Toxicology Health Article

Definition

Toxicology is the scientific study of poisons or toxins. The National Library of Medicine describes toxicology as "the study of the adverse effects of chemicals or physical agents on living organisms." How these toxins affect humans is based in understanding these basic relationships.

Description

The Swiss physician and alchemist Philippus Aureolus, also known as Paracelsus (1493–1541) and said to be the father of the modern science of toxicology, wrote, "All things are poison, and nothing is without poison, the dose alone makes a thing not a poison." In other words, if poisoning is to be caused, an exposure to a potentially toxic chemical must result in a dose that exceeds a physiologically determined threshold of tolerance. Smaller exposures do not cause poisoning.

The dose of toxin is a crucial factor to consider when evaluating effects of a toxin. Small quantities of a substance like strychnine taken daily over an extended period of time might have little to no effect, while one large dose in one day could be fatal. In addition, some toxins may only affect a particular species of organism, such as pesticides and antibiotics killing insects and microorganisms with significantly less harmful effects to humans.

Organisms vary greatly in their tolerance of exposure to chemicals. Even within populations of the same species great variations in sensitivity can exist. In rare cases, some individuals may be extremely sensitive to particular chemicals or groups of similar chemicals, a phenomenon known as hypersensitivity. Organisms are often exposed to a wide variety of potentially toxic chemicals through medicine, food, water, and the atmosphere.

The study of the disruption of biochemical pathways by poisons is a key aspect of toxicology. Poisons affect normal physiology in many ways; but some of the more common mechanisms involve the disabling of enzyme systems, induction of cancers, interference with the regulation of blood chemistry, and disruption of genetic processes.

Toxic agents may be physical (for example, radiation), biological (for example, poisonous snake bite), or chemical (for example, arsenic) in nature. In addition, biological organisms may cause disease by invading the body and releasing toxins. An example of this is tetanus, in which the bacterium Clostridium tetanus releases a powerful toxin that travels to the nervous system.

Toxic agents may also cause systemic or organ-specific reactions in the body. Cyanide affects the entire body by interfering with the body's capacity for utilizing oxygen. Lead has three specific target organs: the central nervous system, the kidneys, and the hematopoeitic (blood-cell generating) system. The target organ is affected by the dose and route of the toxin. For example, the initial effects of a chemical may affect the nervous system; repeated exposure over time might cause chronic damage to the liver.

Function

The toxicologist employs the tools and methods of science to understand more completely the consequences of exposure to toxic chemicals. Toxicologists typically assess the relationship between toxic chemicals and environmental health by evaluating such factors as:

• Risk—To assess the risk associated with exposure to a toxic substance, toxicologists first measure the exposure characteristics and then compute the doses that enter the human body. Then they compare these numbers to derive an estimate of risk, sometimes based on animal studies. In cases where human data exist for a toxic substance, such as benzene, more straightforward correlations with human risk of illness or death are possible.
• Precautionary strategies—Given recommendations from toxicologists, government agencies sometimes decide to regulate a chemical based on limited evidence from animal and human epidemiological studies that the chemical is toxic.
• Clinical data—Some toxicologists devise new techniques and develop new applications of existing methods to monitor changes in the health of individuals exposed to toxic substances. For example, one academic research group in the United States has spent many years developing new methods for monitoring the effects of exposure to oxidants (for example, free radicals) in healthy and diseased humans.
• Epidemiological evidence—Another way to understand the environmental factors contributing to human illness is to study large populations that have been exposed to substances suspected of being toxic. Scientists then attempt to tie these observations to clinical data. Ecological studies seek to correlate exposure patterns with a specific outcome. Case-control studies compare groups of persons with a particular illness with similar healthy groups, and seek to identify the degree of exposure required to bring about the illness. Other studies may refine the scope of environmental factor studies; or, examine a small group of individuals in which there is a high incidence of a rare disease and a history of exposure to a particular chemical.
• Evidence of bio-accumulation—When a chemical is nonbiodegradable, it may accumulate in biosystems, resulting in very high concentrations accumulating in animals at the top of food chains. Chlorinated pesticides such as dieldrin and DDT, for example, have been found in fish in much greater concentrations than in the seawater where they swim.