Iodine, Iodide
(endorsed 2011)
Guideline
Iodide: Based on health considerations, the concentration of iodide in drinking water should not exceed 0.5 mg/L.
Iodine: No guideline value has been set for molecular iodine.
General description
The element iodine is present naturally in seawater, nitrate minerals and seaweed, mostly in the form of iodide salts. It may be present in water due to leaching from salt and mineral deposits. Iodide can be oxidised to molecular iodine with strong disinfectants such as chlorine.
Molecular iodine solutions are used as antiseptics and as sanitising agents in hospitals and laboratories. Iodine is occasionally used for the emergency disinfection of water for field use but is not used for disinfecting larger drinking water supplies. Iodide is used in pharmaceutical and photographic materials.
Iodine has a taste threshold in water of about 0.15 mg/L.
Iodide occurs in cows’ milk and seafood. Some countries add iodide to table salt to compensate for iodide-deficient diets.
Typical values in Australian drinking water
Concentrations of iodide in Australian source or treated water ranges from 0.005 to 2.9 mg/L (median 0.03 mg/L, mean 0.1 mg/L).
Treatment of drinking water
It is unlikely that the concentration of iodine or iodide in drinking water would ever be high enough to justify water treatment.
Measurement
The iodine or iodide concentration in drinking water can be determined using the Leuco crystal violet method (APHA 4500-I-, Part B 1992). The limit of determination is approximately 0.01 mg/L.
A test for iodide alone may not determine all the iodide present at the time of sampling (given the equilibrium between iodine species in water – molecular iodine, iodide, iodate, and hypoiodite). A screening test for total iodine may be carried out by inductively coupled plasma mass spectrometry. If total iodine is found to be elevated, then analysis for iodide may be carried out by ion selective electrode, with further investigation and sampling if necessary.
Health considerations
Iodine is an essential trace element for humans and is used in the synthesis of thyroid hormones. Tolerable upper intake levels for iodine in children and adults recommended by Food Standards Australia and New Zealand (FSANZ 2008) are 0.2 and 1.1 mg/day respectively.
Iodine is efficiently absorbed by the gastrointestinal tract and deposited in the thyroid gland, the eye, and muscle tissue. More than 70% is found in the thyroid gland.
High oral doses (more than 30 mg/kg bodyweight) of iodine can be lethal. Lower doses (3.3 mg/kg bodyweight) have been used to treat asthmatic patients without adverse effects.
Chronic exposure to high amounts of iodide in the diet (over 2 mg/day) can result in a condition known as iodism. Symptoms resemble those of a sinus cold. Long-term consumption of iodinated drinking water has not been associated with adverse health effects in humans. Prisoners drinking water containing up to 1 mg/L iodine for five years showed no signs of iodism or hypothyroidism, but some changes in uptake of iodine by the thyroid gland were observed.
Animal studies using chickens susceptible to autoimmune thyroiditis reported an increase in the incidence of the disease when they were given high doses of iodide in their drinking water (200 mg/L). Excessive iodide consumption may increase the incidence of this disease in humans.
Iodide has not been shown to increase the incidence of cancer of the thyroid in laboratory animals. No data are available on the mutagenic activity of iodine.
Derivation of guideline
In healthy adults, sub-clinical hypothyroidism is associated with intakes of 1.7 to 1.8 mg/day, and for children with intakes of 1.15 mg/day (EFSA 2006, FSANZ 2008). Chronic iodine intakes of approximately 1 mg/day, however, appear to be well tolerated by healthy adults. This is consistent with the provisional maxium tolerated daily intake of 1 mg/day established by the Joint FAO/WHO Expert Committee on Food Additives (JECFA 1989), and the nutrient reference value and tolerable upper intake level of 1.1 mg/day respectively recommended by the NHMRC (2006) and Food Standards Australia New Zealand (FSANZ 2008) for iodine intake by adults in Australia and New Zealand.
The majority of iodine is taken in via the diet. Food Standards Australia New Zealand noted that Australian and New Zealand dietary intakes of iodine were the same (FSANZ 2008). The daily dietary intake of New Zealanders 5 years old and over is 0.1 mg/person (FSANZ 2008). It has been proposed that table salt and salt used in bread be mandatorily fortified with iodine; if this occurs, intake would rise to 0.2 mg/person.
Applying the method of International Program on Chemical Safety (IPCS 2002) for essential trace elements, a drinking water guideline of 0.5 mg/L (rounded up) can be calculated:
Where:
1.1 mg/person is the tolerable daily intake for iodine.
0.2 mg/person is the background dietary intake assuming mandatory salt fortification.
2 L/day is the estimated maximum amount of water consumed by an adult.
Recent data from studies on rats indicate that the effects of molecular iodine in drinking water on thyroid hormone concentrations in the blood differ from those of iodide. The guideline value therefore applies only to iodide. No guideline value can be established for molecular iodine.
References
APHA Method 4500-I-Part B (1992). Iodide: Leuco crystal violet method. Standard Methods for the Examination of Water and Wastewater, 18th edition. American Public Health Association, Washington.
EFSA (European Food Safety Authority) (2006). Tolerable upper intake levels for vitamins and minerals. EFSA.
FSANZ (Food Standards Australia New Zealand) (2008). Final Assessment Report: Proposal P230. Consideration of mandatory fortification with iodine for New Zealand. FSANZ.
IPCS (International Programme on Chemical Safety) (2002). Principles and methods for the assessment of risk from essential trace elements. Environmental Health Criteria 228. World Health Organization, IPCS, Geneva, Switzerland.
JECFA (Joint FAO/WHO Expert Committee on Food Additives) (1989). Toxicological evaluation of certain food additives and contaminants: Iodine. World Health Organization Food Additive Series, 24, 267-294. The 33rd meeting of the Joint FAO/WHO Expert Committee on Food Additives, Geneva, Switzerland.
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