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Chlorine and Iodine (Halogens)

Chlorine and Iodine (Halogens)

Iodine and chlorine, which work by oxidizing the cellular structures of microorganisms, are the most widely used chemical treatments.

Often referred to as halogens, they have been used for decades against bacteria and viruses however recently their inability to kill Cryptosporidium has been a topic of great concern. Halogens do not affect parastic worms eggs and are thus ineffective for treatment of water infected with parastic worm eggs.

Iodine and chlorine products do not neutralize or remove chemicals, elements or heavy metals from water or make seawater drinkable. They also leave residual chemicals in drinking water after treatment is complete.

Factors Affecting the Effectiveness of Chemical Treatments

Many environmental factors affect the effectiveness of chemical treatments in neutralizing pathogenic microbes.

Concentration and Exposure

Most important with iodine and chlorine are concentration, measured in parts per million (ppm) or milligrams per litre (mg/l) of water, and the exposure time of organisms to the halogen, usually measured in minutes.

Concentration and exposure have a direct relationship meaning that a greater halogen concentration reduces the time required to kill organisms, or alternatively, a longer time period allows for a reduction in the halogen concentration.

Water Temperature and pH

The colder the water, the higher the concentration needed, or the longer the contact time required.

The general rule is that for every 10°C increase in water temperature the ability of halogens to disinfect increases at a rate of two to three times. Thus, warm water is significantly better than cold water. The optimum pH for halogen use is 6.5 to 7.5 and most naturally occuring drinking water is within the acceptable range.

Contact Time

Generally the longer the contact time the halogen has with the water the better. Thus taken together, higher concentrations of halogens in warmer water for longer periods of time are more effective than lower concentrations in colder water for a shorter period of time.

Iodine and Chlorine Concentrations Required to Kill Bacteria and Inactivate Viruses at Different Contact Times and Temperatures
Concentration of halogen Contact time in minutes to temperature
5°C 15°C 30°C
2 mg/l (2 ppm) 240 min 180 min 60 min
4 mg/l (4 ppm) 180 min 60 min 45 min
8 mg/l (8 ppm) 60 min 30 min 15 min

*Note: data is correct for all bacteria, viruses, Giardia and E. histolytica but is not applicable to Cryptosporidium or parasitic worm eggs and larvae, which are impervious to halogens at typical treatment concentrations.

Turbidity and Other Matter

Water with high turbidity (suspended particles) from spring run-off or glacial rock flour can shield organisms and demands longer contact times. More importantly, vegetative matter such as humus, nitrogen rich organic and inorganic substances such as amino acids from faeces and nitrites and nitrates from fertilizer, can adsorb or react with halogens. This reduces the halogen concentration and thus the halogen’s ability to kill pathogens.

In basic terms, halogens work best in clear water.

Typical Halogen Demand for Different Types of Water
Water Source
Halogen Demand (mg/l)
Clear alpine water 0.3
Cloudy river water 3–4
Pond water 5–6
Municipal wastewater (sewage) 20–30

*Note: This information depicts only average conditions and should not be taken as a guide for local conditions, which can vary widely.

Pathogen Type

While bacteria and viruses are killed relatively quickly by halogens, protozoa, such as Giardia and E. histolytica, are more resistant and require greater halogen concentrations or contact times. Cryptosporidium, and most helminth larvae and eggs, are too resistant to be killed reliably by halogens under most field conditions.

Exposure to Air and UV

Exposure to air and sunlight (UV radiation) greatly reduces the effectiveness of halogens.

Purification tablets that rely on chlorine are extremely susceptible to UV radiation and exposure to air. They can lose their potency in hours or days. Tablets should be keep tightly sealed in the manufacturer’s container and should be replaced before every major trip or every six months. In other words, do not use last year’s tablets this summer.

Adequate Halogen Doses

When using halogens while hiking few factors are controllable. Warm clear water is infinitely preferable to cold cloudy water rich in vegetative matter or sediment. Follow the instructions provided with whatever product you are using for your water conditions, or consult the following chart for guidelines of concentrations and waiting times. Liquid iodine solutions and chlorine, in the form of household bleach, should be added to water with an eyedropper.

Iodine and Chlorine Doses
Halogen Type
Target Dosage
4 mg/l (4 ppm)
Heavy Dosage
8 mg/l (8 ppm)
Iodine tablets, Portable Aqua (trade name) ½ tablet 1 tablet
2% Iodine solution (tinture) 0.2 ml, (5 drops) 0.4 ml, (10 drops)
10% Providone iodine solution 0.35 ml, (8 drops) 0.70 ml, (16 drops)
Saturated iodine crystals in water, Polar Pure
(trade name)
13 ml 26 ml
Chlorine household bleach (5% sodium hypochlorite) 0.1 ml, (2 drops) 0.2 ml, (4 drops)

*Note: Iodine and chlorine are effective against bacteria, viruses, Giardia, E. histolytica, but not against Cryptosporidium or parasitic worm eggs and larvae.

Health Risks of Halogens

Iodine is a potentially dangerous substance that can have both short and long term effects on human health. While the body requires iodine in very small amounts, large doses— two to four grams of free iodine—can be lethal.

Even much smaller amounts can cause severe problems for people allergic to iodine, those with thyroid disease, and fetuses in pregnant women. Opinions vary, but authorities seem to agree that most people, with the exception of those just mentioned, can safely ingest iodine at levels required for water treatment for periods ranging from three weeks to two months without affecting their health.
Chlorine, whether in the form of water treatment tablets or household bleach, is not generally toxic even at levels as high as 90 ppm. Long term low intensity use has no known affects on humans as evidenced by the worldwide use of chlorine in municipal water treatment systems, although recently some researchers have tentatively linked chlorine use to kidney cancer. At very high concentrations, such as in raw household bleach, chlorine is corrosive to human flesh and direct contact should be avoided.

Odour and Taste

Sensitivity to the odour and taste of iodine and chorine varies widely among people. Some people are unable to choke down chemically-treated water while others guzzle it unfazed. Odour and taste can be overcome or masked in a variety of ways.

  • Decrease the halogen dose and increase the exposure time. This allows for a lower concentration of halogen in the treated water. Letting cloudy water settle before treatment and warming cold water in the sun will help reduce the dose required to disinfect.
  • Using a granular activated carbon filter after treatment will remove virtually all traces of iodine and chlorine from treated water.
  • Resting an open water container in the sun will allow UV radiation to reduce chlorine concentrations in treated water.
  • Add a dehalogen to the water after treatment to remove halogens. Dehalogens include hydrogen peroxide, sodium thiosulfate and ascorbic acid (vitamin C) often found in drink crystals, which has the added bonus of flavouring the water.
  • Hold your breath and swallow.