One Decimal Place

Four backcountry water devices. Here are their pore sizes.

• Platypus QuickDraw: 0.2 microns
• LifeStraw Peak Series Straw: 0.2 microns
• MSR Guardian: 0.02 microns
• Grayl UltraPress: not a pore at all, electroadsorption

That single decimal place is the entire difference between the words filter and purifier, and it decides whether the device is adequate where you are standing.

Why viruses go through

Waterborne pathogens come in three size classes. Protozoa like giardia and cryptosporidium are several microns across. Bacteria are a fraction of a micron. Viruses are smaller again by roughly another factor of ten.

A 0.2 micron membrane is a sieve with holes bigger than a virus. Rotavirus, hepatitis A and norovirus pass through it the way sand passes through a tennis net. This is not a quality failure or a manufacturing tolerance. It is what the device is.

Take the pore to 0.02 microns, as the MSR Guardian does, and viruses no longer fit. Or abandon sieving altogether, as the Grayl does, and catch them by electrical charge instead. Either way you have a purifier, and the word is a functional claim rather than a marketing upgrade.

What the decimal place costs

The weights in our comparison were not close.

There is nothing between 68 grams and 354. The virus step is not an increment, it is a different class of machine, and it costs somewhere between 300 and 425 grams. That is a meaningful fraction of a light pack, and it is the reason most backpackers carry a filter and accept exactly what it does not do.

Geography, not remoteness

The instinct is that wilder water is safer water, and for viruses that instinct is roughly correct but for the wrong reason. Viral contamination of water comes overwhelmingly from human sewage, so the risk tracks human density and sanitation rather than how impressive the valley looks.

In most temperate backcountry, a 0.2 micron filter covers the pathogens that actually make hikers ill, which are protozoa and bacteria. Travel somewhere with poor sanitation, or drink downstream of a town anywhere, and the same filter is no longer sufficient while feeling exactly as reassuring in the hand.

That is the uncomfortable part. A filter gives you clear, cold, good-tasting water either way. It cannot tell you which situation you are in, and neither can the water.

The category name does the damage

Search for a backpacking water filter and you get both types on one results page, described in overlapping language, with pore size buried in a specification panel that several brands collapse by default. We have written before about one category name covering unrelated products. This is the highest-stakes example we have found, because the failure mode is not disappointment, it is illness.

Credit where it is due: every brand in our comparison publishes what its device does. Platypus gives the pore size, the media and the cartridge life. LifeStraw publishes removal percentages by pathogen class and names the standards. MSR gives 0.02 microns and cites NSF P248. Grayl lists the actual viruses by name. Nobody is hiding anything.

They are just not answering the question in the same units, on the same shelf, under the same word.

The check

Before buying, find the pore size. If it is around 0.2 microns, you have a filter, whatever the box says, and viruses are not covered. If there is no pore size, look for an explicit virus claim with a percentage attached, and treat its absence as a no rather than an oversight.

Then answer the geography question honestly, because it is the one that decides which of these two products you are actually shopping for. In the comparison we recommended a 68 gram filter for most people and a 354 gram purifier for travel, and the gap between those two numbers is the whole story.

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