yvon said:
Pristine takes only 20 minutes before you can drink the water. Several agencies, who have done trek in Nepal for several years, recommand this product. On our trek, everybody have use Pristine, and nobody have been sick.
Yes, that is a standard contact time for chlorine dioxide (ClO2). It kills bacteria, but it is not long enough (4 hrs) to kill cysts (crypto and giardia).
Both iodine and ClO2 are good at killing both viruses and bacteria, but are much slower to kill parasitic cysts. Boiling kills all three.
Iodine is a time tested method that, combined with filtration, kills or removes all pathogens. It, however, leaves a taste that some find unpleasant--thus commercial tour companies would have incentive to use a method that leaves less taste (eg ClO2). I don't have any authoritative references that rank iodine vs ClO2 for safety.
In the USA and Canadian hiking areas, viruses (and likely cysts) are not much of a problem if one exercises reasonable care in choosing one's water source. (It appears that one is more likely to get crypto or giardia from other hikers than from drinking water.) However, in the third world, one should protect against all three forms of pathogen in drinking water.
Medicine for Mountaineering by James A. Wilkerson is an authoritative reference with a good section on water purification. (Latest edition is 2001.) It covers boiling, filtration, iodine, and chlorine (bleach), but not the newer ClO2 or UV systems. (
http://en.wikipedia.org/wiki/Chlorine_dioxide appears to be a reasonable reference on ClO2.)
So, ClO2 (from any of a number of suppliers) is probably a reasonable method of purification in Nepal as is iodine or boiling. However, with ClO2 or iodine, some may prefer to add filtering to eliminate cysts as well.
Doug