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 hits are otherwise normal weapon strikes that randomly deal increased damage on enemies. When a weapon critically hits (or crits), the damage is displayed as a yellow number on the HUD instead of the usual white coloring (though crits on shields are still blue).