Well it's not really a matter of opinion. Some gases are deadlier than others although it depends on the context. Which do I personally think has the worst effects? Probably Mustard Gas. Apart from fucking up your skin it blows up the veins in your lungs and causes you to drown in your own blood. Btw KingWolf Anthrax is a virus, not a gas. It's spread by inhaling the spores which are airborne.
Because, it feeds flames. When we inhale Oxygen, we exhale Carbon Dioxide. Certain processes take place in the body during that period. If the oxygen's concentration would be higher, we'd ignite from the inside!
Of course, if its concentration is also lower, we might suffocate.
This is one of those "you can't live without it, but can't continue living with it" cases.
Infrequently active. Don't expect an immediate response. Best to contact me at a different locale. If I create a thread, expect me to be quite active.