After reading this essay you will learn about:- 1. Concept of Environmental Toxicology 2. Toxicological Chemistry 3. Metals.
Essay on the Concept of Environmental Toxicology:
Our surrounding is filled up with a large number of toxicants in different forms. They contaminate our water, land and atmosphere where we live.
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Metals, pesticides and large number of complex organic chemicals, which are prepared through man-made activities, pose serious threat to the life of mankind. Most of the toxicants enter the human body through a number of routes—either through drinks, breath, food, medicines or even occupational or accidental exposures (Fig. 25.1).
‘Toxicology’ is the study of chemical or physical agents that produce adverse responses in the biological systems with which they interact. The responses may be sudden and short span (i.e., acute toxicity) or long term constant or continuous (i.e., chronic toxicity).
The chemical agent or toxicant that can produce an adverse effect in a biological system may do this either through an alteration of normal function or the destruction of life.
Dose-Response Relationship:
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A particular toxicity test is said to exhibit a dose-response relationship when a consistent mathematical relationship describes the proportion of test organisms responding for a given dosage interval for a given exposure period.
Not only dose response to a chemical varies with different species of test organisms, response also varies within a group of test organisms of the same species. In general, responses of biological system were expressed against the cumulative long values of doses.
Dosages are often described as lethal (LD), in test where the response is mortality; effective dose (ED) in tests where the response is a desirable effect, or toxic dose (TD) where the response is an undesirable toxicity other than death. LD50, LC50 or LT50 can also be estimated form such dose response curve, where the dosage is lethal to 50% of the test organisms.
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The dose response curve has a number of utility values:
1. Help in determining toxicity of substances;
2. Identification of threshold limit of toxicity;
3. Help in determination of LD50, LC50, LT50 & ED50 etc.
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The dose-response relationship of any substance thus depends on the following aspects:
(i) Physical and chemical properties of substance,
(ii) Exposure conditions;
(iii) Genetic makeup of test systems;
(iv) Environmental conditions.
The fate of chemical toxicants is diverse after entry into the body (Fig. 25.1, Table 25.2). In general, body physiological functions attempt to eliminate out the toxic substance through a variety of excretion pathways, otherwise the toxicants are deposited in storage sites.
The toxicants not only affect human health, but also cause serious threat to the survivality of various biota.
The details of route of uptake of different pollutants and their sources is shown in Table 25.2:
Among the various pollutants two major groups were discussed at length—metals & pesticides.
Essay on Environmental Toxicological Chemistry:
A variety of chemical substances (gaseous, soluble or peculates) have distinct health borders.
Selected chemicals are discussed:
i. Ozone:
It has toxic effects (above > 1 ppm). Inhalation of ozone at 1 PPm level causes severe irritation and headache. Ozone States the eyes, upper respiratory system and lungs. Inhalation of ozone can sometimes cause fatal pulmonary oedema. Ozone generates free radicals in tissue.
These reactive species can cause lipid peroxidation of sulfhydryl (-SH) groups and other destructive oxidation processes. The compound that protect organisms from the effects of ozone include radical scavengers, antioxidants, and compounds that protect organisms from the effects of ozone include radical scavengers, antioxidants and compounds containing sulfhydryl groups.
ii. Carbon Monoxide:
It is a common cause of accidental poisonings. At CO levels in air of 100 ppm, impairment of judgement and visual perception occur; exposure to 100 ppm causes dizziness headache 3 weariness; loss of consciousness occurs at 250 PPm and inhalation of 1,000 ppm -suits in rapid death.
Chronic long term exposures to low levels of carbon monoxide are suspected of causing disorders of the respiratory system and the heart. After entering the bloodstream through the lungs, carbon monoxide reacts with haemoglobin (Hb) to convert oxyhaemoglobin (O2Hb) to carboxyhaemoglobin (COHb).
iii. Elemental Halogens:
Fluorine (F2), Chlorine (Cl2), Bromine (Br2) and Iodine (I2) are strong oxidant and that causes irritation on inhalation and subsequent respiratory distress and finally lead to death above > 1,000 ppm exposure.
iv. Cyanide:
Both hydrogen cyanide (HCN) and cyanide salt are rapidly acting poisons; a dose of only 60 to 90 mg is sufficient to kill a man.
Air pollutants viz., NOx, SOx, HF are also too toxic to human and plants.
v. Phosphine (PH3):
It is a colourless gas that undergoes a auto regulation at 100°C, is a potential hazard in industrial processes and in the laboratory.
vi. Polyaromatic Hydrocarbons (PAHS):
Various kinds of aromatic and polyaromatic hydrocarbons are carcinogenic. They also act by forming adducts with DNA.
vii. Acrylamide Toxic Potato Chips:
In 2002, Swedish investigators reported finding potentially harmful amounts of acrylamide in fried and baked foods, but not in boiled food. Acrylamide in known to be a neurotoxin and is a carcinogen.
viii. Pyridine:
It is a colourless & odorous chemical used in industrial processes. On inhalation, it causes anorexia, nausea, fatigue and chronic poisoning.
ix. Acetonitrile (CH3CN):
It is a highly toxic chemical, which shown marked toxicity in man on inhalation for prolong period.
x. Nitrosamine:
It is a carcinogenic chemicals found in beer, whisky and other similar liquors. Chronic exposure leads to carcinoma.
xi. Toxic Natural Products:
Bacterial toxins or fungal toxins often causes serious health disorder or ingestion. They may be neurotoxic or carcinogenic or teratogenic and thermo stable.
US Department of Health and Human Services, Public Health Service Agency, published a document for toxic substances and disease registry (HTSDR’s Toxicological Profile).
Metals in Environment:
Metals are introduced into aquatic systems as a result of the weathering of soils and rocks, from volcanic eruptions and from a variety of human activities involving mining, processing, or use of metals and/or substances that contain metal contaminants.
Although some metals such as manganese, iron, copper and zinc are essential micronutrients, others such as mercury, cadmium and lead are not required even in small amounts by any organism. Virtually all metals, including the essential metal micronutrients, are toxic to aquatic organisms as well as humans if exposure levels are sufficiently high.
The fact that metal concentrations in aquatic organisms are typically several orders of magnitude higher than concentrations of the same metals in water has led to some speculation that metals may become progressively concentrated at higher trophic levels in aquatic food chains due to biological magnification.
It is also true, in general, that the concentration of metals may differ greatly between one organism and another and between different parts of the same organism.
In general, one ascribes these concentration differences to differences in the tendency of metals to bind to the various molecule groups formed within the cells of each organism, as well as to the degree of the organism’s exposure to the metal as influenced by its metabolic characteristics and its position in the food chain (Table 25.3).
The term ‘heavy metals’ has been used extensively in the past to describe metals which are environmental pollutants. For a metal to be considered ‘heavy’ it must have a density relative to water greater than 5. However, this term is now replaced by toxic metals.
Metals are non-biodegradable. Unlike some organic pesticides, metals cannot be broken down to less harmful components.
Detoxification by organisms consists of ‘hiding’ active metal ions within a protein such as metallothionein (binding covalently to sulphur), or depositing them to an insoluble form in intracellular granules for long-term storage or excretion in the feces.
Among the metals, some appear to be essential (at low concentration) for biota growth and reproduction, while there are a few non-essential but toxic, also a good number lies in the borderline:
Contamination of our environment by toxic metals may arises in a number of ways. Industrial waste disposal is one of the major routes for metal contamination in aquatic ecosystem (Table 25.4).
In addition, there are other sources of metal contamination into water bodies. The global input of major metals annually is shown in Table 25.5. Some metals were discussed in detail with respect to their sources and toxicity in preceding sections.
