The defensive system on the third level seems to have upgraded from the previous two, in addition to the original facilities, there is now a poison gas defense. The gas covers the entire third level, but the prison cells seem to isolate the gas, so they've actually become the safest places. Prisoners considering escape have to weigh how long they could survive in poison gas first.
Seeing the level of toxins on his body increase, Mu You quickly switched to his vampire skin.
[You switched to your Vampire Skin. Under the corrosion of the poison gas, the 'Survival of the Fittest' Talent comes into play, and your body begins to evolve rapidly to adapt to the toxin. Your HP decrease rate has slowed down.]
Luckily, Mu You had just merged with the Survival of the Fittest Talent a few days ago.
The poisonous gas is definitely one of the harsh environments, but this talent does not help him directly isolate the gas, but helps him adapt to it.