air. Concentrations higher than 1,000 ppm will cause discomfort in more than 20% of occupants, and the discomfort will increase with increasing CO2 concentration. The discomfort will be caused by various gases coming from human respiration and perspiration, and not by CO2 itself. At 2,000 ppm the majority of occupants will feel a significant degree of discomfort, and many will develop nausea and headaches. The CO2 concentration between 300 and 2,500 ppm is used as an indicator of indoor air quality.

Acute carbon dioxide toxicity is sometimes known as by the names given to it by miners: blackdamp (also called choke damp or stythe). Miners would try to alert themselves to dangerous levels of carbon dioxide in a mine shaft by bringing a caged canary with them as they worked. The canary would inevitably die before CO2 reached levels toxic to people. Carbon dioxide caused a great loss of life at Lake Nyos in Cameroon in 1986, when an upwelling of CO2-laden lake water quickly blanketed a large surrounding populated area. The heavier carbon dioxide forced out the life-sustaining oxygen near the surface, killing nearly two thousand people.

Carbon dioxide ppm levels (CDPL) are a surrogate for measuring indoor pollutants that may cause occupants to grow drowsy, get headaches, or function at lower activity levels. To eliminate most Indoor Air Quality complaints, total indoor CDPL must be reduced to below 600. NIOSH considers that indoor air concentrations that exceed 1,000 are a marker suggesting inadequate ventilation. ASHRAE recommends they not exceed 1,000 inside a space. OSHA limits concentrations in the workplace to 5,000 for prolonged periods. The U.S. National Institute for Occupational Safety and Health limits brief exposures (up to ten minutes) to 30,000 and considers CDPL exceeding 40,000 as "immediately dangerous to life and health." People who breathe 50,000 for more than half an hour show signs of acute hypercapnia, while breathing 70,000 – 100,000 can produce unconsciousness in only a few minutes. Accordingly, carbon dioxide, either as a gas or as dry ice, should be handled only in well-ventilated areas.