A toxic substance is one that is harmful to living organisms or cells. Toxicity is the level of harm a toxin can cause, so you would need only a small amount or a short exposure to a substance with a high toxicity for it to be harmful, while for a substance with a low toxicity, you need to have a high dose or a long exposure for it to be harmful.
Toxic substances can be:
chemical – found naturally in the environment (for example, lead), manufactured by humans (for example, dioxin) or made by organisms (for example, chemicals made by fungi that act as toxins to insects, snake venom, or poison ivy)
biological – an infection by bacteria, viruses or fungi can be toxic to cells or an organism
physical – the toxic effect of radiationheat and cold.
How do we measure toxicity?
The toxicity of a substance can be measured by using a bioassay – a type of test that uses a living organism. In the old days, when miners went into coal mines, they would take a canary in a cage with them. If there was toxic carbon monoxide present, the canary would die, and the miners would know they had to get out of the mine. This is a very simple type of bioassay – now days, scientists often use cells grown in a laboratory to test how toxic a substance is.
The toxicity measured is expressed as an LD50. This is the median level of amount of toxin that kills 50% of the tested population. For a toxin such as snake venom, the LD50 would show the number of milligrams of venom, and for a biological toxin, the LD50 might be the number of bacteria you need to ingest to kill half the tested population.
Obviously, scientists can’t test toxicity on humans so they test on smaller animals or on cells and an estimate is made by comparing the relative size and type of animal.
In the example of the miners using canaries as a bioassay, the bird is small, maybe 100 times smaller than a human, so a person might need 100 times the dose for it to be lethal. But because the canary is a bird and humans are mammals, this factor could be different. A bird may be 10 times more susceptible to this particular toxin than humans, so the person might, in fact, need 1000 times the dose than that which kills the canary.
Use this site to carry out a simple bioassay experiment to test for toxicity