Chemical water treatment: what each disinfectant kills, and what it misses

Chemical disinfectants kill viruses and bacteria, but CDC says they may not kill parasites. The three disinfectants CDC names are unscented household chlorine bleach, iodine and chlorine dioxide tablets. Each has a different reach against parasites, so the choice depends on what the water may carry and on whether a filter is used first. Disinfection does not make water with chemical contamination usable.

What each disinfectant kills and misses

CDC's travel page says chlorine bleach and iodine kill bacteria and viruses, but do not work well against parasites. It says chlorine dioxide tablets can kill Giardia and are somewhat effective against Cryptosporidium. CDC's emergency page says iodine and chlorine tablets kill most germs but will not kill Cryptosporidium. Those two parasites are where the three disinfectants differ.

CDC also describes salt-based on-site generators, which make a mix of chemicals, primarily chlorine. Chlorine from that source kills viruses and bacteria and does not work well against parasites, so it sits under the same limits as bleach. EPA lists calcium hypochlorite, tincture of iodine and disinfection tablets as other options. The doses below are EPA's figures for household bleach only; follow the label for any other product.

Household bleach: the EPA figures

EPA says to use only regular, unscented chlorine bleach that is suitable for disinfection. The label may give 6% or 8.25% sodium hypochlorite. Do not use scented bleach, colour-safe bleach or bleach with added cleaners, and use bleach that is fresh and has been stored at room temperature for less than a year. The table gives the amount to add to clear water.

The gallon conversion is 1 US gallon = 3.785 litres, so 2 gallons is 7.57 litres, rounded to 7.6 litres in the table. The figures are EPA's for emergency use, and a product label that differs should be followed.

clear water     2 US gal: 16 drops of 6%  or  12 drops of 8.25%
cloudy water    double:   32 drops of 6%  or  24 drops of 8.25%
stand           30 min, then check for a slight chlorine odour
no odour        repeat the dose, stand another 15 min

Iodine: who should avoid it

CDC's advice on iodine is specific. If you are pregnant, have thyroid problems, or have a hypersensitivity to iodine, avoid drinking water disinfected with iodine. No one should drink water disinfected with iodine for more than a few weeks at a time. CDC's emergency page names iodine tablets alongside iodine, and says neither will kill Cryptosporidium.

The limit is on use at a time, so a trip longer than a few weeks should plan another method for part of it. Anyone in the excluded groups should avoid drinking iodine-disinfected water, however short the trip.

Chlorine dioxide and Cryptosporidium

Chlorine dioxide tablets can kill Giardia, which CDC states plainly. For Cryptosporidium, the two CDC pages differ in emphasis. The travel page says the tablets are somewhat effective. The emergency page says they will kill the parasite as long as the instructions are followed correctly. The cautious reading, the travel page's, is the safer one to plan with.

Chlorine dioxide tablets are made for specific amounts of water. Follow the instructions on the package rather than adjusting the amount. The travel page also says it is still safest to filter the water first to remove these parasites. See the filter guide for what a filter removes.

Taste, odour and the wait

Both CDC and EPA say to let disinfected water stand before use. For bleach, EPA says 30 minutes. CDC's emergency page says at least 30 minutes before drinking. If the chlorine taste is too strong, EPA suggests pouring the water between clean containers and letting it stand for a few hours before use.

Filter first, then disinfect

CDC says to filter water before disinfecting it. Its emergency guidance says to add the disinfectant to the filtered water, and says that the disinfectant will kill any viruses and bacteria left in it. The filter's job is the parasites, which chlorine and iodine handle poorly. The disinfectant's job is the viruses and bacteria that most portable filters do not remove.

EPA adds that disinfection does not work as well when the water is cloudy or coloured. Settling or filtering through a clean cloth, paper towel or coffee filter is therefore a step before the disinfectant. The cloudy water guide covers that step in detail.

What disinfection leaves out

EPA says boiling and disinfection will not destroy heavy metals, salts or most other chemicals. CDC says water containing harmful chemicals, toxins or radioactive material cannot be made usable by boiling or disinfecting, and it recommends bottled water or a different source. CDC's emergency page says the same for water suspected of containing fuel or toxic chemicals, and advises contacting the health department for specific advice.

The water decisions tool returns "choose a different water source" whenever a chemical hazard is unknown or suspected. Treat that result as the decision, and do not treat a disinfectant as a fix for it. For the other methods, see the boil guide and the UV guide. For the wider packing list, see the ten essentials guide.