Toxicity:

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).

According to the Australian Maritime Safety Authority, "Prolonged exposure to moderate concentrations can cause acidosis and adverse effects on calcium phosphorus metabolism resulting in increased calcium deposits in soft tissue. Carbon dioxide is toxic to the heart and causes diminished contractile force. At concentrations of three per cent by volume in air, it is mildly narcotic and causes increased blood pressure and pulse rate, and causes reduced hearing. At concentrations of about five per cent by volume it causes stimulation of the respiratory centre, dizziness, confusion and difficulty in breathing accompanied by headache and shortness of breath. At about eight per cent concentration it causes headache, sweating, dim vision, tremor and loss of consciousness after exposure for between five and ten minutes."

Due to the health risks associated with carbon dioxide exposure, the U.S. Occupational Safety and Health Administration says that average exposure for health adults during an eight-hour work day should not exceed 5,000 ppm (0.5%). The maximum safe level for infants, children, the elderly and individuals with cardio-pulmonary health issues is significantly less. For short-term (under ten minutes) exposure, the U.S. National Institute for Occupational Safety and Health (NIOSH) and American Conference of Government Industrial Hygienists (ACGIH) limit is 30,000 ppm (3%). NIOSH also states that carbon dioxide concentrations exceeding 4% are immediately dangerous to life and health.

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.

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.^[32] 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.