Toxicity


Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ 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. Sometimes the word is more or less synonymous with poisoning in everyday usage.
A central concept of toxicology is that the effects of a toxicant are dose-dependent; even water can lead to water intoxication when taken in too high a dose, whereas for even a very toxic substance such as snake venom there is a dose below which there is no detectable toxic effect. Considering the limitations of this dose-response concept, a novel has been proposed recently. DTI redefines drug toxicity, identifies hepatotoxic drugs, gives mechanistic insights, predicts clinical outcomes and has potential as a screening tool. Toxicity is species-specific, making cross-species analysis problematic. Newer paradigms and metrics are evolving to bypass animal testing, while maintaining the concept of toxicity endpoints.

Types

There are generally four types of toxic entities; chemical, biological, physical and radiation:
Toxicity can be measured by its effects on the target. Because individuals typically have different levels of response to the same dose of a toxic substance, a population-level measure of toxicity is often used which relates the probabilities of an outcome for a given individual in a population. One such measure is the. 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" are added to account for uncertainties in data and evaluation processes. For example, if a dose of a toxic substance is safe for a laboratory rat, one might assume that one-tenth that dose would be safe for a human, allowing a safety factor of 10 to allow for interspecies differences between two mammals; if the data are from fish, one might use a factor of 100 to account for the greater difference between two chordate classes. Similarly, an extra protection factor may be used for individuals believed to be more susceptible to toxic effects such as in pregnancy or with certain diseases. Or, a newly synthesized and previously unstudied chemical that is believed to be very similar in effect to another compound could be assigned an additional protection factor of 10 to account for possible differences in effects that are probably much smaller. Obviously, this approach is very approximate; but such protection factors are deliberately very conservative, and the method has been found to be useful in a deep variety of applications.
Assessing all aspects of the toxicity of cancer-causing agents involves additional issues, since it is not certain if there is a minimal effective dose for carcinogens, or whether the risk is just too small to see. In addition, it is possible that a single cell transformed into a cancer cell is all it takes to develop the full effect.
It is more difficult to determine the toxicity of chemical mixtures than a pure chemical because each component displays its own toxicity, and components may interact to produce enhanced or diminished effects. Common mixtures include gasoline, cigarette smoke, and 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.
The preclinical toxicity testing on various biological systems reveals the species-, organ- and dose-specific toxic effects of an investigational product. The toxicity of substances can be observed by studying the accidental exposures to a substance in vitro studies using cells/ cell lines in vivo exposure on experimental animals. Toxicity tests are mostly used to examine specific adverse events or specific end points such as cancer, cardiotoxicity, and skin/eye irritation. Toxicity testing also helps calculate the No Observed Adverse Effect Level dose and is helpful for clinical studies.

Classification

For substances to be regulated and handled appropriately they must be properly classified and labelled. Classification is determined by approved testing measures or calculations and have determined cut-off levels set by governments and scientists. Pesticides provide the example of well-established toxicity class systems and toxicity labels. While currently many countries have different regulations regarding the types of tests, numbers of tests and cut-off levels, the implementation of the Globally Harmonized System has begun unifying these countries.
Global classification looks at three areas: Physical Hazards, Health Hazards and environmental hazards.

Health hazards

The types of toxicities where substances may cause lethality to the entire body, lethality to specific organs, major/minor damage, or cause cancer. These are globally accepted definitions of what toxicity is. Anything falling outside of the definition cannot be classified as that type of toxicant.

Acute toxicity

Acute toxicity looks at lethal effects following oral, dermal or inhalation exposure. It is split into five categories of severity where Category 1 requires the least amount of exposure to be lethal and Category 5 requires the most exposure to be lethal. The table below shows the upper limits for each category.
Method of administrationCategory 1Category 2Category 3Category 4Category 5
Oral: measured in mg/kg of bodyweight5503002 0005 000
Dermal: LD50 measured in mg/kg of bodyweight502001 0002 0005 000
Gas Inhalation: LC50 measured in ppmV1005002 50020 000Undefined
Vapour Inhalation: LC50 measured in mg/L0.52.01020Undefined
Dust and Mist Inhalation: LC50 measured in mg/L0.050.51.05.0Undefined

Note: The undefined values are expected to be roughly equivalent to the category 5 values for oral and dermal administration.

Other methods of exposure and severity

Skin corrosion and irritation are determined though a skin patch test analysis. This examines the severity of the damage done; when it is incurred and how long it remains; whether it is reversible and how many test subjects were affected.
Skin corrosion from a substance must penetrate through the epidermis into the dermis within four hours of application and must not reverse the damage within 14 days. Skin irritation shows damage less severe than corrosion if: the damage occurs within 72 hours of application; or for three consecutive days after application within a 14-day period; or causes inflammation which lasts for 14 days in two test subjects. Mild skin irritation is minor damage within 72 hours of application or for three consecutive days after application.
Serious eye damage involves tissue damage or degradation of vision which does not fully reverse in 21 days. Eye irritation involves changes to the eye which do fully reverse within 21 days.

Other categories

An Environmental hazard can be defined as any condition, process, or state adversely affecting the environment. These hazards can be physical or chemical, and present in air, water, and/or soil. These conditions can cause extensive harm to humans and other organisms within an ecosystem.

Common types of environmental hazards

The EPA maintains a list of priority pollutants for testing and regulation.

Occupational hazards

Workers in various occupations may be at a greater level of risk for several types of toxicity, including neurotoxicity. The expression "Mad as a hatter" and the "Mad Hatter" of the book Alice in Wonderland derive from the known occupational toxicity of hatters who used a toxic chemical for controlling the shape of hats. Exposure to chemicals in the workplace environment may be required for evaluation by industrial hygiene professionals.
Hazards for small businesses
Hazards from medical waste and prescription disposal
Hazards in the arts
Hazards in the arts have been an issue for artists for centuries, even though the toxicity of their tools, methods, and materials was not always adequately realized. Lead and cadmium, among other toxic elements, were often incorporated into the names of artist's oil paints and pigments, for example, "lead white" and "cadmium red".
20th-century printmakers and other artists began to be aware of the toxic substances, toxic techniques, and toxic fumes in glues, painting mediums, pigments, and solvents, many of which in their labelling gave no indication of their toxicity. An example was the use of xylol for cleaning silk screens. Painters began to notice the dangers of breathing painting mediums and thinners such as turpentine. Aware of toxicants in studios and workshops, in 1998 printmaker Keith Howard published Non-Toxic Intaglio Printmaking which detailed twelve innovative Intaglio-type printmaking techniques including photo etching, digital imaging, acrylic-resist hand-etching methods, and introducing a new method of non-toxic lithography.

Mapping environmental hazards

There are many environmental health mapping tools. TOXMAP is a Geographic Information System from the Division of Specialized Information Services of the United States National Library of Medicine that uses maps of the United States to help users visually explore data from the United States Environmental Protection Agency's Toxics Release Inventory and Superfund programs. TOXMAP is a resource funded by the US Federal Government. TOXMAP's chemical and environmental health information is taken from NLM's Toxicology Data Network
and PubMed, and from other authoritative sources.

Aquatic toxicity

testing subjects key indicator species of fish or crustacea to certain concentrations of a substance in their environment to determine the lethality level. Fish are exposed for 96 hours while crustacea are exposed for 48 hours. While GHS does not define toxicity past 100 mg/l, the EPA currently lists aquatic toxicity as "practically non-toxic" in concentrations greater than 100 ppm.
ExposureCategory 1Category 2Category 3
Acute≤ 1.0 mg/L≤ 10 mg/L≤ 100 mg/L
Chronic≤ 1.0 mg/L≤ 10 mg/L≤ 100 mg/L

Note: A category 4 is established for chronic exposure, but simply contains any toxic substance which is mostly insoluble, or has no data for acute toxicity.

Factors influencing toxicity

Toxicity of a substance can be affected by many different factors, such as the pathway of administration, the time of exposure, the number of exposures, the physical form of the toxicant, 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 toxicant over an extended period of time, often measured in months or years; it can cause irreversible side effects.

Etymology

"Toxic" and similar words derive from Greek τοξον toxon, a reference to the use of poisoned arrows as weapons. This root was chosen because the transliteration of 'ιον ion, the usual Classical Greek word for "poison", was not distinctive enough from the English word "ion," itself derived from a similar but unrelated Greek root. The literal meaning of the root toxo- is reflected in such biological names as Toxodon.