@Alphabets Calculation is incomplete. You have to factor in reaction time, which on average is 200ms, and also lag (another 30-50ms). If you have only 25 health, popping a syringe in the gas is death, unless you start healing right before the first gas tick.
Anyway, it makes no sense to camp inside gas. Good Caustic users take advantage of enhanced vision and the fact that the enemy gets slowed, very few rely on the gas damage alone. You're going to get fried if you sit in my gas, that's for sure.