Recent researches have shown that people worldwide are consuming water which has unsafe level of natural arsenic in it. Arsenic is a semi-metal element in the periodic table. 
It is odorless and tasteless. It enters drinking water supplies from natural deposits in the earth or from agricultural and industrial practices. High arsenic levels can also come from certain fertilizers and animal feeding operations. Industry practices such as copper smelting, mining and coal burning also contribute to arsenic in our environment.
World Health Organization guidelines set a safe limit of 10 parts per billion of arsenic in water supplies but, as per researchers huge number of people in the world drink unsafe water above that level. The magnitude of the problem is now coming in the picture. As per new research presented at the annual Royal Geographical Society meeting in London, the problem effects nearly 140 million people in more than 70 countries.
Human exposure to arsenic can cause both short and long term health effects. Short or acute effects can occur within hours or days of exposure. Long or chronic effects occur over many years. Long term exposure to arsenic has been linked to cancer of the bladder, lungs, skin, kidneys, nasal passages, liver and prostate. It also may cause birth defects and reproductive problems.
In order to avoid drinking surface water, which can be contaminated with bacteria causing diarrhoea and other diseases, aid agencies had been promoting the digging of wells, not suspecting that well water would emerge with elevated levels of arsenic.
Via:IOL
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