Water Treatment Methods Compared: Filter, Chemical, UV, Boil

Clear-looking stream water can still carry germs that make people sick. The Centers for Disease Control and Prevention (CDC) groups those germs into three broad types: parasites (protozoa such as Giardia and Cryptosporidium), bacteria and viruses. Every treatment method handles a different mix of the three. Here's how the four common methods compare, based on CDC guidance, with additional detail from the Environmental Protection Agency (EPA).

Quick comparison

Based on CDC's guidance for hiking, camping and traveling:

| Method | Parasites | Bacteria | Viruses | Main limits | |---|---|---|---|---| | Boiling | Yes | Yes | Yes | Needs fuel and time; won't remove chemicals | | Portable filter (1 micron or smaller) | Yes | Not all | No | Pore size and certification matter | | Filter (0.3 micron or smaller) | Yes | Yes | No | Still doesn't remove viruses | | Chlorine bleach or iodine | Not well | Yes | Yes | Weak against parasites; cloudy or cold water needs adjustment | | Chlorine dioxide | Giardia yes; Cryptosporidium somewhat | Yes | Yes | Long contact time; follow product instructions | | UV light | Yes | Yes | Yes | Clear water only; small amounts; needs batteries or power |

Boiling, filtering or disinfecting can't make water safe if it contains harmful chemicals, toxins or radioactive materials, according to CDC. If you suspect that kind of contamination, use bottled water or find a different source.

Boiling

CDC says boiling is the best way to kill germs in water, and that it kills all types, including viruses, bacteria and parasites.

How to do it, per CDC: bring clear water to a rolling boil for 1 minute. At elevations above 6,500 feet, boil for 3 minutes. Let the water cool before drinking so you don't get burned.

If the water is cloudy: CDC's emergency water guidance says to filter it first through a clean cloth, paper towel or coffee filter, or let it settle and draw off the clear water, before boiling.

Note on altitude: EPA's emergency guidance uses a lower threshold, saying to boil for three minutes at altitudes above 5,000 feet. If you're camping between 5,000 and 6,500 feet, the longer boil meets both agencies' guidance.

Limits: Boiling takes fuel and time, which matters on longer trips and in winter, when you may already be melting snow. It also does nothing about chemical contamination, and EPA notes it won't remove heavy metals, salts and most other chemicals.

Filters

CDC says most portable water filters remove parasites but not viruses or bacteria. The details depend on the filter:

• A filter with an absolute pore size of 1 micron or smaller, or one certified to NSF Standard 53 or 58 for cyst reduction or removal, removes parasites, according to CDC, but not viruses or all bacteria.
• A filter with an absolute pore size of 0.3 micron or smaller removes bacteria and parasites, but not viruses.
• Reverse osmosis filters remove parasites, bacteria and viruses, and also remove salt.
• CDC's Yellow Book, its travel-health reference, says portable microfilters don't reliably remove enteric viruses such as norovirus, while ultrafilters with a pore size of 0.01 micron should be effective against viruses, bacteria and parasites.

Check the label. Pore size and certification are the details that tell you which germs a filter is designed to handle. CDC says to follow the instructions on any product you use so that it works properly.

Pair it up. Because most portable filters don't stop viruses, CDC recommends filtering water and then disinfecting it, which covers both parasites and viruses.

Chemical disinfection

CDC says chemical disinfectants kill viruses and bacteria but may not kill parasites. The common options behave differently:

• Chlorine bleach and iodine kill bacteria and viruses but don't work well against parasites, according to CDC. For bleach, use only regular, unscented chlorine bleach, not scented, color-safe or "with added cleaners" products, per EPA. CDC's emergency guidance says to let bleach-treated water stand at least 30 minutes before drinking; EPA's dosing table gives drop counts by bleach strength.
• Iodine comes with health cautions. CDC says people who are pregnant, have thyroid problems or are hypersensitive to iodine should not drink iodine-treated water, and no one should drink it for more than a few weeks at a time.
• Chlorine dioxide tablets can kill Giardia and are somewhat effective against Cryptosporidium, according to CDC's hiking guidance; its emergency water page says they will kill Cryptosporidium when the instructions are followed correctly. CDC's Yellow Book notes that chlorine dioxide tablets and drops need a prolonged contact time of several hours.

Cloudy water: CDC's Yellow Book explains that cloudy water contains substances that neutralize chemical disinfectants, so it needs higher concentrations, longer contact times or clarifying first by settling or filtering.

Cold water: CDC and EPA both say to double the amount of bleach if the water is cloudy, colored or very cold. For tablets and other products, EPA says to follow the label, since each product may have a different strength.

Contact time is the step people cut short. Read the product's stated wait time before you rely on it, and plan for the longest time listed for your conditions.

UV light

CDC says portable UV units can kill parasites, bacteria and viruses in small amounts of clear water. The limits are practical ones:

• Clarity: UV doesn't work well in cloudy water, because small particles can shield germs from the light. CDC says to filter water before using UV.
• Power: CDC's Yellow Book notes that UV devices require batteries or a power source. Carry spare batteries or a backup treatment method.
• Verification: The Yellow Book also notes you can't tell whether a device is actually delivering the required UV dose, which is another reason to keep a second method on hand.

Combining methods

No single portable method is perfect in every situation. CDC recommends filtering before disinfecting, and its Yellow Book describes a microfilter plus a chemical disinfectant as an option for clear surface water near human or animal activity. For cloudy water, the Yellow Book suggests clarifying it first and then following up with a second step, such as heat, ultrafiltration or a chemical disinfectant.

A practical setup for many trips is a filter for everyday use, a chemical disinfectant or UV as a backup or second step, and a stove that lets you boil when conditions call for it.

Before you go

Check with the park or forest you're visiting about water sources and any current advisories. Read the instructions for your filter, tablets or UV unit at home, not at the creek. Pack a backup method in case your primary one fails, freezes or runs out of battery.

Sources

• CDC, About Water Treatment Options When Hiking, Camping, or Traveling: https://www.cdc.gov/drinking-water/prevention/water-treatment-hiking-camping-traveling.html
• CDC Yellow Book, Water Disinfection for Travelers: https://www.cdc.gov/yellow-book/hcp/preparing-international-travelers/water-disinfection-for-travelers.html
• CDC, How to Make Water Safe in an Emergency: https://www.cdc.gov/water-emergency/about/index.html
• EPA, Emergency Disinfection of Drinking Water: https://www.epa.gov/ground-water-and-drinking-water/emergency-disinfection-drinking-water

Last verified: November 1, 2026.