In simple words, a toxicant may be defined as an agent that causes an adverse effect or response in a biological system, seriously damaging its structure or function or producing death. The adverse effect or response may be defined in terms of a measurement that is outside the normal range for healthy organisms.
In view of the National Institute for Occupational Safety and Health a toxicant may be defined as “a substance which demonstrates the potential to induce cancer, produces long term disease or bodily injury; affects health adversely; produces acute discomfort; or endangers the life of man or animals through exposure via the respiratory tract, skin, eyes, mouth, or the other routes.”
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Introduction of Toxicants into the Ecosystem:
A toxicant or foreign substance may be introduced deliberately or accidently into the ecosystem, impairing the quality of environment and making it unfavorable for organisms.
Toxicants find their way in the ecosystem from:
i. Non-point sources such as agricultural runoff from land, contaminated ground water and bottom sediments, urban runoff, dredged sediment disposal, and atmospheric fall out, and
ii. Point sources such as discharges (effluents) from manufacturing plants, hazardous waste disposal sites, and municipal waste treatment plants.
Survey and Classification of Toxicants:
There are a number of chemical substances in the environment — some of these are-toxic and rest non-toxic. The toxicants are released from various sources into air, water and soil. They get into the human food chain from the environment. Once the toxicants enter our biological system, they interfere with the biochemical processes and may lead, in some, cases to fatal results.
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According to the International Register of Potentially Toxic Chemicals of the UNEP (United Nations Environment Programme), there are four million known chemicals in the world today and another 30,000 new chemical compounds are added to this list every year. Among these, 60,000 – 70,000 chemicals are very commonly used. Apart from their benefit in increasing production, living standards and health, many of them are potentially toxic.
Survey of toxicants in various sections of environment may precisely by explained in the following points:
1. Toxicants in air
2. Toxicants in water
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3. Toxicants contaminating food
4. Others viz., noise and radiation pollution.
1. Toxicants in Air:
Quite a large number of chemicals presumably pose serious problems of health when present in air. It is, therefore, necessary to exercise strict control during their manufacture and handling.
Environmental Protection Agency (EPA), Duluth, U.S.A. and Occupational Safety and Health Administration and Consumer Product Safety Commission (1978), listed 24 extremely hazardous substances in the atmosphere which are:
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1. Arsenic
2. Acrylonitrate
3. Asbestos
4. Berilium
5. Benzene
6. Cadmium
7. Chlorinated Solvents
8. Chromates
9. Chromates
10. Chlorofluorocarbons
11. Diethylstilbestrol
12. Dibromochloropropane
13. Ethylene dibromide
14. Ethylene oxide
15. Lead
16. Mercury
17. Ozone
18. Nitrosoamines
19. PCBs
20. PBBs
21. Sulphur dioxide
22. Vinyl Chloride
23. Radiations
24. Toxic waste disposal emissions and leachates.
The common toxicants present in air are:
i. Carbon monoxide
ii. Nitrogen gases, viz., NO, NO2, NH3, and N2O
iii. Sulphur gases, viz., SO2 and H2S
iv. Hydrocarbons viz., CH4
v. Photochemical oxidants viz., O3, and other photochemical products like benzopyrene, peroxybenzoyl nitrate (PBZN) and peroxyacetyl nitrate (PAN).
vi. Lead from automobile emission.
2. Toxicants in Water:
A large number of toxicants — intentionally or otherwise — reach in the aquatic environment.
The principal sources of toxicants in aquatic environment are:
i. Sewage and other wastes of domestic nature
ii. Agricultural discharges
iii. Industrial effluents, and
iv. Wastes from thermal and nuclear power plants.
Toxicants in aquatic environment may, however, be grouped into two principal categories:
i. Toxic trace elements and heavy metals found in natural and waste waters
ii. Pesticides.
i. Toxic Trace Elements and Heavy Metals found in Natural and Waste Waters:
Some of these are essential at low level, serving as nutrients for animal and plant life, but certainly deleterious at higher levels.
ii. Pesticides in Water:
The aquatic bodies contain a large number of pesticides, primarily from the drainage of agricultural field.
3. Toxicants Contaminating Food:
A food additive may be defined as “a substance or mixture of substances other than a basic food, which is present in food as a result of any aspect of production, processing, storage or packaging”. In other words, food additive is any substance not normally present in the food in question and which is added, either deliberately or incidentally, in order that some quality of the food may be improved.
Food additives may be of two types, namely, Intentional additives, for example, sodium benzoate, sorbic acid, lecithin, monosodium glutamate, saccharin etc., and Incidental additives, which are mixed with food during processing or packaging etc.
Food additives are used to provide better colour, flavour and texture and as a preservative. Several food additives have been found associated with the toxic hazards and health risks. Many of the additives like cyclamates, safrole and diethyl stilbestrol (DES) have been banned. Several others like saccharin, monosodium glutamate, nitrite, nitrate, etc. are under close investigation and are likely to be banned.
4. Others (Noise and Radiation Pollution):
Noise is an environmental pollutant, but it differs from air and water pollutants in that it disappears fast and thus does not remain in environment for long. Presence of unwanted sound in the atmosphere that produces adverse effect in human beings is termed as ‘Noise Pollution’.
Vehicles do not just cause air pollution, they are also a major cause of noise pollution, exposure to which over an extended period of time can even lead to hearing loss.
Actually, sound, as such, is not considered as a pollutant. Many sounds are agreeable and pleasing to human ear. But the constant presence of unavoidable and irritating sound, for periods longer than the tolerance limit of human ear, is now considered as one of the serious air pollutants.
Noise can interfere with all our activities, such as work, rest, recreation, and sleep. It is not only a nuisance but also a threat to our health and, thus deserves the attention of all concerned.
Sources of noise are many. The main sources of man-made noise in developed urban areas are mechanized vehicles such as trucks, buses, cars, scooters, fire engines, tractors, generators etc. Industrial machinery, trains, aeroplanes, and accessory noise producers such as horns, sirens, loud-speakers etc. also contribute to noise pollution. Musical instruments, TV, radio, washing machines, vacuum cleaners, food mixers, air-conditioners, shouting, social gatherings, etc. are some other sources of noise pollution.
The main menace of noise, however, comes from transport. It mainly includes noise from road traffic, rail traffic, aircraft, in addition to vehicle horns. The number of road vehicles, particularly the diesel engine vehicles, has increased enormously in recent years.
A survey conducted in metropolitan cities has shown that the average noise level in Delhi, Mumbai, and Kolkata is as high as 90 dB. Inhabitants of the major cities, subjected continuously to this level of noise, run the risk of impairing and gradually losing their hearing ability.
Noise is transmitted as sound energy through any elastic medium like air, water or steel by the vibrations of the molecules in the medium. When there are no molecules, as in a vacuum, sound cannot travel. Sound travels faster in solids than in air. The speed of sound in air, at 20°C, is nearly 344 m/s, while the speed of sound in wood is 3,962 m/s and in steel 5,029 m/s.
Sound is usually measured either by:
(i) Sound pressure, or
(ii) Sound intensity.
There are two important parameters for describing sound:
1. The Sound Wave Frequency:
As sound waves travel through air, the air undergoes periodic increase and decrease in density (compression and rarefaction). The number of compressions and rarefactions per unit time is known as the sound wave frequency. It determines the pitch of the sound. Its unit is Hertz (Hz) or cycles per second. The human auditory system is normally able to discriminate pitches in the range of frequencies between 16 and 20,000 Hz — called the audio-frequency range.
2. The sound wave amplitude or magnitude:
This is a measure of loudness. Since audible sounds may vary in intensity by many levels of magnitude, a logarithmic scale is used to measure sound intensity. This unit is decibel (dB), named after Alexander Graham Bell. The threshold for detection of sound is defined as 0 dB.
One dB is equal to the faintest sound a human ear can hear. Normal conversation generates about 50 to 60 dB, a city street has a sound level around 75 dB, and an auto horn at close range is about 90 dB. Sounds become painful and potentially damaging to auditory receptors at 130 dB. The most immediate and acute effect of noise pollution is impairment of hearing. Long exposure to loud noise can cause some hearing loss that may become permanent.
Generally, hearing loss is a result of neural damage involving injury to the hair cells. Two theories are offered to explain noise-induced injury. The first is that excessive hearing forces mechanically damage the hair cells. The second is that intense noise stimulation forces the hair cells into high metabolic activity that overdrives them to the point of metabolic failure and consequent cell death. Once destroyed, hair cells are not capable of regeneration. It has been established that a person exposed to a noise level of 90 dB for more than eight hours per day is liable to severe hearing problems after 20 years.
This is also true in case of traffic police personnel on duty and also for shopkeepers and residents near noise- polluted intersections. In addition to hearing damage, continuous noise can induce non-auditory physiological effects. Noise pollution can interfere with speech communication, sleep, acoustic privacy and cause annoyance — thus affecting human health, comfort and efficiency.
Noise pollution is also known to increase the rate of heart beat. It causes constriction of blood vessels, makes the skin pale, and muscles to contract leading to nervous breakdown and tension. It increases the digestive spasms and causes dilatation of the pupils of the eye.
Fluctuations in arterial blood pressure, decrease in heart output, impairment of night vision, and decrease in the rate of colour perception have also been reported. Noise also interferes in inter-personal communication. It causes headache, irritability (annoyance) and extreme emotional disturbances. It aggravates existing diseases by disturbing the peace of mind and sleep.
Effects of noise on the fetus are not fully known. Medical scientists have found that an unborn child moves and kicks about when there is loud noise. It also responds with a sudden increase in the heart beat as though it were disturbed or frightened. However, more research is required to establish a noise index consistent with human comfort and well- being.
The atoms of certain elements are known to undergo spontaneous disintegration with the emission of atomic particles, i.e. radiation. Such elements or substances that emit radiations due to spontaneous nuclear disintegration are termed radioactive substances.
The well-known radioactive elements are:
Carbon14, Thorium234, Uranium238, Phosphorus32, Potassium40, Cesium137, Radon222, Radium226 etc.
The radioactive rays and subatomic particles enter the environment mainly from two sources:
i. Natural sources
ii. Anthropogenic (man-made) sources.
The non-ionizing ultraviolet rays from the sun, cosmic rays and natural decomposition of radioactive substances emit radiation and sub-atomic particles.
ii. Anthropogenic (man-made) radiation:
a. X-rays used in medical practice
b. Radiation therapy in treatment of cancer
c. Application of radio-isotopes in research work, medical practice and archeological dating
d. Atomic power plant
e. Nuclear generator
f. Radioactive fall-out resulting from nuclear weapons testing.
Contamination of the environment with the radioactive waves may be by:
(i) Accidental Spilling:
Example—Chernobyl (Russia), Three Miles Island (U.S.A.) Nuclear disasters.
(ii) Deliberate Action:
Example — Atomic explosion as in Hiroshima, Nagasaki (Japan) and explosives in wars.
(iii) Radioactive Substances:
Example — Cesium, Uranium, Zirconium, Tritium, Strontium, Radium, Plutonium etc.
(iv) Radio-Isotopes:
Example — Isotopes of Uranium238, Carbon14, Cobalt60, Manganese54 and Strontium90.
(v) Ignorance:
Example — Disposal of nuclear wastes indiscriminately and cutting of forests.
Radiations are chiefly of two types:
(a) Non-ionizing UV
(b) Ionizing (X-rays, alpha, beta, neutrons and gamma radiations).
Ionizing radiations are of two types:
(i) Electromagnetic Radiation:
It is in the form of short wave, high energy radiation similar to X-rays. Example, gamma radiation.
(ii) Corpuscular or Particulate Radiation:
It consists of subatomic particles which transfer their energy to whatever object they strike. Example, alpha and beta radiations.
(i) Non-Ionizing Radiation:
The radiations which do not possess ionizing effect are called non-ionizing radiation.
UV Rays:
UV rays emitted by sun may penetrate living tissues up to 0.1 mm thickness, depending upon the length of exposure and are specifically absorbed by nucleic acids and generate pyrimidine dimers. These suppress separation of DNA strand during replication. These may produce mutation.
(ii) Ionizing Radiations:
Very high-energy radiations that are able to remove electrons from atoms and attach them to other atoms, thereby producing +ve and -ve ion pairs, are known as ionizing radiations. Isotopes of elements that emit ionizing radiations are called radionuclides or radioisotopes.
(a) X-Rays:
The X-rays possess high speed almost equal to the speed of light — and have ionizing power but do not penetrate deep in the living tissues.
(b) Gamma rays:
The gamma rays possess enormous penetrating power and travel with a speed that equals speed of light. It may penetrate a lead sheet of about 15 cm (6 inch) thickness. It also damages living tissues.
Classification of Toxicants on the basis of Organ/System Affected:
From the human health view-point and the organ/system affected, the toxicants may be of the following types:
1. Environmental Carcinogens:
In various countries, cancer is one of the leading cause of death. It is supposed to be induced by a variety of agents and also certain life styles, e.g., smoking cigarettes, drinking alcohol, chewing tobacco etc. However, toxicants in the community and at work places, besides certain therapeutic agents, also play certain role in inducing cancer.
Attempts at reducing cancer incidence are, therefore, aimed at minimizing these hazardous life styles along with identifying these chemical carcinogens, in the hope that human exposure to them can be eliminated or minimized. Chemical carcinogens may thus be defined as any chemical which initiates carcinogenicity.
2. Cardiotoxicants:
Heart may not be a common target organ, yet it is damaged by a variety of chemical or drugs referred to as cardiotoxicants. They act either directly on the myocardium or indirectly through the nervous system or blood vessels.
3. Immunotoxicants:
Actually, the immune system serves an important role in the body’s defense against infection by microbes viz., viruses, bacteria, fungi and unicellular and multicellular parasites as well as against neoplastic cells.
It has now become an established fact that a variety of toxicants (chemicals) can impair the function of the system. A variety of chemicals may induce hypersensitivity reactions. The site of such reactions are mainly confined to the respiratory tract and the skin. Dean et al (1979) reported that a variety of substances may adversely affect the immune system.
4. Teratogens:
Chemical agents causing teratogenesis are called teratogens. Teratogenesis is the formation of congenital defects.
5. Occupationally Inhaled Toxicants:
Following industrialization, since the Industrial Revolution begun in 1970, the human respiratory system is increasingly exposed to airborne toxicants. Airborne substances exist in the form of gases, vapours, liquid droplets and solid particulate matters of different sizes. The uptake and effects of inhaled toxicants, therefore, depend not only on their toxic nature but also on their physical characteristics.
6. Hepatotoxicants:
Some of the hepatic (liver) injuries are produced by hepatotoxicants like CCl4; Halothane; α-naphthylisocyanate (ANIT); Chloroform, Ethanol; Bromobenzene; Bromsulphathalein (BSP); Aflatoxins and Phosphorus. These hepatotoxicants induce several types of injuries like steatosis, necrosis, cirrhosis, neoplasia etc. These injuries are relatively easily produced in experimental animals.
7. Mutagenic Agents:
Mutagenesis can occur as a result of interaction between mutagenic agents (like As, Cd, Co, Hg, Ni, Pb and Pt) and the genetic material, i.e., DNA of organisms. While spontaneous mutation and natural selection are the major means of evolution, a number of toxicants in recent decades have been reported to be mutagenic.