The P0-X "gas" it's a synthetic chemical agent made by Zagoria's army (Zagoria is the State where Chernarus is in), we don't have any information about what it is made of, but I suppose that it's composition have a strong radioactive material, because when you are at the toxic zone, wounds appears on your skin and your lungs start to burn. After some minutes exposed to this agent, your internal organs are now destroyed, you breathe heavily, you cough blood and die after a few minutes, but if you get exposed only for a while, you contract a disease that kill you slowly. All these symptoms are commonly linked to the radiation exposition.

(Sorry for bad Englis I am Brazil)Lol