Exposure to a chemical harmful to health is function of the amount (or concentration) of the chemical involved and the time of it interaction. Effects may vary depending upon the nature of exposure, continuous or repeated short intervals, and may depend on the substance's ability to accumulate in the body and on the age of the exposed person and / or state of health.
The potential to cause damage or toxicity can vary enormously from substance to substance e.g. LD50 values (the single dose that can be expected to kill 50% of a population) can vary from 7000mg/kg of body weight for Ethanol to 0.02mg/kg for Dioxins.