Final answer:
Toxic substances can enter the human body through the gastrointestinal tract by being ingested through the mouth and entering the digestive system.
Step-by-step explanation:
Toxic substances can enter the human body through the Gastrointestinal tract. They can be ingested through the mouth and enter the digestive system. This route of exposure is particularly vulnerable because particles, including microorganisms, can be constantly ingested into the body.