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What is NSF? - About NSF ratings

All serious manufacturers of water filtration equipment rely on independent certification by NSF (  The reason: Since users can’t see, taste or smell whether water-borne contaminants are actually removed, companies could make any claims they want about their filters.  So the top end of the industry has come to rely on the stringent regulations established by NSF.  (It’s the industry equivalent to UL for electrical products.)  If a manufacturer claims that their systems are tested “according to” NSF (or similar language) that’s a dodge.  What counts is whether the claims made are actually certified by NSF.

At the same time, it’s also important to understand how NSF works:

1. They only test for what a manufacturer asks them to.  So if you look at the ratings of filters posted on their site, some filters will appear to take out more contaminants than others.  But that’s not necessarily the case.  Say there is something called “globolob2”.  It might never be present in water, harmless, and something even the crudest filter would take out.  A company that makes a basic filter notices that no one ever asked NSF to test for globolob2 reduction.  So they commission NSF, and the result? “We make the only filter NSF-certified to reduce globolob2!”  Big deal.

2.  When NSF rates capacity – how many gallons the filter is “good” for – it means that after the rated number of gallons the filter is still able to remove 200% (twice as much) of all the claimed contaminants.  That doesn’t mean that the filter is only good for that amount of water.  It means that according to NSF testing protocol, after the stated number of gallons passes though the filter, it begins to lose its capacity to take out twice as much as claimed of that one contaminant that presents the greatest challenge.

3.  Just because a contaminant isn’t NSF-certified for reduction does not mean that the contaminant is not taken out.  Manufacturers know, for example, that the same filtration media used to reduce VOCs works for THMs.  VOC reduction is actually harder to achieve, so a company may elect to pay NSF only for the tougher VOC certification on the valid assumption that the product is just as effective (if not more so) against THMs.

The public generally doesn’t know this, of course.  So in New York City, for instance, where the water supply almost always has THMs but never VOCs, a filter listed for NSF-certified VOC (but not THM) reduction really means that you are getting both.

4.  Why do we use the word “reduction” instead of “removal”?  Because NSF doesn’t like “remove” – they use “reduce”.  We want to comply with their terms.  And technically, they are right.  Taking contaminants out of water is always about probabilities, percentages.  Nothing can be assured 100%.  Whether it says “removes 99.99%” or “reduces by 99.99%” doesn’t matter.  Both mean the same thing.

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