Toxicity
Toxicity (from
Greek τοξικότητα –
poisonousness) is a measure to the degree to which something is toxic or
poisonous. The study of poisons is known as
toxicology. Toxicity can refer to the effect on a whole
organism, such as a human or a
bacterium or a
plant, or to a substructure, such as the
liver. By extension, the word may be
metaphorically used to describe
toxic effects on larger and more complex groups, such as the
family unit or "society at large".
In the science of toxicology, the subject of such study is the effect of an external substance or condition and its deleterious effects on living things:organisms, organ systems, individual organs, tissues, cells, subcellular units. A central concept of toxicology is that effects are
dose-dependent; even
water is toxic to a human in large enough doses, whereas for even a very toxic substance such as
snake venom there is a dose for which there is no toxic effect detectable.
There are generally three types of toxic entities; chemical, biological, and physical.
*
Chemicals include both
inorganic substances such as
lead,
hydrofluoric acid, and
chlorine gas, as well as
organic compounds such as
ethyl alcohol, most medications, and poisons from living things.
*Biological toxicity can be more complicated to measure, as the "threshold dose" may be a single organism, as theoretically this one
virus, bacterium or
worm can reproduce to cause a serious
infection. However, in a host with an intact
immune system the inherent toxicity of the organism is balanced by the host's ability to fight back; the effective toxicity is then a combination of both parts of the relationship. A similar situation is also present with other types of toxic
agents. In particular, toxicity of cancer-causing agents is problematic, since for many such substances it is not certain if there is a minimal effective dose or whether the risk is just too small to see; here too the possibility exists that a single cell transformed into a cancer cell is all it takes to develop the full effect. Mixtures of chemicals are more difficult to assess in terms of toxicity, such as
gasoline,
cigarette smoke, or
industrial waste. Even more complex are situations with more than one type of toxic entity, such as the discharge from a malfunctioning sewage treatment plant, with both chemical and biological agents.
* Physically toxic entities include things not usually thought of as such by the lay person: direct blows, concussion, sound and
vibration,
heat and
cold, non-ionizing
electromagnetic radiation such as infrared and visible light,
ionizing non-particulate radiation such as
X-rays and
gamma rays, and particulate radiation such as
alpha rays,
beta rays, and
cosmic rays.
Toxicity can be measured by the effects on the target (organism, organ, or tissue). Because individuals typically have different levels of response to the same dose of a toxin, a population-level measure of toxicity is often used which relates the probability of an outcome for a given individual in a population. One such measure is the
LD50, "LD" standing for "lethal dose", which is a concentration measure for a toxin at which fifty-percent of the members of an exposed population dies from exposure. When such data does not exist, estimates are made by comparison to known similar toxic things, or to similar exposures in similar organisms. Then "
safety factors" must be built in to protect against the uncertainties of such comparisons, in order to improve protection against these unknowns.
Toxicity of a substance can be affected by many different factors, such as the pathway of administration (is the toxin applied to the skin, ingested, inhaled, injected), the time of exposure (a brief encounter or long term), the number of exposures (a single dose or multiple doses over time), the physical form of the toxin (solid, liquid, gas), the genetic makeup of an individual, an individual's overall health, and many others. Several of the terms used to describe these factors have been included here.
;acute exposure: a single exposure to a toxic substance which may result in severe biological harm or death; acute exposures are usually characterized as lasting no longer than a day.
chronic exposure: continuous exposure to a toxin over an extended period of time, often measured in months or years*
Biological activity*
Biological warfare*
Homeopathy*
Nephrotoxicity*
Neurotoxicity*
Ototoxicity*
Paracelsus*
Physiologically-based pharmacokinetic modelling.
*
Poison*
RTECS, a toxicity database
*
Toxin*
Carcinogen*
Mutagen*
Soil contamination*
Teratogen*
Agency for Toxic Substances and Disease Registry*
Chemical and Other Safety Information – Oxford University
