The world is full of toxins and for the acute porphyria patients, the toxins are often lethal.
Toxicology is the study of the way that toxicants work.
Toxicology studies compounds that are harmful to living tissue. The potency of various toxicants can be compared by comparing LD50 values obtained for the various toxicants in similar animals similarly exposed to the toxicants. The lower the LD50 value, the more potent the toxin. Before human are often studied, plant life is studied. Cu usually allows normal growth.