NSF Standard 61 is a certification for material safety. It means that the physical media used in a water filter system has been tested to be free of hazardous materials such as lead.
Some water filter systems will say "NSF Certified for Material Requirements Only." This means NSF 61.
Some key points about NSF 61:
1. It certifies safety of materials only, not tested system performance
2. All water filter systems sold, distributed, or manufactured in North America are required to comply with NSF 61
3. All NSF Certified systems that are actually performance tested and Certified for the other standards listed below, must be made with 100% NSF 61 Certified materials.
NSF Standard 42 primarily concerns what are considered aesthetic-concern contaminants. These include chlorine, chloramines, particulates, and taste/odor.
NSF Standard 53 is testing for contaminants of health concern. These are known health-hazards like mercury, lead, asbestos, cysts, giardia, chlordane (pesticide), MTBE (gasoline additive), PCB's (industrial electric products component), toxaphene (toxic insecticide that is banned), radon (radioactive odorless gas), and a long list of volatile organic compounds (VOC's). Trihalomethanes (THM's) and Haloacetic Acids (HAA's) are both part of that list of VOC's.
NSF Standard 401 is a newer protocol that was created in response to consumer demand for independent testing on what are known as emerging contaminants. These are contaminants like pesticides, insecticides, and pharmaceuticals that have leached into water supplies in trace amounts in recent years.
The contaminants tested in this category are tested in trace quantities, standards set by NSF to reflect real-world conditions for these contaminants. Some of the compounds on this list are quite familiar to most, such as Ibuprofen and DEET (bug spray, anyone?).
It's important to note that a system may be certified under one of these standards, but not for all of the contaminants.
There are many products out there that can correctly state that they are Certified under NSF 42 and 53, but not for very many contaminants.
Recently I saw a product that stated this, and when I looked up their NSF listing, under Standard 42, they are certified for chlorine reduction only, and under Standard 53, for cysts only.
So while that's still better than many filter systems on the market, just seeing certification for NSF 42 or 53 doesn't tell the whole story. You have to look at which specific contaminants are listed under the certification.
You can always see that on NSF's water filter listings.
The WQA and IAPMO are two other certification bodies that test to true NSF protocols.
See the videos below for how to search for water filters on all 3 of these certification directories.