Re-boiling clean uncontaminated drinking will water not cause cancer or any other health problems.
Boiling water contaminated with chemicals such as nitrates or toxic elements such as arsenic is a bad idea, so there may be circumstances when re-boiling water is a problem This is really only a problem in a few rural areas where water quality isn't monitored.
Ground water may contain nitrates, which are chemicals often found in fertilizer, and arsenic that may occur naturally or from farm or industrial waste. Nitrates, along with nitrites, are also found in processed meats.
Consuming water contaminated with nitrates can cause a blood disorder called methemoglobinemia and some research shows that nitrates in drinking water may be associated with some types of cancer.
Exposure to arsenic may result in arsenic poisoning and increased risks of several types of cancer.
Nitrates and arsenic are not like bacteria, so boiling your water doesn't remove them. In fact, boiling water makes it worse -- the longer the water is boiled, the higher the concentration of these substances will be.
The levels of both nitrates and arsenic are regulated and water supplies must be monitored in towns and cities, so if your tap water comes from a city water source, it's safe. People who have private wells in rural areas must have their well water tested for nitrates and arsenic.
Centers for Disease Control. "Nitrate and Drinking Water From Private Wells." Accessed May 12, 2009. http://www.cdc.gov/ncidod/dpd/healthywater/factsheets/nitrate.htm.
Centers for Disease Control. "Arsenic and Drinking Water." Accessed May 12, 2009http://www.cdc.gov/ncidod/dpd/healthywater/factsheets/arsenic.htm."
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United States Environmental Protection Agency. "Consumer Factsheet on: Nitrates/Nitrites." Accessed May 12, 2009. http://www.epa.gov/safewater/contaminants/dw_contamfs/nitrates.html.
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World Health Organization. "Arsenic in Drinking Water. "Accessed May 12, 2009. http://www.who.int/water_sanitation_health/dwq/arsenic/en/."
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