Skip to content
Tom's HikingTom’s Hiking

Stoves and water

Micron rating

Also called pore size, absolute micron rating

What is micron rating?

A micron rating states the largest particle a filter lets through, in microns. Backpacking filters are typically rated 0.1 or 0.2 microns, which stops bacteria and protozoa. Viruses are around 0.02 microns and pass straight through.

Micron rating at a glance

Protozoa
About 1 to 15 microns
Bacteria
About 0.2 to 5 microns
Viruses
About 0.02 microns

The scale that matters

A micron is a thousandth of a millimetre. The scale that matters: protozoan cysts like giardia and cryptosporidium run from about 1 to 15 microns, bacteria like E. coli and salmonella from about 0.2 to 5, and viruses down around 0.02. A 0.2 micron filter catches the first two categories comfortably and misses the third entirely.

Why viruses are the exception

That's why almost every backpacking filter sold in North America is a filter rather than a purifier. Waterborne viruses in wild backcountry water are rare in that setting, and the pathogens that actually make hikers ill are the protozoa and bacteria a 0.1 or 0.2 micron filter removes. Travel to somewhere with human sewage in the water supply and the calculation changes.

Absolute against nominal ratings

There's also a wording trap. An absolute rating means nothing larger than that size gets through; a nominal rating means most of it doesn't. Reputable backpacking filters quote absolute ratings, and a filter advertising a nominal figure is making a much weaker claim than it appears to.

What this means when you're choosing

  • An absolute rather than a nominal rating, which are not the same claim
  • 0.1 or 0.2 microns for bacteria and protozoa
  • A purifier instead if viruses are a real risk where you are going

The common mistake

Assuming a fine filter handles viruses

Viruses are around 0.02 microns, an order of magnitude below what these filters catch. In remote North American backcountry that is usually fine. Anywhere human sewage reaches the water, it is not, and you need chemistry or a purifier.

Common questions

Is 0.1 micron better than 0.2 micron?

Marginally, and not in a way that changes what you're protected against. Both stop bacteria and protozoa and neither stops viruses, so pick on flow rate, form factor and durability instead.

What does absolute vs nominal mean on a filter?

Absolute means nothing bigger than the rating passes. Nominal means most of it doesn't, which is a much weaker guarantee. For drinking water, only absolute ratings are meaningful.

Where this is defined

Where this comes up

Guides on Tom's Hiking where micron rating changes which product is right for you.

More in stoves and water

Canister stove

A canister stove screws directly onto a sealed cartridge of pressurised isobutane-propane. It lights instantly, simmers well and needs no priming, which is why it's the default backpacking stove almost everywhere fuel canisters are sold.

Integrated stove system

An integrated stove system pairs a burner with a matched pot that locks on, usually with a heat exchanger ring on the base. It boils water faster and uses less fuel than a separate stove and pot, at the cost of versatility.

Regulated valve

A regulated valve holds a stove's fuel pressure steady as the canister empties or cools. Without one, output drops off through the canister's life and falls sharply in the cold; with one, the flame stays consistent much further down.

Isobutane-propane blend

Isobutane-propane is the fuel blend inside standard threaded backpacking canisters, usually around 80 percent isobutane and 20 percent propane. Propane provides cold-weather pressure, isobutane provides most of the burn time.

Hollow fiber filter

A hollow fiber filter passes water through a bundle of tiny straw-like tubes whose walls are perforated with pores around 0.1 to 0.2 microns. It's the technology in almost every modern backpacking filter, and a single freeze can destroy it invisibly.

Flow rate

Flow rate is how fast a filter delivers clean water, usually quoted in liters per minute. The advertised figure is measured on a brand-new filter with clean water, and real flow drops as the element clogs with sediment.

Backflushing

Backflushing pushes clean water backwards through a filter to dislodge the sediment clogging its pores. It's routine maintenance on a hollow fiber filter, and it's the difference between one that lasts a season and one that lasts years.

Purifier vs filter

A filter physically strains out bacteria and protozoa. A purifier also deals with viruses, which are too small to strain, using chemistry, UV light or a much finer element. Most backcountry hiking needs a filter; international travel often needs a purifier.

Soft flask

A soft flask is a collapsible bottle, usually 250 to 750 ml, carried in the front pockets of a hydration vest. It shrinks as you drink so it doesn't slosh, and you can see exactly how much water you have left.

Hydration bladder

A hydration bladder is a flat water reservoir carried inside a pack with a drinking hose routed over the shoulder. It holds 1.5 to 3 liters, packs flat against your back and lets you drink without stopping or taking the pack off.

Bite valve

A bite valve is the silicone mouthpiece on the end of a hydration reservoir's drink tube. It stays shut until your teeth compress a slit in it, then self-seals when you let go, so water only flows while you are drinking.

Quick disconnect

A quick disconnect is the two-part coupling in a hydration hose that lets you unclip the tube from the reservoir without threading it back through your pack. Both halves usually self-seal, so you can pull the bag out to refill and neither end drips.

Hard-anodized aluminum

Hard-anodized aluminum is aluminum whose surface has been grown into a thick, very hard oxide layer by an electrochemical bath. In cookware it gives a pot that spreads heat far more evenly than titanium and resists scratching far better than bare aluminum.

Last reviewed . Back to the glossary