IUPAC Glossary of Terms Used in Toxicology – Terms Starting with H
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haem
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:
- For a first-order reaction, the half life of the reactant may be called the half life of the reaction.
- 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
[2]
half time,
t1/2
See half life
haploid
monoploid
State in which a cell contains only one set of chromosomes.
haplotype
- 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).
- Set of single nucleotide polymorphisms found to be statistically associated on a single chromatid.
hapten
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]
harm
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.
hazard
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.
- State of complete physical, mental and social well-being, and not merely the absence of disease or infirmity.
- 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.
- 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.
- 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.
helminthagogue
See anthelmint(h)ic
helminthic
See anthelmint(h)ic
hematemesis
Vomiting of blood.
hematoma
Localized accumulation of blood, usually clotted, in an organ,
space, or tissue, due to a failure of the wall of a blood
vessel.
hematotoxicity
Adverse changes in blood caused by exposure to chemicals.
hematuria
Presence of blood in the urine.
heme
haem
Complex consisting of an iron ion coordinated to a porphyrin acting as a tetradentate ligand, and to
one or two axial ligands.
After [3]
hemochromatosis
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.
hemodialysis
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.
hemoglobin
Heme-containing protein in red blood cells with an important
function in transporting oxygen from the lungs to body
tissues.
hemoglobinuria
Presence of free hemoglobin in the urine.
hemolysin
Substance that damages the membrane of erythrocytes causing the
release of hemoglobin.
hemolysis
Release of hemoglobin from erythrocytes, and its appearance in
the plasma.
hemoperfusion
Passing blood through a column of charcoal or adsorbent resin for
the removal of drugs or
toxins.
hemosiderin
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.
[13]
hepatic
Pertaining to the liver.
hepatotoxic
Poisonous to liver cells.
herbicide
Substance intended to kill plants.
heterozygote
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]
histamine
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.
histology
Study (usually microscopic) of the anatomy of tissues and their
cellular and subcellular structure.
histopathology
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.
homeostasis
Normal, internal stability in an organism maintained by
co-ordinated responses of the organ systems that automatically
compensate for environmental changes.
homology
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.
homozygote
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.
hormone
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.
hydrolysis
Chemical reaction of a substance with water, usually resulting in the formation of one or more new compounds.
hydrophilic/ adj., -ity
n.
lipophobic
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.
hygiene
Science of health and its
preservation.
hyper-
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”.
hyperemia
Excessive amount of blood in any part of the body.
hyperalimentation
Ingestion or administration of nutrients in excess of optimal
amounts.
hyperbilirubinemia
Excessive concentration of bilirubin in
the blood.
hypercalcemia
Excessive concentration of calcium in
the blood.
hyperglycemia
Excessive concentration of glucose in
the blood.
hyperkalemia
Excessive concentration of potassium in
the blood.
hypernatremia
Excessive concentration of sodium in the
blood.
hyperparathyroidism
Abnormally increased parathyroid gland activity that affects, and
is affected by, plasma calcium
concentration.
hyperplasia
Abnormal multiplication or increase in the number of normal cells
in a tissue or organ.
hyper-reactivity
Term used to describe the responses of (effects on) an individual
to (of) an agent when they are qualitatively those expected, but
quantitatively increased.
hyper-reflexia
Exaggeration of reflexes.
hypersensitivity
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.
hypersusceptibility
Excessive reaction following exposure to a given amount or
concentration of a
substance as compared with the large majority of other
exposed subjects.
hypertension
Persistently high blood pressure in the arteries or in a circuit,
for example pulmonary hypertension or hepatic portal
hypertension.
hypertrophy
Excessive growth in bulk of a tissue or organ through increase in
size but not in number of the constituent cells.
hypervitaminosis
Condition resulting from the ingestion of an excess of one or
more vitamins.
hypo-
Prefix meaning under, deficient: when used with the suffix
“-emia” refers to blood and with the suffix
“-uria” refers to urine, for example
“hypocalcemia”.
hypocalcemia
Abnormally low calcium concentration in the
blood.
hypokalemia
Abnormally low potassium concentration in the
blood.
hypomagnesemia
abnormally low magnesium concentration in the blood.
hyponatremia
Abnormally low sodium concentration in the
blood.
hypovolemic
Pertaining to an abnormally decreased volume of circulating fluid
(plasma) in the body.
hypoxemia
Deficient oxygenation of the blood.
- Abnormally low dioxygen content or tension.
- Deficiency of dioxygen in the inspired air, in blood or in tissues, short of anoxia.
hypoxic
Dioxygen deficient.