Which water filters work in an emergency, and which don't?

Short answer. A filter with an absolute pore size of 1 micron or smaller removes parasites such as Giardia and Cryptosporidium; one of 0.3 micron or smaller also removes bacteria; but most portable filters do not remove viruses ¹. The CDC says to add a chemical disinfectant after filtering ¹. No filter makes water with fuel or toxic chemicals safe: use bottled water instead ¹.

What does a filter's pore size tell you?

Germs come in three size classes. The CDC puts it simply: "Parasites and ameba are larger than bacteria, which are larger than viruses" ². A filter only stops what is bigger than its holes, which is why the word "absolute" matters on the label: an absolute 1-micron filter has every pore at 1 micron or smaller, while a "nominal" 1-micron filter only averages that size ².

The gap that matters most in an emergency is viruses: the CDC says portable filters "will not remove viruses" ¹.

What do the NSF/ANSI numbers on a filter mean?

NSF notes that no federal regulations exist for residential filters, purifiers and reverse osmosis systems in the United States; voluntary standards set the minimum requirements instead ⁴. The numbers reflect the order in which standards were written, not a ranking ⁴.

• NSF/ANSI 42: reduces aesthetic impurities such as chlorine, taste and odour ⁴. It is not a health claim.
• NSF/ANSI 53: reduces a contaminant with a health effect, as set by the EPA and Health Canada ⁴. For germs, the relevant claim is "cyst reduction" ².
• NSF/ANSI 55: ultraviolet systems. Class A units disinfect contaminated water; Class B units only reduce non-disease-causing bacteria in water that is already disinfected ⁴⁵.
• NSF/ANSI 58: reverse osmosis ⁴.
• NSF P231: microbiological water purifiers, tested against the EPA's 1987 guide standard for purifiers ⁴.

Health Canada explains that a cyst-reduction certification under standards 53, 58 or 62 requires removal of 3 logs or better, which is 99.9% ⁶. It also warns that devices certified to standard 53 for cysts "will not remove viruses or bacteria from water" ⁵. Read the full claim on the box, not just the number: NSF says certification does not mean a product reduces all possible contaminants ⁴.

Does a charcoal pitcher filter make emergency water safe?

No. The CDC says most pitcher and fridge filters "are not designed to remove germs from your water," and that NSF 42 and activated carbon improve taste and smell, not safety ². They are made for water that is already treated. In an outage or after a flood, treat the water first; a carbon filter can then take out the chlorine taste.

What about hiking filters, UV and reverse osmosis?

Hiking and camping filters are usually pore-size filters, so the table above applies: they handle parasites, and bacteria if rated 0.3 micron or smaller, but not viruses ¹. A 2025 systematic review of published studies found that microfiltration had the lowest virus performance of the membrane types studied, a mean of 0.85 log (about 86%) ⁸.

Ultraviolet units can inactivate bacteria, viruses and cysts when certified as Class A ⁴⁶, but Health Canada says such units should be installed in visually clear water and are "not designed to treat wastewater or water contaminated with raw sewage" ⁶. The CDC adds that portable UV units work on small amounts of clear water and not well in cloudy water ¹, so settle and pre-filter first ¹³.

Reverse osmosis removes parasites, bacteria and viruses ² and reduces some chemicals such as lead, copper and sodium ².

What does no filter remove?

The CDC is direct: "You cannot make water containing fuel, toxic chemicals, or radioactive materials safe by boiling or disinfecting it" ¹. The EPA says boiling or disinfection will not destroy heavy metals, salts and most other chemicals ³. For water that may contain fuel or toxic chemicals, the CDC's advice is to use bottled water or another source and contact your health department ¹.

The CDC also notes that "filters that remove germs often do not remove chemicals, and vice versa" ².

How do you combine a filter with disinfection?

Filtering and disinfecting do different jobs, so use both:

1. If the water is cloudy, let it settle and pour it through a clean cloth, paper towel or coffee filter ¹³.
2. Run it through your filter to remove parasites: the CDC notes that chemical disinfectants "may not kill parasites" ¹.
3. Add a chemical disinfectant to kill the viruses and bacteria left over ¹.

Doses, contact times and boiling rules are in the Phaenyx guide on making water safe to drink. If your utility has told you to boil water, the boil-water notice guide covers what that means for your household.

What official agencies recommend

• United States (CDC, EPA): A filter alone is not enough. Use absolute pore size 1 micron for parasites, 0.3 micron for bacteria, then disinfect for viruses ¹. Never rely on boiling, disinfection or a filter for water with fuel or toxic chemicals ¹³.
• United States (NSF): Choose a product certified to the standard and contaminant claim you need; no single certification covers everything ⁴.
• Canada (Health Canada): Buy a device certified to the applicable NSF/ANSI standard ⁵. Cyst claims require at least 3-log removal ⁶. Standard 53 cyst filters do not remove viruses or bacteria ⁵.
• World Health Organization: WHO tests household treatment products and rates them 3-star, 2-star or 1-star according to how well they remove bacteria, viruses and protozoa ⁷.

Where they differ. The CDC lists nanofiltration among the filters that remove viruses ². The 2025 review found a mean virus reduction of 3.1 log for nanofiltration and reverse osmosis combined, below the top-tier target of at least 4 log it cites ⁸. Treat any filter's virus claim with caution and disinfect anyway.

Frequently asked questions

Is a 0.2-micron filter enough after a flood?

It is small enough for bacteria and parasites under the CDC's 0.3-micron rule ¹, but not for viruses, so disinfect afterwards ¹. If the floodwater may contain fuel or chemicals, do not drink it at all ¹.

Do I still need to disinfect if my filter says "purifier"?

Check what it is certified for. NSF P231 covers microbiological purifiers ⁴, but agency advice for ordinary portable filters is to disinfect after filtering ¹. When in doubt, do both.

Can a filter remove lead or other chemicals from my tap water?

Some can, but that is a different claim from germs. The CDC says reverse osmosis reduces lead and copper, and to check the label and NSF certification for the specific chemicals a filter removes ². NSF/ANSI 401 covers some emerging contaminants not yet regulated ⁴. These are everyday-water questions, not emergency ones.

How often should I change the cartridge?

Follow the manufacturer's directions ²⁵. The CDC advises wearing gloves and washing your hands after a change, and says people with weakened immune systems should avoid changing filters themselves and talk to a healthcare provider about using one ².

Know your local water risks

See which water risks agencies flag where you live, country by country.

Your Phaenyx plan's "Water and air" section lists the water and air risks for your postal code, so you know whether floods, outages or boil-water advisories are the likely problem. The Phaenyx Safe app keeps the response sheets offline, including the filter-then-disinfect steps. Get your Phaenyx plan.

Sources

1. Centers for Disease Control and Prevention — How to Make Water Safe in an Emergency — https://www.cdc.gov/water-emergency/about/index.html (accessed 2026-10-27)
2. Centers for Disease Control and Prevention — About Choosing Home Water Filters — https://www.cdc.gov/drinking-water/prevention/about-choosing-home-water-filters.html (accessed 2026-10-27)
3. U.S. Environmental Protection Agency — Emergency Disinfection of Drinking Water — https://www.epa.gov/ground-water-and-drinking-water/emergency-disinfection-drinking-water (accessed 2026-10-27)
4. NSF — NSF Standards for Water Treatment Systems — https://www.nsf.org/consumer-resources/articles/standards-water-treatment-systems (accessed 2026-10-27)
5. Health Canada — Questions And Answers On Drinking Water Treatment Devices — https://www.canada.ca/en/health-canada/services/environmental-workplace-health/water-quality/questions-answers-drinking-water-treatment-devices.html (accessed 2026-10-27)
6. Health Canada — Guidelines for Canadian Drinking Water Quality: Guideline Technical Document - Enteric Protozoa: Giardia and Cryptosporidium (page 6) — https://www.canada.ca/en/health-canada/services/publications/healthy-living/guidelines-canadian-drinking-water-quality-guideline-technical-document-enteric-protozoa-giardia-cryptosporidium/page-6-guidelines-canadian-drinking-water-quality-guideline-technical-document-enteric-protozoa-giardia-cryptosporidium.html (accessed 2026-10-27)
7. World Health Organization — International Scheme to Evaluate Household Water Treatment Technologies — https://www.who.int/tools/international-scheme-to-evaluate-household-water-treatment-technologies (accessed 2026-10-27)
8. Rao G., Wells E., Reynolds C., Yoo R., Kowalsky E., DeFrance J., Linden K., Brown J. — Systematic Review of the Microbiological Performance of Household Water Treatment Technologies, Environmental Science & Technology 59(41), 2025 — https://pmc.ncbi.nlm.nih.gov/articles/PMC12550818/ (accessed 2026-10-27)