Water filters and purifiers: what a pore size removes, and what it misses

A portable filter removes parasites, but CDC says most portable filters do not remove viruses or bacteria. The label decides what a given filter can do, so match its stated pore size and any certification against the thresholds CDC uses. The word purifier is a separate question: CDC's guidance does not use it as a category.

What CDC says a filter removes

CDC's starting point is that most portable water filters remove parasites but not viruses or bacteria. Two labels change that picture for parasites. The first is an absolute pore size of 1 micron or smaller. The second is certification to NSF standard 53 or 58 for cyst reduction or removal. CDC ties the 1-micron threshold to parasites such as Giardia and Cryptosporidium. A filter that meets either label removes parasites, but not viruses and not every bacterium.

Bacteria need a tighter threshold. CDC names an absolute pore size of 0.3 micron or smaller as the level that removes bacteria and parasites. Viruses are still not removed at that size. A filter rated only at 1 micron may therefore let some bacteria through, even though it removes the parasites.

Pore size and certification on the label

The table sets the methods side by side. The filter rows follow CDC's thresholds exactly, and the boiling and disinfecting rows follow CDC and EPA guidance. Reverse osmosis is the only filter type that CDC describes as removing viruses.

The chlorine dioxide row carries a caveat, covered in the chemical treatment guide. CDC's two pages describe its effect on Cryptosporidium in different terms.

Absolute pore size and why the term matters

All of CDC's filter thresholds are written as absolute pore size. A rating that is not described as absolute does not match any of those thresholds, so it cannot be used to decide whether a filter removes parasites or bacteria. Check the manufacturer's specification for the word absolute before you rely on a number. The site's filter versus boiling comparison makes the same point: a pore-size label is not a treatment plan on its own.

Label A: 0.3 micron absolute
  bacteria    removed (CDC: 0.3 micron or smaller)
  parasites   removed
  viruses     not removed, so disinfect after filtering

Label B: 1 micron absolute, no NSF 53 or 58
  parasites   removed (CDC: 1 micron or smaller)
  bacteria    not all removed
  viruses     not removed

Purifier claims and what to check

CDC's guidance does not define the word purifier, so a product with that name carries no CDC meaning. Claims that a product is a purifier usually refer to testing against viruses as well as bacteria and protozoa. Protozoa is the group that includes Giardia and Cryptosporidium.

Check which organisms the product was tested against. Compare that list with the three groups above, and look for the word absolute next to any pore size. A test against bacteria and protozoa alone would not show that the product removes viruses, and a pore-size figure does not show what the product was tested against. Where a product's claims stop at a pore size, CDC's emergency guidance still calls for a disinfectant after filtering, because filters do not remove viruses.

Filter first, then disinfect

CDC describes filtering followed by disinfecting the filtered water as the next best option after boiling. The reason is in the table. Chlorine bleach and iodine do not work well against parasites, so the filter handles those. The disinfectant then covers the viruses and bacteria that a filter of this type leaves behind. CDC's emergency guidance lists the same order: filter, then add a chemical disinfectant to the filtered water.

CDC says to filter before disinfecting, and it says the same before using UV light. Chlorine and iodine are the disinfectants that most need the filter's help with parasites. EPA also says disinfection does not work as well when water is cloudy or coloured, so a filter that clears the water helps whichever disinfectant follows.

What filters leave in the water

CDC's filter criteria are about germs. They say nothing about salt, dissolved chemicals or other contaminants, so a filter that meets them should not be relied on for chemical contamination. Reverse osmosis is the exception CDC names, because it removes salt. EPA says boiling and disinfection do not destroy heavy metals, salts or most other chemicals, and a filter that is not reverse osmosis cannot be assumed to remove them either.

For water that may contain harmful chemicals, toxins or radioactive material, CDC says boiling and disinfecting cannot make the water usable, and its filter criteria do not cover chemicals or toxins either. Use bottled water or a different source instead.

Care, freezing and storage

Follow the manufacturer's instructions on cleaning, replacement, freezing and storage. Neither CDC nor EPA gives a general freezing rule for filters; the manufacturer's instructions are the reference. For the pack itself, a filter counts toward the water category in the base weight ledger, which lists bottles, filter and treatment items when they are empty.

The UV guide covers the other portable option, and the boil guide covers heat. Neither removes chemical contamination, so a source with a chemical concern needs a different source, whichever method is carried.