Carbon dioxide is a natural gas found in our atmosphere. By volume, dry air contains approximately 78% nitrogen, 21% oxygen, 1% argon, but only 0.04% carbon dioxide and small amounts of other gases.
At 0.04% CO2 (400 parts per million) under normal conditions, you'll never have to worry about CO2.
However, there are 3 ways carbon dioxide levels can be dangerous to humans and animals.
Breathing in a sealed environment
The most common way is breathing in a sealed environment. Exhaled breath contains about 3% CO2. As we breath in a sealed environment, oxygen is slowly converted into CO2. The oxygen level falls while the CO2 level rises. The Occupational Safety and Health Administration, OSHA, has determined the optimal breathing range to be between 19.5 and 23.5 percent oxygen. Serious side effects will occur if the oxygen levels are outside of the safe zone. At levels at or below 17 percent, your mental abilities become impaired.
In a sealed environment, as oxygen levels drop from 21% to 17%, the CO2 level will rise to 4%. This level of CO2 can result in symptoms like dizziness, confusion and headache. At higher levels of CO2, it can be life-threatening.
A real world example of this was Apollo 13, where the CO2 buildup was a more urgent problem than the oxygen shortage.
Natural out-gassing of CO2
The second way CO2 can be dangerous is a sudden out-gassing of CO2 from the ground. Under the right conditions, mines, volcanoes, or fissures in the earth’s surface can suddenly leak tremendous quantities of CO2. The heavier-than-air carbon dioxide settles into low areas and becomes a death trap for any living organisms inside it.
For example, in 1986 Lake Nyos in Cameroon emitted a large cloud of CO2, which suffocated 1,700 people and 3,500 livestock in nearby towns and villages.
Leaking compressed CO2 system
The third way CO2 can be dangerous is a leak in a compressed CO2 system. Virtually every restaurant, bar or brewery in the country stores pressurized tanks of carbon dioxide on premises. The gas is odorless, colorless and tasteless. A CO2 leak inside an enclosed space becomes a potential death trap for anyone caught inside.
While death by CO2 leaks are rare, they do happen. To protect the public, state and local municipalities around the country are writing new codes that require the use of CO2 safety alarms in buildings where compressed CO2 is stored.
Signs of CO2 poisoning
Remember the signs of CO2 poisoning: disorientation, increased heart rate, muscle tremors, and shortness of breath. In addition, carbon dioxide reacts with saliva, resulting in carbonic acid and an acidic taste inside the mouth.Image by: Häggström, Mikael. "Medical gallery of Mikael Häggström 2014". Wikiversity Journal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 20018762.