A filter is a parasite and bacteria tool. CDC says most portable water filters remove parasites, and those with an absolute pore size of 0.3 micron or smaller also remove bacteria, but they do not remove viruses. A device that also handles viruses is a purifier. No filter on this page makes chemically contaminated water safe.
For a household emergency, a filter solves a specific problem: reducing the parasites and bacteria in clear-but-untrusted water from a stream, a rain barrel or a suspect tap to the levels each manufacturer publishes for its device. It pairs with, and does not replace, the boiling and disinfection methods that handle viruses. Filter first, then disinfect: CDC's own sequence, because bleach and iodine work poorly against parasites and cloudy water defeats them.
How to read a filter's specifications
- Absolute versus nominal pore size. "Absolute 1 micron" means every pore is 1 micron or smaller. A "nominal" rating means the average pore is that size and larger holes exist. CDC's parasite guidance is absolute 1 micron or smaller; its bacteria guidance is absolute 0.3 micron or smaller.
- NSF/ANSI 53 or 58 with cyst reduction. A certification to these standards that specifically names cyst reduction or removal covers Cryptosporidium and Giardia. A standard number alone, without the cyst claim, does not.
- NSF P231 or the EPA purifier guide standard. These protocols test microbiological performance against bacteria, protozoa and, for purifiers, viruses. A manufacturer that names the protocol and the organisms is giving you something checkable.
- Viruses and chemicals. CDC lists only nanofiltration and reverse osmosis among home methods that remove viruses by filtration; portable purifiers reach viruses with electroadsorption, chemical or UV stages instead. Activated carbon can reduce some chemicals, tastes and odors, but no portable device on this page is a solution for fuel, pesticides or metals in an emergency.
Compare four types, not four brands
| Option | Type | Rated pore size | Bacteria | Parasites | Viruses | Stated lifespan | Weight |
|---|---|---|---|---|---|---|---|
| Sawyer Squeeze | Squeeze or gravity, hollow fiber | 0.1 micron absolute | 99.99999% | 99.9999% | No claim | Not published | 3 oz filter, 8.19 oz kit |
| LifeStraw Personal | Straw, membrane microfilter | 0.2 micron | 99.999999% | 99.999% | No claim | Up to 1,000 gal (4,000 L) | 1.6 oz |
| Platypus GravityWorks 4.0L | Gravity bag system, hollow fiber | 0.2 micron | 99.9999% | 99.9% | No | 1,500 L; 1.75 L per minute | 11.5 oz |
| GRAYL GeoPress | Press purifier bottle, electroadsorption and carbon | Not pore-rated | 99.9999% | 99.9% | 99.99% | 350 presses (about 65 gal) or 3 years | 15.9 oz |
The percentages are the manufacturers' figures, not our test results, and they describe laboratory challenge water. Read them as "designed and tested for this organism class" rather than a guarantee for any given source. The most important column is Viruses: three of the four leave that job to boiling or disinfection.
A straw serves one person at a time. A gravity system can fill the household's containers while nobody is holding it. Choose by daily volume before choosing by brand.
Calculate your household's daily water Photo: charlesdeluvio / UnsplashWhich type fits your household
- Everyone has a straw in their bag; the house has something bigger. A straw filter cannot fill a pot. It is an evacuation-bag item, not the household's water treatment plan.
- A gravity system for volume. If you need to treat several gallons a day for cooking, hygiene and drinking, a hanging gravity system is the least work. The Sawyer Squeeze can also run as a gravity setup with its included tubing.
- A purifier where viruses are plausible. Water shared by many people, or travel outside areas with reliable sanitation, are virus scenarios. A purifier bottle or filter-then-disinfect is designed for them; a plain filter is not.
- Stored water first. A stored reserve in clean containers is easier and safer than treating water from an unknown source. Filters are for when the reserve runs out or when you had to leave it behind.
Whatever you choose, run it once at home. Learn how to backflush or replace the cartridge, how to keep the dirty and clean sides apart, and whether it can be stored after freezing; hollow-fiber filters can be damaged by ice, and manufacturers' instructions cover that.
Next: review the disinfection doses and boil times you will pair with a filter, then size your stored water reserve so the filter is the backup, not the plan.
Sources and selection method
We compared published filtration ratings, certification claims, lifespan and household use cases. We have not laboratory-tested these products. Retail listings and bundles change; confirm the exact model and included parts before ordering.