Toxicology- Definition, Branches, Terminologies & Measures Of Toxicity

‘Toxicology’ is ‘the study of the detection, occurrence, properties, effects, and regulation of toxic substances.’

If we talk about the etymology of the word ‘toxicology,’ it is a combination of two words, ‘toxico+logy,’ which comes from the Ancient Greek words, toxikos, meaning poisonous and logos, subject matter. We should first know how to define the term ‘poison’ before going in the details of ‘Toxicology.’

Toxicology, toxic, Poison
The international pictogramme for poisonous substances.

Poison is any substance that causes a harmful effect when administered, either by accident or design, to a living organism. It is a quantitative concept. Almost any substance being harmful at some doses but, at the same time, being without any harmful effects at some lower dose. So, in simple words, we can say that ‘it is the dose that differentiates between a poison and a medicine.

Paracelsus, a physician-alchemist, formulated many revolutionary views that form the basic structure of toxicology, pharmacology, and therapeutics today. ‘Paracelsus is often known as the ‘Father of Toxicology.’ He said-“Dosis facit venenum,” which implies: “All substances are poisons; there is none which is not a poison. The right dose differentiates a poison from a remedy.”

Toxicology- Definition, Branches, Terminologies & Measures Of Toxicity

A view initiated by Paracelsus that became a forever contribution held as a great outcome can be summed up in the following points.

  • Experimentation is essential in the examination of responses to chemicals.
  • One should make a distinction between the therapeutic and toxic properties of chemicals.
  • These properties are sometimes but not always indistinguishable except by dose.
  • One can ascertain a degree of specificity of chemicals and their therapeutic or toxic effects.

These, above four points, can be referred to as the ‘principles of toxicology’.

Branches Of Toxicology

Toxicology is divided into several branches depending on the subject under consideration. The basic branches can be described as:

  • Forensic Toxicology

It is concerned with the medicolegal aspects of the adverse effects of various toxins on humans. It primarily deals with the identification of the type of poisoning in lethal and sub-lethal cases. Forensic Toxicology is a hybrid of analytical chemistry and fundamental toxicological principles. (Read More on Forensic Toxicology)

  • Environmental Toxicology

It deals with the study of the adverse effects of various toxic substances on the environment, including both biotic and biotic factors.

  • Clinical Toxicology

It studies clinical implications of various toxicants on living organisms, the methods of detection, diagnosis, and treatment of such toxicity. Generally, clinical toxicologists are physicians who receive specialized training in emergency medicine and poison management.

  • Analytical Toxicology

It deals with how analytical chemistry methodologies can be used for the qualitative and quantitative estimation of various toxic agents.

  • Developmental Toxicology

It deals with the study of adverse effects of various toxicants on developing organisms, anytime during their lifespan, be it at the time of conception, or the intrauterine life or post-natal life, till one reaches puberty.

  • Regulatory Toxicology

It deals with the conduction of various toxicological studies on products as per the content and characteristics prescribed by the regulatory agencies. We can also divide it either to general toxicology and individual (specialized on particular substances or groups of substances) or by the field it is mainly used in, ecotoxicology, toxicology of foods/food additives/food industry, industrial toxicology, military toxicology, occupational toxicology, etc., the list is never-ending.

A toxicologist is trained to examine and communicate the nature of the effects on human, animal, and environmental health. Toxicological research examines the cellular, biochemical, and molecular mechanisms of action as well as functional effects such as neurobehavioral and immunological, and assesses the probability of their occurrence. Fundamental to this process is characterizing the relation of exposure (or dose) to the response.

(You may also like to read- What is Forensic Science? | Applications & Sub-Disciplines | Specialization)

Some Important Terminologies

  • Lethal Dose (LD): Dose resulting in the death of an organism.
  • Effective dose (ED): Desired response is observed at the dose tested. Also, it can be a therapeutic dose.
  • Toxic dose: dose at which toxicity is observed.
  • Acute toxicity: Toxicity elicited immediately after exposure (generally refers to a single dose).
  • Chronic toxicity: Toxicity caused by long-term exposure to a toxic substance (generally by multiple doses).
  • No adverse effect level (NOAEL): Highest dose level where no adverse effect is observed.
  • Lowest observed adverse effect level (LOAEL): Lowest dose level that produces a statistically significant effect.


Dose-Response Relationship

The dose can be defined as the amount of chemical/toxicant/drug entering the body. It is usually given as mg of substance/kg of body weight => mg/kg. The dose can be dependent upon: 

  • The environmental concentration
  • The properties of a substance
  • The duration of exposure
  • The exposure pathway
  • The frequency of exposure

The degree and spectra of the response generated in an organism depend upon the dose and the condition of the organism. It describes the exposure conditions with the description of the dose.

Dose-response assumes that a response to a toxic agent can be measured, and the effects increase in some relationship with the dose. “It is the most pervasive concept in toxicology.” The dose-response principle can be used effectively to determine the toxic dose for a particular substance as well as the safe dose for the same substance. It can also be used to determine the margin of safety for drugs.

Measures Of Toxicity

Toxicity is measured as clinical “endpoints” which include-

  • Mortality (death)
  • Teratogenicity (ability to cause birth defects)
  • Carcinogenicity (ability to cause cancer)
  • Mutagenicity (ability to cause a heritable change in the DNA)

An important concept in measuring mortality is that of LD50.

The LD50, the lethal dose for 50% of the animals tested, of poison is usually expressed in milligrams of chemical per kilogram of body weight (mg/kg). A chemical with a small LD50 (like 5 mg/kg) is very highly toxic. A chemical with a large LD50 (1,000 to 5,000 mg/kg) is practically non-toxic.

LD50 Of Some Substance

Agent  LD50(mg/kg)
Ethyl Alcohol 10,000
Sodium Chloride 4,000
Ferrous Sulfate 1,500
Morphine Sulfate 900
Phenobarbital Sodium 150
Picrotoxin 5
Strychnine Sulfate 2
Nicotine 1
d-Tubocurarine 0.5
Hemicholinium-3 0.2
Tetrodotoxin 0.10
Dioxin(TCDD) 0.001
Botulinum Toxin 0.00001

The LD50 says nothing about non-lethal toxic effects though. A chemical may have a large LD50 but may produce illness at minimal exposure levels. It is incorrect to say that chemicals with small LD50 values are more dangerous than chemicals with large LD50. They are more toxic. The danger, or risk of the adverse effect of chemicals, is mostly determined by how they are used (exposed), not by the inherent toxicity of the chemical itself.

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