Q: In our locality, many salesmen come to sell water purification systems. They say we are drinking poison and this is dangerous. While some do certain tests and turn the water black and say that it is all impurities. I want to know that the water that we drink looks to be pure and even tastes sweet. Do you thin what the salesman says could be true? Every sales man claims his product to be the best and criticises other products. So whom to trust? Do you think these water purification products can benefit our health?

A:The United Nations has categorised India amongst the worst countries for poor quality of water, in a report. The Asian rivers are the most polluted in the world, with three times as many bacteria from human waste as the global average. These rivers also have 20 times more lead than those of the industrialised countries. The quality of water resources is steadily declining because of population growth, pollution, illegal booster pumps and climate change. To answer your specific questions: 1) Even if the water looks quite good and tastes sweet, testing of water quality would be ideal to confirm the suitability for drinking purposes. 2) If the water is contaminated, as it can be in certain areas, then it can have deleterious effects on human health. But this can only be ascertained after suitable testing. 3) This is true. The latest water purification technologies involve reverse osmosis, which also takes care of chemical impurities. Some companies do indulge in marketing gimmicks as well. 4) If your water contains high levels of lead, arsenic or fertilisers, they would have some effect on your health, though there are no studies to indicate that their removal will prolong your life. 5) Boiling or chemical disinfection (using chlorine tablets) is only suitable for prevention of infectious diarrhoea, and does not take care of chemical impurities. Here is some more information on water quality standards in India and the US; though whether these are observed in practice (in India) is another question altogether. Indian water quality standards: http://www.elaw.org/assets/word/IndiaWQS.doc US water standards: http://www.epa.gov/safe water/mcl.html