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There are a few questions regarding safety of water purification, one regarding expiration date, another regarding iodine, but I know that there are more than one types of water purficiation tablets.

If I am using a water-pump filter. Is it necessary to combine this with a water purification tablet? Katadyn, a water filter company, suggests doing so.

However, this makes me question the safety of putting such chemicals into my drinking water. In general, is it best to just avoid it? Or are there any water purification tablets that are proven safe?

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If I am using a water-pump filter. Is it necessary to combine this with a water purification tablet?

As discussed in this answer and at greater length here, the need to treat backcountry water before drinking it is largely a myth. Neither the filter nor the tablets are needed. You're better off focusing your efforts on avoiding the real reason that backpackers are likely to get sick, which is hand-to-mouth contamination. Practice good potty hygiene, and don't share pots with your hiking partners (whose gut flora your body hasn't got a tolerance for).

If you're traveling in the third world, or drinking water that you collected downstream from a poultry farm in Arkansas, that's a different matter. Filters work against protozoan cysts. Tablets kill bacteria and viruses. Another good option to consider is a steripen, which treats water using UV light. It kills cysts, bacteria, and viruses, is more compact than a filter, and is faster than tablets.

Katadyn, a water filter company, suggests doing so.

Katadyn sells tablets, so they have an economic interest in propagating myths about the need for water treatment.

However, this makes me question the safety of putting such chemicals into my drinking water. In general, is it best to just avoid it? Or are there any water purification tablets that are proven safe?

Iodine is a nutrient that your body needs. That's why they iodize salt. So the question is not really whether iodine is bad for you but whether a certain amount is good or bad for you. The lethal dose in humans is a couple of grams, and it kills you by oxidizing proteins. A typical concentration used for purifying drinking water is about 3 mg/liter, so by drinking a liter of water you're getting about a thousandth of the lethal dose. For comparison, a gram of iodized salt has about 0.02 mg of iodine.

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Unfortunately, third world countries are on the itinerary. So while I'm in those countries, should I always treat, then filter? –  MarkE Aug 16 '13 at 9:14
    
In that case, I think I would to make sure viruses are killed, and because the speed of medical response is probably lower. –  Don Branson Aug 16 '13 at 9:52
    
@MarkE: In your situation, I'd use a steripen. I've added some info about that to the answer. –  Ben Crowell Aug 16 '13 at 13:53
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-1 for "Iodine is a nutrient that your body needs." Sodium is also an element your body needs. Try eating pure sodium sometime and let me know how that works out for you. (Disclaimer: That's a joke, don't actually eat it, you will die.) Also, the dose is crucially important: eat enough of any "essential nutrient" and you will also die. To evaluate the safety of iodine tablets, you have to consider their exact chemical composition and strength. –  Nate Eldredge Aug 23 '13 at 0:33
    
@NateEldredge: Good comment, thanks. I've added a more quantitative discussion of iodine concentrations. –  Ben Crowell Aug 23 '13 at 14:41
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I currently use a Sawyer Filter without using purification. I carry iodine tablets as a backup. Iodine's been in use for decades, and I don't worry at all about using them when necessary. Ray J. recommends being smart about choosing your water sources, which I am. When I was a kid I drank straight from the streams without any purification or any problems, so perhaps I have some immunities from that. People today are way too worried about water purification, if you ask me. People in some countries survive worse water from their taps than we have from our streams. We also worry too much about chemicals, probably. Water is a chemical, and an extremely dangerous one in pure form.

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Oh, the iodine tablets I use have some kind of citrus flavor to cut the flavor of the iodine, and the purified water tastes fine. It looks rusty, and I wouldn't drink it very day, but it's fine as a backup. –  Don Branson Aug 16 '13 at 1:21
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Just as an addition:

For treatment and filter, I'd filter first, then treat. For 2 reasons:

  • After a while, filters get alive and things start growing.
  • Most of the chemical treatments work by oxidation. No use spending the oxidation capacity on stuff that can be oxidized, but could have been filtered out anyways.
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