Exposure (toxicology)
Anything can be a toxin if enough of it gets into a person's body.  For example, even water can be toxic.  If a person drinks enough water (usually more than 6 gallons in one day), they can get water toxicity (also called "water poisoning" or "water intoxication").  This causes the body's cells to swell up with extra water.  Swelling in the brain's cells can cause brain damage or death.  Water, in normal amounts, helps keep us alive; but if a person is exposed to enough of it, it becomes a toxin, and it can kill the person.