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Should be checked toxic substance in water before used.
Toxic substances are a feature of the natural world - many plants contain chemical compounds that make them anything from mildly distasteful to lethally poisonous to animals that might eat them. Some animals are equipped to deal with dangerous plants in their environment and possess detoxification mechanisms that break down harmful compounds.
During human cultural evolution, cooking techniques have developed that destroy toxic chemicals in plants. Examples include nerve poisons in the lentils from which dahl is made in India, and cyanide in cassava, a staple crop in Africa and South America.
There are limits, however, to what cooking can achieve and the environment contains a huge array of chemical compounds, some of them of human manufacture, that are harmful to human health and survival.
The latter are said to be xenobiotic (zen-oh-bye-to-i). Literally meaning ‘alien to nature’, in the context of this unit this word refers to chemicals 'of human origin'.
Water is said to be polluted, or contaminated, whenever any harmful or undesirable change in its physical, chemical or biological quality results from the release into it of synthetic or naturally occurring chemicals, radioactivity or organic matter. (Organic means arising from the bodies of plants, animals or other organisms.)
Pollution often refers to the results of human activity but there are significant natural causes of contamination, such as volcanic eruptions, which release a variety of chemicals, and tsunamis, which mix salty seawater with freshwater. For example, much of the groundwater obtained from boreholes in parts of Bangladesh and West Bengal is contaminated with naturally occurring arsenic, released from rocks deep underground.
Most familiarly, pollutants can be released into rivers or into the sea, but they can also be released into groundwater by pollution of the soil.
Some pollutants enter the water cycle from the atmosphere. For example, acid rain is caused by the mixing of water vapour with gaseous pollutants such as sulfur dioxide, released by burning fossil fuels, and a variety of nitrogen compounds from agricultural fertilisers.
Pollution may be acute or chronic.
Acute pollution events refer to the sudden release of large quantities of a contaminant, usually leading to very obvious harmful effects.
An example is provided by the accidental release of a large quantity of aluminium sulfate, a substance used in water treatment, into the water supply of Camelford, in Cornwall, UK in July 1988.
Chronic pollution refers to the slow and persistent contamination of water through the sustained release of a pollutant and is, in many ways, a more serious concern, for three reasons:
1. Chronic pollution may go undetected for a long time;
2. It is generally more difficult to rectify than an acute pollution event;
3. Chronic pollution is also serious because, unlike most acute pollution events, it is often not confined, as the Camelford incident was, to a small area.
As a result of the risks arising from pollution, the water supply in high-income countries is carefully monitored to ensure that levels of contaminants do not exceed specified concentrations that are considered to be safe.
The determination of safe levels is quite a complex process that involves the science of toxicology (the study of toxins and their effects on living organisms).
Toxicology involves exposing animals to a toxic compound to determine its lethal dose. Among the animals used for toxicological testing of water-borne chemicals are tadpoles of the African clawed frog (Xenopus laevis).
Xenopus is widely used for this purpose as it breeds readily in captivity and produces huge numbers of tadpoles. Frogs and amphibians are regarded as particularly sensitive indicators of environmental damage.
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