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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.

Role in human health

Humans are exposed to complex mixtures of chemicals, many of which are synthetic and have been either deliberately or accidentally released into the environment. In some cases, people actively expose themselves to chemicals that are known to be toxic, such as smoking cigarettes, drinking alcohol, or taking recreational drugs. Voluntary exposure to chemicals also occurs when people take medicines to deal with illness, or when they choose to work in an occupation that involves routinely dealing with dangerous chemicals. Most exposures to potentially toxic chemicals are inadvertent, and involve living in an environment that is contaminated with small concentrations of pollutants, such as those associated with pesticide residues in food, lead from gasoline combustion, or sulfur dioxide and ozone in the urban atmosphere.

Drugs given to improve health can lead to toxicity even when given in appropriate doses. Conditions such as dehydration and other forms of physiological compromise can make the patient more vulnerable to toxicity. Drugs like digoxin, lidocaine, and lithium are common examples of drugs with potentially toxic effects. Interactions of substances in the body may also produce toxic effects. For example, if two central nervous system depressants are taken at once, as in the case of combining alcohol and a tranquilizer, the effects are additive and could lead to extreme depression of the central nervous system functions.

The health care system's role related to toxicology includes education and prevention as well as treatment of both acute and chronic effects of toxins. Agencies such as the Food and Drug Administration (FDA) and the Occupational Safety and Health Administration (OSHA) work with health care and industry to offer guidelines and restrictions on the manufacture and use of pharmaceuticals, foods, and other substances.

Health care workers are involved by being aware of these regulations, and staying informed. They also provide education, such as, teaching new parents about the dangers of lead paint consumption by children, and help prevent exposure to toxins, such as, tetanus vaccination, or monitoring for signs of lithium toxicity. The Poison Control Center uses nurses and other allied health workers to inform the public of immediate actions to take in the event of a poisoning emergency. Emergency interventions at the hospital include blood and urine tests,

gastric lavage with administration of absorbent activated charcoal, and administration of antidotes when available.

Common diseases and disorders

Toxicologists have ranked the most commonly encountered toxic chemicals in the United States. In descending order of frequency of encounter, they are as follows:

• Arsenic—Toxic exposure occurs mainly in the workplace, near hazardous waste sites, or in areas with high natural levels. A powerful poison, arsenic can, at high levels of exposure, cause death or illness.
• Lead—Toxic exposure usually results from breathing workplace air or dust, or from eating contaminated foods. Children may be exposed to lead from eating lead-based paint chips or playing in contaminated soil. Lead damages the nervous system, kidneys, and the immune systems.
• Mercury—Toxic exposure results from breathing contaminated air, ingesting contaminated water and food, and possibly having dental and medical treatments. At high levels, mercury damages the brain, kidneys, and developing fetuses.
• Vinyl chloride—Toxic exposure occurs mainly in the workplace. Breathing high levels of vinyl chloride for short periods can produce dizziness, sleepiness, unconsciousness, and, at very high levels, death. Breathing vinyl chloride for long periods of time can give rise to permanent liver damage, immune reactions, nerve damage, and liver cancer.
• Benzene—Benzene is formed in both natural processes and human activities. Breathing benzene can produce drowsiness, dizziness, and unconsciousness. Long-term exposure affects the bone marrow and can produce anemia and leukemia.
• Polychlorinated biphenyls (PCBs)—PCBs are mixtures of chemicals. They are no longer produced in the United States, but remain in the environment. They can irritate the nose and throat, and cause acne and rashes. They have been shown to cause cancer in animal studies.
• Cadmium—Toxic exposure to cadmium occurs mainly in workplaces where cadmium products are made. Other sources of exposure include cigarette smoke and cadmium-contaminated foods. Cadmium can damage the lungs, cause kidney disease, and irritate the digestive tract.

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KEY TERMS

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Antidote—A substance that combats the effects of a poison or toxin.

Gastric lavage—The act of emptying out the stomach via orogastric or nasogastric tube.

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Katherine Hauswirth, A.P.R.N.