The adjective toxic refers to the damage caused by a poison. It is used to describe both the poison itself having the ability to damage, and the body that has been damaged.
It comes from the Greek word toxon, which means “bow”, as in bow and arrow. The Greek term toxikon pharmakon means “poison arrows” made with the natural poison from the poison dart frog. These were used in hunting, the poison being an aid to killing the animal. So something “toxic” has the character of a bow, being a weapon to kill things.
The noun toxic (usually used in plural as toxics) is a manmade toxic agent, a poison. These are also called toxicants. Toxicologists and maybe doctors call them toxicants. I generally use the word toxics or toxic chemicals for a group, and toxicant if I am referring to one single manmade toxic agent.
[Note that a toxin is a naturally-occurring poisonous substance that is a specific product of the metabolic activities of a living organism, notably toxic when introduced into the tissues, such as Botulinus toxin.
Frequently this word is used incorrectly to refer to a manmade poison when the word toxic or toxicant should have been used.] Doctors may (or may not) know this definition and so think you are talking about toxins when you are really talking about toxics.
Probably the most correct way to refer to indoor air pollutants are “toxins and toxicants,” which would cover both living things like mold and manmade toxic chemicals.
To intoxicate is to have manmade poisons introduced from the outside inside one’s body, especially in amounts that cause body functions to be impaired. Though this word also has the definitions of causing someone to lose control of their faculties or behavior, and to excite or exhilarate, and it is generally exclusively used in reference to alcoholic beverages, the original root of the word is “to put a poison into.” And this applies to all poisons. We quite literally are intoxicated by toxic chemicals we are exposed to every day.
The toxicity of a poison is the relative degree of harm caused by it.
Toxicology is the science of poisons and their effects, particularly on living systems.