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.
Safeopedia explains Critical Injury. A critical injury may be fatal if not dealt with urgently. The patient should be given immediate medical emergency assistance or first aid and taken to the nearest hospital without any delay. Response time is vital in cases of critical injury. The scene of a critical injury, especially when work related, should be secured for: