Toxicity is a measurement of the dosage needed of a particular substance to damage a living organism. A substance becomes toxic at the dose which begins to damage an organism. Contrary to popular belief, all substances have a certain toxicity. Even water and oxygen are dangerous to organisms at certain concentrations. Furthermore, different species experience toxins in different ways. The toxicity of a certain substance, like sulfur for instance, will vary with the species. To humans, large doses of sulfur are fatal. However, to the organisms living in the heat of volcanic vents at the bottom of the ocean, sulfur is a necessary and welcome nutrient.
Toxicity is determined by an organisms reactions to various dosages of a chemical. The lethal dose is determined by a test in which organisms are dosed with the chemical in question. The dosage which kills half of the population is considered the lethal dose. This is referred to as an LD50 test, and used to be a standard measure of toxicity. However, the ethics and reliability of this test have been called into question in recent decades. Once it was understood that different toxins can effect similar organisms in vastly different ways, it was no longer reliable to use laboratory animals to predict human toxicity levels. New tests and measures are being developed to study and determine toxicity in ethical and reliable ways. The field of studying the toxicity of different chemicals is called Toxicology.
The most important thing to remember about toxicity is that everything is a toxin, and only the dosage matters. Toxins work in many different ways, and toxicology has many means of measuring and documenting the damage that different toxins due. While some toxins seem extremely potent because they deliver a lot of damage at once, other toxins which seep into the body slowly can do as much or more damage.
Toxicity and Exposure
Certain chemicals or substances can be toxic even in a minor, or one-time exposure. This is known as an acute exposure, and all substances have an acute toxicity. Some substances can be very toxic acutely, even in a single exposure. Consider snake venom. To be efficient for the snake, a very small amount of venom must incapacitate their prey. It would cost them too much water and energy to produce large amounts of venom, and it would also be hard to inject in a single dosing.
However, venom and poisons are not the only acute toxins. Acute toxins include things like carbon dioxide and nitrous oxide. Carbon dioxide is produced by your cells as they create ATP, and nitrous oxide is the gas dentists use to put their patients under before a surgery. Both of these gases are potentially deadly at a certain pressure and concentration in the body. The body must actively work to dispel these gases, or it will undergo gas narcosis, a condition of euphoria and then unconsciousness. Scuba divers who venture too deep also experience this, as the gases more easily establish narcosis under pressure. In effect, the pressure increases the acute toxicity of the gas.
Chronic toxicity is the opposite of acute toxicity. It is a measure of how toxic a substance is over a longer period of time. This could be anything from weeks to years, but it is just as significant to understand the chronic toxicity of a substance. Many substances we use in consumer products are new to science. It is easy to test their acute toxicity, because it is easy to administer a single dose and observe and organism for a week or less. When observing chemicals for signs of being a chronic toxin, the observer must watch the system for the entire life of an organism.
For this reason, the chronic toxicity of many products used in households is not well understood. The Food and Drug Administration (FDA), as well as other regulatory agencies, actively work to keep toxic chemicals out of the hands of consumers. However, with the number of new chemicals and products appearing every year, it is virtually impossible for these organizations to police everything. Combined with the subtle and sometimes hidden illness associated with chronic toxins, this makes finding and measuring chronic toxicity difficult. Scientists use the sciences of statistics and epidemiology to track and understand chronic toxicants from products, the environment, and other sources.