Kills or removes 99.9999% of all known microorganisms. Which is another way of saying that it lets .0001% through. Which is one virus in every ten thousand. That's ten for every million. With some water born diseases that is more than sufficient to ensure infection. It is a pocket FILTER. Not a PURIFIER. It removes hard shelled microorganisms, bacteria etc from your drinking water. It DOES NOT remove viruses.
Why?
Because they are too small.
The Pen Filter - and they are all the same basic design - has a microporous filter whose holes are between 0.2 and 0.12 micrometers across(1 micrometer is 1/1000 of a mm). Which means that they are small enough to stop everything bigger than 0.12 micrometers across. Viruses are 0.1 micrometers across. That means that viruses pass straight through. Some of the nastier illnesses are caused by viruses.
However.
Viruses are easily killed by chemical purification. Some of the bigger wee nastys are not. Unless you happen to have access to MP-1 tablets.
So:
If you want to use one of these you should treat the water chemically first with chlorine tablets following the instructions on the packaging. You would be well advised to purchase the neutralising tablets at the same time. Drinking water that tastes like a swimming pool gets real old, real fast. Make sure that you use them in the correct order. Its good practice to store them in different pockets so as not to confuse them.
The pocket filter is a good idea. With in it's limitations. Which are the same as those for any travel filter. Correctly used they are a useful addition to any survival/travel kit.
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I don't do dumb & helpless.