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IUPAC Glossary of Terms Used in Toxicology - Terms Starting with H

See heme

half life, t1/2
half time
Time required for the concentration of a reactant in a given reaction to reach a value that is the arithmetic mean of its initial and final (equilibrium) values. For a reactant that is entirely consumed it is the time taken for the reactant concentration to fall to one half of its initial value.
Note: The half life of a reaction has meaning only in special cases:

  1. For a first-order reaction, the half life of the reactant may be called the half life of the reaction.
  2. For a reaction involving more than one reactant, with the concentrations of the reactants in their stoichiometric ratios, the half life of each reactant is the same, and may be called the half life of the reaction.

If the concentrations of reactants are not in their stoichiometric ratios, there are different half lives for different reactants, and one cannot speak of the half life of the reaction.
See also biological half life, elimination half life

half time, t1/2
See half life

State in which a cell contains only one set of chromosomes.


  1. Contraction of the phrase “haploid genotype”, the genetic constitution of an individual with respect to one member of a pair of allelic genes: haplotype can refer to only one locus or to an entire genome (a genome-wide haplotype would comprise half of a diploid genome, including one allele from each allelic gene pair).
  2. Set of single nucleotide polymorphisms found to be statistically associated on a single chromatid.

Low-molecular-mass species which is not itself antigenic unless complexed with a carrier, such as a protein. Once bound, it presents an epitope that can cause the sensitization of lymphocytes.
After [1]

adverse effect
Damage or adverse effect to a population, species, individual organism, organ, tissue or cell.

harmful occupational factor
Component of the work environment the effect of which on a worker under certain conditions leads to ill health or reduction of working ability.

harmful substance
noxious substance
Substance that, following contact with an organism can cause ill health or adverse effects either at the time of exposure or later in the life of the present and future generations.

Set of inherent properties of a substance, mixture of substances or a process involving substances that, under production, usage or disposal conditions, make it capable of causing adverse effects to organisms or the environment, depending on the degree of exposure; in other words, it is a source of danger.
See also risk

hazard assessment
Determination of factors controlling the likely effects of a hazard such as the dose-effect and dose-response relationships, variations in target susceptibility, and mechanism of toxicity.

hazard communication standard
US OSHA standard requiring all employers to inform employees of the hazard of substances in the workplace and the steps necessary to avoid harm.

hazard evaluation
Establishment of a qualitative or quantitative relationship between hazard and benefit, involving the complex process of determining the significance of the identified hazard and balancing this against identifiable benefit.
Note: This may subsequently be developed into a risk evaluation.

hazard identification
Determination of substances of concern, their adverse effects, target populations, and conditions of exposure, taking into account toxicity data and knowledge of effects on human health, other organisms and their environment.

hazard index (HI)
Sum of the hazard quotients for substances that affect the same target organ or organ system.
Note: Ideally, hazard quotients should be combined for pollutants that cause adverse effects by the same mechanism. Aggregate exposures below a hazard index of 1.0 were unlikely to result in adverse health effects over a lifetime of exposure. A hazard index greater than 1.0 does not necessarily suggest a likelihood of adverse effects. The hazard index cannot be translated to a probability that adverse effects will occur, and is not likely to be proportional to risk.

hazard quotient (HQ)
Ratio of toxicant exposure (estimated or measured) to a reference value regarded as corresponding to a threshold of toxicity.
Note: If the hazard quotient exceeds unity, the toxicant may produce an adverse effect but normally this will require a hazard quotient of several times unity; a hazard quotient of less than one indicates that no adverse effects are likely over a lifetime of exposure.

hazardous production factor
hazard at work
hazardous occupational factor
Production factor the effect of which on a worker under certain conditions results in injury or some impairment of health.


  1. State of complete physical, mental and social well-being, and not merely the absence of disease or infirmity.
  2. State of dynamic balance in which an individual's or a group's capacity to cope with the circumstances of living is at an optimal level.
  3. State characterized by anatomical, physiological and psychological integrity, ability to perform personally valued family, work and community roles; ability to deal with physical, biological, psychological and social stress; a feeling of wellbeing; and freedom from the risk of disease and untimely death.
  4. In ecology, a sustainable steady state in which humans and other living organisms can coexist indefinitely.

health advisory level (HAL)
In the USA, non-regulatory health-based reference level of chemical traces (usually in ppm, i.e., mg L-1) in drinking water at which there are no adverse health risks when ingested over various periods of time.
Note: Such levels are established for one day, 10 days, long-term and life-time exposure periods. They allow for a wide margin of safety.

health-based exposure limit
Maximum concentration or intensity of exposure that can be tolerated without significant effect (based on only scientific and not economic evidence concerning exposure levels and associated health effects).

health hazard
Any factor or exposure that may adversely affect health.

health surveillance
Periodic medico-physiological examinations of exposed workers with the objective of protecting health and preventing occupationally related disease.

healthy worker effect
Epidemiological phenomenon observed initially in studies of occupational diseases: workers usually exhibit lower overall disease and death rates than the general population, due to the fact that the old, severely ill and disabled are ordinarily excluded from employment. Death rates in the general population may be inappropriate for comparison, if this effect is not taken into account.

heat shock proteins
stress proteins
Group of proteins whose synthesis is increased by increased transcription when cells are exposed to elevated temperatures.
Note: Production of high levels of heat shock proteins can also be triggered by exposure to different kinds of environmental stress conditions, e.g., infection, inflammation, exposure of the cell to chemicals (such as ethanol, arsenicals, or certain metal species), ultraviolet light, starvation, hypoxia (oxygen deprivation), nitrogen deficiency (in plants), or water deprivation. Hence, the alternative name, stress proteins. Their upregulation is sometimes described more generally as part of the stress response.

heavy metal
toxic metal
Erroneous terms used commonly in the toxicological literature but having no generally agreed meaning, sometimes even applied to nonmetals, and therefore a source of confusion and to be avoided. The term “metal” is adequate without the qualifying adjective but may be misleading since it implies a solid material when toxicological concern is mostly for the ionic form or another chemical species.

See anthelmint(h)ic

See anthelmint(h)ic

Vomiting of blood.

Localized accumulation of blood, usually clotted, in an organ, space, or tissue, due to a failure of the wall of a blood vessel.

Adverse changes in blood caused by exposure to chemicals.

Presence of blood in the urine.

Complex consisting of an iron ion coordinated to a porphyrin acting as a tetradentate ligand, and to one or two axial ligands.
After [3]

Hereditary disorder affecting iron metabolism in which excessive amounts of iron accumulate in the body tissues.
Note: The disorder is characterized by diabetes mellitus, liver dysfunction, and a bronze pigmentation of the skin.

Use of an artificial kidney to remove toxic compounds from the blood by passing it through a tube of semipermeable membrane.
Note: The tube is bathed in a dialysing solution to restore the normal chemical composition of the blood while permitting diffusion of toxic substances from the blood.

Heme-containing protein in red blood cells with an important function in transporting oxygen from the lungs to body tissues.

Presence of free hemoglobin in the urine.

Substance that damages the membrane of erythrocytes causing the release of hemoglobin.

Release of hemoglobin from erythrocytes, and its appearance in the plasma.

Passing blood through a column of charcoal or adsorbent resin for the removal of drugs or toxins.

Insoluble iron(III) hydroxide-based pigment deposited in cells in conditions of iron overload.

Henderson–Hasselbalch equation
Equation of the form:
pH = pKa – lg([HA]/[A])
for the calculation of the pH of solutions where the ratio [HA]/[A] is known and HA and A- are the hydronated and dehydronated forms of an acid, respectively.
Corrected from [3]

Henry's law constant
At constant temperature and pressure, the ratio of the partial pressure of a gas above a liquid to its solubility in the liquid and therefore a measure of its partition between the gas phase and the solute phase.
Note 1: The solubility may be expressed in any convenient units, such as amount fraction, molality or amount (substance) concentration. The exact definition used should always be given.
Note 2: Rigorously, the Henry’s law constant is the limiting value at zero partial pressure.

Pertaining to the liver.

Poisonous to liver cells.

Substance intended to kill plants.

Organism which has different allelic forms of a specified gene on each of a pair of homologous chromosomes or describing the genome of that organism.
After [9]

Hill plot
Graphical method for analysing binding of a molecule A to a macromolecule P with n binding sites. A Hill plot of lg[Θ/(1-Θ)] vs lg[A] has a slope of 1 if binding is non-co-operative and >1 for co-operative binding, where Θ = [A]bound/n[P]total is the fraction of sites occupied.

2-(1H-imidazol-4-yl)ethan-1-amine, an amine derived from histidine by decarboxylation and released from cells in the immune system as part of an allergic reaction: it is a powerful stimulant of gastric secretion, constrictor of bronchial smooth muscle, and vasodilator.

histogenic origin
Germ cell layer of the embryo from which a given adult tissue develops.

Study (usually microscopic) of the anatomy of tissues and their cellular and subcellular structure.

Microscopic pathological study of the anatomy and cell structure of tissues in disease to reveal abnormal or adverse structural changes.

hit-and-run effect
Toxicity which follows a single exposure to a substance.

Normal, internal stability in an organism maintained by co-ordinated responses of the organ systems that automatically compensate for environmental changes.

Degree of identity existing between the nucleotide sequences of two related but not complementary DNA or RNA molecules.
Note 1: 70% homology means that on the average 70 out of every 100 nucleotides are identical in a given sequence.
Note 2: The same term is used in comparing the amino acid sequences of related proteins.

Organism which has the same allelic form of a specified gene on each of a pair of homologous chromosomes or describing the genome of that organism.
After [9]

horme/sis n., -tic adj.
Benefit at low dose of a substance that is harmful at a higher dose.

Substance formed in one organ or part of the body and carried in the blood to another organ or part where it selectively alters functional activity.

human ecology
Interrelationship between humans and the entire environment - physical, biological, socio-economic, and cultural, including the interrelationships between individual humans or groups of humans and other human groups or groups of other species.

human equivalent dose
Human dose of an agent that is believed to induce the same magnitude of a toxic effect that the known animal dose has induced.

human exposure threshold (of toxicological concern)
Generic value of exposure to a substance, or a group of substances falling within a defined structural class, below which there is expected to be no appreciable risk to human health.

Chemical reaction of a substance with water, usually resulting in the formation of one or more new compounds.

hydrophilic/ adj., -ity n.
antonym hydrophobic
Describing the character of a substance, material, molecular entity or group of atoms which has an affinity for water.

hydrophobic/ adj., -ity n.
antonym hydrophilic
Describing the character of a substance, material, molecular entity or group of atoms which is insoluble or confers insolubility in water, or resistance to wetting or hydration.

Science of health and its preservation.

antonym hypo-
Prefix meaning above or excessive: when used with the suffix “-emia” refers to blood and with the suffix “-uria” refers to urine, for example “hyperbilirubinemia”.

Excessive amount of blood in any part of the body.

Ingestion or administration of nutrients in excess of optimal amounts.

Excessive concentration of bilirubin in the blood.

Excessive concentration of calcium in the blood.

Excessive concentration of glucose in the blood.

Excessive concentration of potassium in the blood.

Excessive concentration of sodium in the blood.

Abnormally increased parathyroid gland activity that affects, and is affected by, plasma calcium concentration.

Abnormal multiplication or increase in the number of normal cells in a tissue or organ.

Term used to describe the responses of (effects on) an individual to (of) an agent when they are qualitatively those expected, but quantitatively increased.

Exaggeration of reflexes.

State in which an individual reacts with allergic effects following exposure to a certain substance (allergen) after having been exposed previously to the same substance.
Note: Most common chemical-induced allergies are Type I (IgE-mediated) and Type IV (cell-mediated) hypersensitivity.

Excessive reaction following exposure to a given amount or concentration of a substance as compared with the large majority of other exposed subjects.

Persistently high blood pressure in the arteries or in a circuit, for example pulmonary hypertension or hepatic portal hypertension.

Excessive growth in bulk of a tissue or organ through increase in size but not in number of the constituent cells.

Condition resulting from the ingestion of an excess of one or more vitamins.

Prefix meaning under, deficient: when used with the suffix “-emia” refers to blood and with the suffix “-uria” refers to urine, for example “hypocalcemia”.

Abnormally low calcium concentration in the blood.

Abnormally low potassium concentration in the blood.

abnormally low magnesium concentration in the blood.

Abnormally low sodium concentration in the blood.

Pertaining to an abnormally decreased volume of circulating fluid (plasma) in the body.

Deficient oxygenation of the blood.


  1. Abnormally low dioxygen content or tension.
  2. Deficiency of dioxygen in the inspired air, in blood or in tissues, short of anoxia.

Dioxygen deficient.