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Different factors contribute to the toxicity of a substance. The main factor would have to be the way the substance enters the body whether it was inhaled, ingested or even applied to the skin. The time of exposure and amount of time the worker is actually exposed to the substance plays an important part. The two main points are:
Acute exposure -
Acute exposure is usually referred to as a 'one-off' dose from a harmful chemical substance. However, some acute doses may result in extreme harm and even death, such as an acid spill.
Chronic exposure -
Chronic exposure is when a worker is exposed to a chemical throughout the entire workday. These exposures are calculated over months and even years and in most cases present with irreversible side effects.
Furthermore, the shape or physical form of the substance such as solid, gas or liquid combined with the individuals overall health and gender contribute to the severity of the acute and chronic side effects.
The main factor that presents itself is dosage. A quote from Paracelsus (1493-1541) describes: "All things are poison and nothing is without poison, only the dose permits something not to be poisonous". However, other factors also contribute such as exposure routes, form, sex, age, metabolism condition and excretion and presence of other substances. The dosage is the most important and critical factor in determining if a substance will be an acute or a chronic toxicant. Acute and chronic toxicity derives from whether certain toxic mechanisms are targeted and its duration to that substance. In large doses, all chemicals can be acute toxicants however, target organs and toxic mechanisms differ between acute and chronic exposure.
Exposure route is important in determining toxicity. Some chemicals may be highly toxic by one route but not by others. Two major reasons are differences in absorptionÂ and distribution within the body. For example:
Ingested chemicals, when absorbed from the intestine, distribute first to the liver and may be immediately detoxified. Inhaled toxicants immediately enter the general blood circulation and can distribute throughout the body prior to being detoxified by the liver. Different routes of exposure target different target organs in most cases.
Reactions and effects of the chemical depends majority of the time on species and most of the major differences of species and their effects depend on quality of metabolism of the person. Other people that are exposed may show physiological differences instead. In animals for example, a study shows that rats cannot get rid of inhaled or ingested toxic therefore leaving them extremely vulnerable to disease and even death. However, human and dogs can induce vomiting therefore limited them of harmful side effects.
Selective toxicity is the ability of a chemical or drug to kill a microorganism without harming its host.
Using an insecticide on plants and crop can kill all insects and other harmful parasites but with be relatively non-toxic to larger animals.
AntibioticsÂ are used to help the host by targeting either specific organs and getting rid of selective toxic microorganisms.
In most cases of toxicity in harmful chemicals and its effects, age could be an important factor in determining the response to toxicants. Some chemicals are more toxic to infants or the elderly than to young adults. For example:
Parathion, a potent insecticide and acaricide, is more toxic to young animals than more mature animals.
Nitrosamines are more carcinogenic to newborn or young animals
Although uncommon, toxic responses can vary depending on gender.
In studies done, it's evident that majority of male rats are approximately 10 times more sensitive than females to liver damage fromÂ chemicals such as dichlorodiphenyltrichloroethane (DDT). However, female rats are twice as sensitive to parathion as male rats. For systematic toxicity to occur, chemicals absorption ability is very important. Some chemicals such as alcohols, when ingested are readily absorbed however, other chemicals such as 'polymers' are poorly absorbed and lead to toxic effects. All chemicals are dependent on the rates and extent of absorption which may vary greatly depending on the form and the route of the chemical.
It is harder for ethanol, alcohol based or not, to be absorbed through the skin however, very easily absorbed through the gastrointestinal system.
Inorganic lead sulphate is not absorbed well or at all through the gastrointestinal tract however organic mercury is.
Biotransformation or also known as metabolism is a major factor in determining toxicity. Metabolites are the main products of metabolism and consist of two types - detoxification and bioactivation. With detoxification a chemical 'xenobiotic' is released allowing the toxin to be converted to a less toxic form and is a natural defence mechanism of the organism. A change in lipid-soluble compounds to polar is usually the detoxification process and bio-activation is the process by which a xenobiotic may be converted to more reactive or toxic forms. The main factor that determines where the toxicity occurs is the distribution of toxicants and toxic metabolites. Furthermore, the lipid solubility is one the best determinant of whether the toxin will damage cells it comes into contact with. The more common storage areas include - FatÂ tissue, liver, kidney, and bone and where blood and lymph serves as the main avenue for distribution.
The site and rate of excretion is another major factor affecting the toxicity of a xenobiotic. Depending on where the toxin lands in the body determines the rate of excretion; the kidney is the primary organ to excrete toxins, second is the gastrointestinal tract and then lungs (in the case of gases).
In relation to the toxicity of chemicals, there are two important types of toxic effects; organ specific toxicity and systemic toxicity. Organ specific refers to substances, when absorbed in the body, affect a particular organ. For example, asbestos mainly targets the lungs and neurotoxicity substances target areas of the nervous system. Systemic toxicity on the other hand could include absorption of acid where abnormal cell growth develops and carcinogenic are formed causing cancer. Both of these types are related to acute and continuous exposure and their dosage.
As described earlier in the article, the physical form which the substance presents itself plays a major role in dose and side effect severity. For example; mercury vapour varies from methyl mercury.
Chemical activity of substances also differs greatly which in turn can determine between two outcomes. Some substances can affect cell structure and life causing gradual onset of side effects such as paralysis. Other more harmful substances immediately alter cell structure at an alarming rate such as hydrogen cyanide resulting in hypoxia causing death in most cases.
A large volume of blood serum is filtered through the kidney. Lipid-soluble toxicants are reabsorbed and concentrated in kidneyÂ cells. Impaired kidney function causes slower elimination of toxicants and increases their toxic potential. TheÂ presence of other chemicalsÂ may decrease toxicity, add to toxicity or increase toxicity. - - - In normal kidney function, the kidney reabsorbs and used kidney cells to concentrate lipid-soluble toxicants that have entered the bloodstream as large volumes of blood pass through. However, if other chemicals or toxins are present in the body or the kidneys are not functioning at their optimum, slower elimination of harmful toxins may occur. This could leave the person affected exposed to chronic illness.
When chemicals or poisons enter the body, they usually affect the pace in which body organs functions. With some poisons, they can either slow the organs like the lungs and cause breathing difficulties or increase their rate causing a faster heart beat leading to sweating or hyperventilation. For example, a person may come into contact with mercury and could end up absorbing it into the body. After a short period of time, this may experience increased sweating due to the body trying to get rid of it. A number of steps follows mercury poisoning and are as follows. The first step is the "biochemical inactivation of an enzyme." After the first step, the second step in the process is there will be a biochemical change leading to a cellular alteration. The cellular modification is then responsible for physiological changes, which are the symptoms of poisoning that are seen or felt in particular organ systems (in this case the sweat glands). The basic progression of effects from biochemical to cellular to physiological occurs in most all cases of poisoning.
Depending on the specific biochemical mechanism of action, a poison may have very widespread effects throughout the body, or may cause a very limited change in physiological functioning in a particular region or organ. Parathion causes a very simple inactivation of an enzyme which is involved in communication between nerves. The enzyme which parathion inactivates however is very widespread in the body, and thus many varied effects on many body systems are seen besides sweating.
Toxicity is a general term used to indicate adverse effects produced by poisons. These adverse effects can range from slight symptoms like headaches or nausea, to severe symptoms like coma and convulsions and death.
Toxicity is normally divided into four types, based on the number of exposures to a poison and the time it takes for toxic symptoms to develop. The two types most often referred to are acute and chronic. Acute toxicity is due to short-term exposure and happens within a relatively short period of time, whereas chronic toxicity is due to long-term exposure and happens over a longer period.
Most toxic effects are reversible and does not cause permanent damage, but complete recovery may take a long time. However, some poisons cause irreversible (permanent) damage. Poisons can affect just one particular organ system or they may produce generalized toxicity by affecting a number of systems. Usually the type of toxicity is subdivided into categories based on the major organ systems affected. Some of these are listed in table 1. Individual Toxicological Information Briefs (TIBs) are available which more fully explain skin and reproductive toxicities. Another is available which covers the formation of tumours and cancer.
Because the body only has a certain number of responses to chemical and biological stressors, it is a complicated business sorting out the signs and symptoms and determining the actual cause of human disease or illness. In many cases, it is impossible to determine whether an illness was caused by chemical exposure or by a biological agent (like a flu virus). A history of exposure to a chemical is one important clue in helping to establish the cause of illness, but such a history does not constitute conclusive evidence that the chemical was the cause. To establish this cause/effect relationship, it is important that the chemical be detected in the body (such as in the blood stream), at levels known to cause illness. If the chemical produces a specific and easily detected biochemical effect (like the inhibition of the enzyme acetyl cholinesterase), the resulting biochemical change in the body may be used as conclusive evidence.
People who handle chemicals frequently in the course of their jobs and become ill and need medical attention should tell their physicians about their previous exposure to chemicals.