Toxicology
Toxicology is the study of how poisons and harmful chemicals affect living things.
Think of it this way
A small amount of hot sauce in a bowl of soup adds flavor, but it does not hurt you. Pour in half the bottle, and your mouth burns — the same sauce is now harmful. Pour in the whole bottle, and the soup becomes painful to eat at all. How much of a substance enters your body is what makes it safe or dangerous.
Explaining toxicology by grade level
Some things can make bugs or people sick. How much of it matters a lot. In one test, a bug spray hurt flies. But adding a special charcoal to the water trapped the spray and kept the flies safe.