Humans can detect CO2. An elevated level of CO2 makes you feel out of breath. Reducing O2 has no impact other than you pass out. If you use fire, the "invisibl

Humans can detect CO2. An elevated level of CO2 makes you feel out of breath. Reducing O2 has no impact other than you pass out. If you use fire, the "invisible" risks are oxygen deprivation or monoxide poisoning. You will be quite aware that something is wrong when there is too much CO2.