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.
That has nothing to do with the nonsense that we should avoid using something powerful or dangerous simply because our human senses are not equipped to detect it. You are literally surrounded by toxicity in modern materials, power transmission, and machinery. 110 volts is enough to arc and create ozone. Basements can accumulate radon. Cooking can release toxic gasses. These are not reasons alone to avoid modern conveniences.
