Poisons mainly invade the human body through the respiratory tract, digestive tract and skin. 1. Respiratory tract: When the respiratory tract inhales toxic gases or dust and other poisons from the outside world, due to the large surface area of the alveoli and the abundant capillaries in the lungs, the poisons can quickly enter the blood circulation and produce poisoning. 2. Digestive tract: The digestive tract is the most common route. Through diet, poison can enter into the digestive tract from the mouth and esophagus, and through the action of digestive juices and digestive enzymes, the poisonous substances can change and enter into the blood circulation, so that the poison can be distributed to the tissues and organs of the whole body. 3. Skin: It enters into the human body through incision or infected wound, conjunctiva and all surface skin absorption. Especially when the human body is accidentally exposed to a toxic environment or cleaned with toxic substances, the poison can invade directly from the skin and spread to the surrounding tissues and organs.