Toxicology
Toxicology is the study of poisons, or toxins, and how they affect living organisms. It also includes investigations of toxins in the environment, how they are distributed, and the risks they present to plants, animals, and people. Toxicology draws on several areas of science including biology, chemistry, mathematics, and physics.
Human awareness of poisons predates recorded history. The earliest humans used toxic plant and animal extracts for both hunting and warfare. In ancient times and during the Middle Ages, knowledge of toxins was well-developed. However, a parallel gain in knowledge also took place with regard to medicines. In the late Middle Ages, Paracelsus (1493-1541), a physician-alchemist wrote: "All substances are poisons; there is none which is not a poison. The right dose differentiates a poison from a remedy." At the time, his views were seen as revolutionary, but they mark the beginnings of modern toxicology.
By the late nineteenth and early twentieth centuries, many scientists were devoting their research efforts to finding out why certain chemicals were poisonous and identifying their effects in the body. Such efforts continue today as new toxins are identified and others are re-evaluated.
Toxins can be classified according to many different systems. For example, they can be grouped according to what organ or organ system they affect. They can also be grouped according to their purpose, such as a pesticide or a food additive. A more general classification scheme groups toxins according to their source--plant, animal, or created by humans. Toxins can also be classified by the type of injury, such as cancer or liver damage, that they cause. Some classification schemes are very specific and zero in on a toxin's molecular mode of action. Examples of such specific modes of action include toxins that inhibit a particular enzyme or toxins that damage genetic material.
Regardless of the classification scheme, toxins are assessed by a standard set of criteria. One of the first sets of questions in evaluating a toxin center on exposure, how it occurs, and at what frequency. How a toxin enters the body can have a great impact on what its effects might be. Toxins can enter the body through the skin, by the mouth, or by being inhaled. Exposure can occur due to a person's job or where that person lives. Exposure can also occur because of an accident or through a deliberate act such as a suicide attempt. Toxicologists divide the frequency of exposure into acute or chronic durations. Acute exposure corresponds to a short time frame; chronic exposure lasts for a long time frame.
The effects of exposure are evaluated by the reactions. For example, exposure to a toxin might cause a person to suffer an allergic reaction. Exposure to a different toxin might cause more serious effects such as trouble breathing, irregular heartbeat, or even death. Effects are not always immediately apparent. The effects of some exposures are not seen for years as in the case of cancer or damage to the nervous system.
Exposure to a toxin and the effects it causes are usually expressed by the dose-response relationship. This relationship describes the effects that certain amounts of toxin will have on a living organism. The concept of the dose-response relationship is seen in Paracelsus's writings; he makes it clear that the dose makes the poison. Any substance in a large enough quantity will have a negative effect. How large a quantity depends on the substance.
A toxin is also evaluated according to its kinetics. The kinetics of a toxin, or toxicokinetics, describes how the toxin is absorbed in the body, where it is distributed, and how the body handles the toxin. For example, if a toxin enters the body through the mouth, it may be absorbed from the stomach or the intestine. Once in the bloodstream, it is carried to the liver.
The liver contains many enzymes—proteins that trigger speedy chemical reactions--that can alter the chemical structure of toxins. Many toxins that enter the body eventually go through the liver, although there are some that will bind to other tissues and remain for a long time. In the liver, enzymatic alterations may lead to the toxin being quickly excreted or they may result in slowing the toxin's exit from the body. Other alterations may either make the toxin harmless or cause it to become harmful.
Determining how the body handles the toxin provides information on its effects and how to stop or reverse them. This information is very useful in making risk assessments and developing risk management plans. There are many natural and human-made chemicals in the environment. Some of these chemicals are toxins, but others are not. Using risk assessment, the ones that are dangerous can be identified. By identifying these chemicals, management efforts can be clearly focused on limiting or preventing exposure to them.
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