Water Purification - Health Advice
Water purification
This is the most reliable method, although stoves, kettles and heating coils may be inconvenient to use while travelling.  Boiling for one minute is sufficient to kill all harmful organisms, even at high altitude when water boils at a lower temperature.

Iodination is a very effective and convenient method for water purification.  Iodine destroys bacteria, viruses and cysts, and its action is dependent on its concentration, the water temperature and duration of contact.  Thus, a concentration of 8 mgs per litre at 20 degrees centigrade, will destroy all pathogens if left for 10 minutes.  Lower concentrations and lower water temperatures require a longer duration of action.

Various preparations of iodine are available, including tincture of iodine, iodine crystals and tablets.  Manufacturers instructions should be followed in each case.  With tincture of iodine, adequate levels are normally achieved by using 4 drops to 1 litre of water or one drop to a glass. As with all halogens it is less effective against cryptosporidia.

Iodine has been used safely for periods of several months, however it is not known whether there are adverse effects associated with longer term use.  Those with a history of thyroid disease or iodine allergy, the pregnant and the very young may be advised against using iodine for water purification.

The taste of iodine can be largely removed by neutralising the iodine, after it has had time to work, with dissolvable vitamin C tablets, lime or lemon juice.  It should be noted that this also stops the disinfectant action, so care should be taken to prevent recontamination of water.

Chlorine and silver
Chlorine and silver based tablets are available from chemists and specialised travel equipment shops. When used correctly they destroy most bacteria (e.g. V.cholerae), but are less effective for viruses and cysts (e.g. hepatitis A virus, giardia and amoebic cysts, cryptosporidia).

Chlorine alone is readily inactivated by organic matter and its action varies with pH. However if used in combination with Phosphoric acid in combination (e.g. Aquamira) it is more effective and this combination will destroy both giardia and cryptosporidia.

A wide variety of filters are available. Some versions have cores which can be replaced. Filters remove sand, clay and other matter as well as organisms by means of small pore size, membranes, adsorption, exchange resins and osmosis. They effectively remove bacteria and parasites but not viruses. Good filters are effective against cryptosporidia and giardia.  Due to the inability to remove viruses, filtered water must also be chemically treated or boiled.

Filtration combined with iodine (or chlorine)
Some modern filters incorporate a method of chemical disinfection to increase their effectiveness.  This is usually achieved by passing water through iodine exchange resins. When negatively charged contaminants contact the iodine resin, iodine is instantly released so killing the microorganisms without large quantities of iodine being in solution.  Such filters should be replaced after a specified volume of water has been treated, although this could sometimes be difficult to keep track of.

Sources of Equipment
Outdoor and adventure holiday shops and pharmacists
Equipment for sterilising water and water filters can usually be purchased at 'outdoor' sports, camping and adventure shops. Chlorine and iodine preparations can often be obtained from local pharmacies. Filters for sterilising water must not be confused with cheaper versions (sometimes available in supermarkets) designed only to remove smells and chlorine from domestic water.

source: Scottish NHS www.fitfortravel.scot.nhs.uk/General/
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Water Purification - Health Advice