Some substances are more toxic than others. The toxicity of a substance is described by the types of effects it causes and its potency. 1  Types of Effects: Different chemicals cause different effects.
I think I remember reading that injured means bruises and lascerations, seriously injured meant hospital but not life threatening, critically injured meant hospital ICU time and fatally injured meant injuries leading to dying sometime later.
Critical means you are in very serious condition. It does not mean you have a 50-50 chance of survival, just that it is very serious and may need surgery or a long hospital stay to survive. You will not be going home for several days or maybe weeks but it does not mean you are at death's door.