Disinfection with iodine

Like chlorine, iodine belongs to the halogens, a group of highly reactive elements. It attacks the proteins and enzymes of microorganisms – the building blocks and tools they need to live. This inactivates bacteria and viruses, which means they can no longer cause disease.

How well this works depends on four things: concentration, contact time, temperature and the cloudiness of the water. Cold water slows the reaction, which is why manufacturers give longer waiting times for cold water. Suspended particles use up part of the iodine and can shield pathogens.

Protozoa such as Giardia have a protective shell. With enough contact time, iodine partly reaches them. The particularly resistant oocysts of Cryptosporidium – a kind of resting form with a very tough shell – are barely affected at usual doses. That is why you need an additional method where this risk exists.

Use only a product explicitly intended for drinking water and supplied with instructions. Dose exactly for the amount of water you are actually treating – never guess. If your bottle has no scale, measure once at home how much it holds.

Let cloudy water settle first and pour it through a cloth. After dosing, close the bottle, shake it and let some water run over the threads and lid rim. Otherwise drops of untreated raw water sit there and end up in your mouth when you drink.

Wait for the full contact time stated by the manufacturer – for cold water, the extended time. Only then add neutraliser tablets, vitamin C or drink powders to improve the taste. If they go into the water too early, they interfere with the disinfection.

Before using iodine in a group, ask: is anyone pregnant or breastfeeding? Does anyone have thyroid disease or an iodine allergy? Then choose a different method for that person, such as boiling or chlorine dioxide, or they talk to a doctor first. Also read the manufacturer’s notes on contraindications and the maximum duration of use.

Iodine is meant for short trips and as a light backup, not for daily use over weeks or months. The thyroid gland uses iodine, and a permanently high intake can put a strain on it.

If Cryptosporidium is a concern – for example with water below grazing livestock – boil the water or use a suitable filter first. Iodine does nothing against chemicals and salt.

Iodine is light, small and needs no fuel. That makes it a practical backup for short trips or travel, in case the filter fails or no stove is available. Combined with a filter that retains protozoa, it also covers the viruses the filter lets through.

It does not make sense as a permanent solution, for people with contraindications, or as the only method where there is a Cryptosporidium risk. Also note that iodine products for water disinfection are not available or approved in every country – check local rules and availability before you travel.

Iodine was long a standard agent for travellers and armed forces, because it is light and still works in colder water. Travel-medicine guidance still describes it today, but points out contraindications and a limited duration of use.

In many kits it has since been supplemented or replaced by chlorine dioxide and filters. The yellowish colour of the water, by the way, is not a sign of dirt but comes from the iodine itself. It disappears when a neutraliser or vitamin C is added after the contact time.