The study of the poisonous properties of substances is known as:

Toxicology is the branch of science that specifically focuses on the study of the harmful effects of chemicals, substances, or biological agents on living organisms and the environment. This science encompasses the assessment of the degree of toxicity and the mechanisms of action of toxins, which is essential for understanding how various substances can cause adverse health effects.

The other choices present related concepts but do not pertain to the study of poisonous properties specifically. Exposure refers to the condition of being subjected to a substance, which does not encompass the systematic study of toxic properties. Threshold Limit Value relates to guidelines established for the acceptable concentration of hazardous substances in the workplace, providing limits for safe exposure rather than delving into toxicological analysis. Permissible Exposure Limits set legal limits on the amount or concentration of a substance in the workplace air, determining what levels are considered safe without specifically studying the properties of the substances themselves. Therefore, toxicology is distinct and directly relevant as the field studying the poisonous properties of substances.