When water is from a questionable source, it should be boiled before consumption, for at least 15 minutes where water contamination is severe. In some areas drinking water may be purified by chlorination, but it should not be relied upon where bacteria and parasites are common. Boiling is best.

In many countries contaminated water spreads cholera, jaundice, typhoid, paratyphoid, bacillary dysentery, and amoebic dysentery, among other diseases. In some places even city water supplies cannot be assumed to be safe. Once water is boiled, store it in clean, covered vessels.

In some areas the filtering of water is also recommended. Filters are available as faucet attachments or as separate units into which water is poured and allowed to drip through unglazed porcelain or other filtering substances. Filtering removes suspended particles and contaminants, but it usually does not get rid of harmful bacteria. However, some new filters and attachments evidently do remove harmful bacteria, although they are relatively expensive, and if they are not regularly changed, they themselves may contaminate. Modern filtering equipment even enabled the astronauts to drink their own urine.

Since it costs money to boil water, many restaurants in developing lands do not boil the drinking water served to their customers, so it is safer not to drink it. Also, avoid juices or drinks that require added water or ice. Bottled drinks or hot drinks are generally safer.

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Water from a questionable source should be boiled