Carbon dioxide (CO₂) is naturally present in the atmosphere at levels of approximately 0.035%. Short-term exposure to CO₂ at levels below 2% (20,000 parts per million or ppm) has not been reported to cause harmful effects. Higher concentrations can affect respiratory function and cause excitation followed by depression of the central nervous system. High concentrations of CO₂ can displace oxygen in the air, resulting in lower oxygen concentrations for breathing. Therefore, effects of oxygen deficiency may be combined with effects of CO₂ toxicity.

Volunteers exposed to 3.3% or 5.4% CO₂ for 15 minutes experienced increased depth of breathing. At 7.5%, a feeling of an inability to breathe (dyspnea), increased pulse rate, headache, dizziness, sweating, restlessness, disorientation, and visual distortion developed. Twenty-minute exposures to 6.5 or 7.5% decreased mental performance. Irritability and discomfort were reported with exposure to 6.5% for approximately 70 minutes. Exposure to 6% for several minutes, or 30% for 20-30 seconds, has affected the heart, as evidenced by altered electrocardiograms.

Workers briefly exposed to very high concentrations showed damage to the retina, sensitivity to light (photophobia), abnormal eye movements, constriction of visual fields, and enlargement of blind spots. Exposure to up to 3.0% for over 15 hours, for six days, resulted in decreased night vision and colour sensitivity.

Exposure to 10% for 1.5 minutes has caused eye flickering, excitation and increased muscle activity and twitching. Concentrations greater than 10% have caused difficulty in breathing, impaired hearing, nausea, vomiting, a strangling sensation, sweating, stupor within several minutes and loss of consciousness within 15 minutes. Exposure to 30% has quickly resulted in unconsciousness and convulsions. Several deaths have been attributed to exposure to concentrations greater than 20%. Effects of CO₂ can become more pronounced upon physical exertion, such as heavy work.

CO₂ gas is not irritating to the skin. Contact with liquefied CO₂ can cause frostbite. Symptoms of mild frostbite include numbness, prickling and itching in the affected area. Symptoms of more severe frostbite include a burning sensation and stiffness of the affected area. The skin may become waxy white or yellow. Blistering, tissue death and gangrene may also develop in severe cases.

Exposure to very high concentrations of the gas may cause a stinging sensation. Inhalation of high concentrations of CO₂ has been reported to produce effects on vision. See "Health Hazards associated when I breath in Carbon dioxide gas" above for details. Direct contact with liquefied CO₂ may cause freezing of the eye. Permanent eye damage or blindness could result.

Ingestion is not a relevant route of exposure for gases or for liquefied CO₂.

Several studies have monitored workers repeatedly exposed to elevated levels of CO₂ gas. Exposure to 1-1.5% for 42-44 days caused a reversible acid-base imbalance in the blood and an increased volume of air inhaled/minute (minute volume). In another study, harmful effects were not observed in 19 brewery cellar workers repeatedly exposed to average concentrations of 1.1% CO₂, with levels occasionally up to 8% for a few moments. Submarine occupants exposed to 3% CO₂, 16 hours/day for several weeks experienced flushing of the skin, an impaired response of the circulatory system to exercise, a fall in blood pressure, decreased oxygen consumption, and impaired attentiveness. Adaptation to some of the effects of long-term exposure to CO₂ has been reported.

There is no human information available. One limited animal study cannot be evaluated.

There is no human information available. No conclusions can be drawn from the limited animal information.

There is no human information available. There is limited evidence in animal studies that exposure to very high levels of CO₂ gas during pregnancy may cause developmental effects. It is not possible to fully evaluate these studies due to lack of information on maternal toxicity.

Animal studies indicate that a combination of CO₂ and carbon monoxide causes an increase in the rate of carbon monoxide-binding to hemoglobin.

Small amounts of CO₂ are produced during cellular metabolism and CO₂ is a normal component of the body. CO₂ is present in the blood as dissolved CO₂, carbonic acid, and the bicarbonate ion. The majority of CO₂  is excreted from the body in exhaled air.