Boron
(endorsed 2011)
Guideline
Based on human health considerations, the concentration of boron in drinking water should not exceed 4 mg/L.
General description
Boron can be present in drinking water through the natural leaching of boron-containing minerals, or by contamination of water sources. The environmental chemistry of boron is not well understood. In water, the predominant form is probably boric acid, which does not dissociate readily.
Boron compounds are used in glass manufacture, cleaners, wood and leather preservatives, flame retardants, cosmetic products, antiseptics, and occasionally food preservatives; and as agricultural fertilisers, algicides, herbicides and insecticides.
In other countries, concentrations of boron in uncontaminated water sources are usually less than 1 mg/L. Concentrations up to 6.5 mg/L have been reported in ground water supplies, but these higher concentrations are associated with seawater intrusion.
Boron is present naturally in many food products, with high amounts found in foods of plant origin, especially fruits, leafy vegetables, nuts and legumes. It has been estimated that intake of boron from food is about 10 times that from water. The daily consumption of boron is 10-25 mg. This value, however, will vary from country to country depending on population dietary habits, geographical area and soil geochemistry. In the United States, average intake values for adults range from 0.87 to 1.34 mg/day and 90 percentile intakes are about 1.5 to 2 mg/day (IOM 2001, USEPA 2008a). In Australia, the estimated dietary intake for boron is 2.2 mg/day (Samman et al. 1998).
Typical values in Australian drinking water
Boron is not often monitored in Australian drinking water supplies, but the limited information available indicates that boron concentrations are less than 0.1 mg/L.
Treatment of drinking water
The concentration of boron in drinking water can be reduced by the use of granular activated carbon, or by lime softening.
Measurement
The boron concentration in drinking water can be determined using inductively coupled plasma emission spectroscopy (APHA Method 4500-B Part D 1992). The limit of determination is approximately 0.05 mg/L.
Health considerations
Boron, as soluble borate (borax) or boric acid, is rapidly and completely absorbed after ingestion. It is widely distributed throughout the body and up to 90% is excreted in urine as unchanged compound.
There have been a number of reported cases of poisoning following the ingestion of high doses of boron. Symptoms include gastrointestinal disturbances, skin eruptions, and central nervous system stimulation and depression. Long-term occupational exposure to boron can lead to similar symptoms.
Short-term studies with rats and dogs reported testicular atrophy at high doses (5000 mg/kg bodyweight) of boric acid and borate. This condition was also observed in longer-term studies with rats, mice and dogs over 2 years. Reproductive studies reported that rats became sterile at the highest doses. No increase in the incidence of tumours was observed in long-term studies using mice.
A tolerable daily intake (TDI) of 0.16 mg/kg bodyweight (mg/kg bw) has been derived by the the World Health Organization (WHO 2004a). This TDI is based on the no-observed-adverse-effects level (NOAEL) of 9.6 mg/kg bw/day for foetal bodyweight effects in a rat developmental study (Price et al. 1996), with an uncertainty factor of 60 (10 for interspecies and 6 for human intraspecies variation).
Tests for mutagenicity using bacteria and mammalian cells have been mostly negative. Neither boric acid nor borate induced chromosomal aberrations in mammalian cells.
Boron is suspected of being a trace nutrient in mammals and some authors argue for such a role in humans (Nielson 1996), however recent reviews concluded that a clear biological function for boron in humans has yet to be established (IOM 2001, USEPA 2008b).
Although boron is an essential trace element for plants, certain plants (e.g. citrus fruit, stone fruit, some nut trees) are sensitive to the toxic effects of boron if irrigation water has concentrations higher than about 0.5 mg/L (Lazarova and Bahri 2005). WHO (2004) indicates that this concentration is below the level that can be achieved by practical treatment methods. Application of waste water containing 0.8–1.3 mg/L to young orange trees for three years was well tolerated (Reboll et al. 2000).
Derivation of guideline
The background intake from diet is given as 2.2 mg/day (Samman et al. 1998) and from consumer products, 0.1 mg/day (WHO 2003). Assuming a bodyweight of 70 kg for an average adult, this is equivalent to a background intake of 0.03 mg/kg bw/day. When subtracted from the TDI of 0.16 mg/kg bw, the remaining 0.13 mg/kg bw/day can be allocated to intake from water.
When the above considerations are applied in the standard equation for deriving a drinking water guideline in Australia, a value of 4 mg/L (rounded down from 4.55 mg/L) is attained.
Where:
0.13 mg/kg bw/day is the tolerable intake allocated to water.
70 kg is taken as the average weight of an adult.
2 L/day is the estimated maximum amount of water consumed by an adult.
References
APHA Method 4500-B Part D (1992). Boron: Inductively Coupled Plasma method. Standard Methods for the Examination of Water and Wastewater, 18th edition. American Public Health Association, Washington.
IOM (Institute of Medicine) (2001). Dietary reference intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Institute of Medicine, National Academy Press, Washington, D.C.
Lazarova V, Papadopoulos I, Bahri A (2005). Chapter 5: Code of successful agronomic practices. In: Lazarova V, Bahri A, (eds), Water Reuse for Irrigation: Agriculture, Landscapes and Turf Grass, CRC Press, Boca Raton, FL, pp. 103-150.
Neilson FH (1996). Evidence for the Nutritional Essentiality of Boron. Journal of Trace Elements in Experimental Medicine, 9:215–229.
Price CJ, Strong PL, Marr MC, Myers CB, Murray FJ (1996). Developmental toxicity NOAEL and postnatal recovery in rats fed boric acid during gestation. Fundamental and Applied Toxicology, 32(2):179-193.
Reboll V, Cerezo M, Roig A, Flors V, Lapena L, Garcia-Agustin P (2000). Influence of wastewater vs groundwater on young Citrus trees. Journal of the Science of Food and Agriculture, 80:1441-1446.
Samman S, Naghii MR, Lyons Wall PM, Verus AP (1998). The nutritional and metabolic effects of boron in humans and animals. Biologic Trace Element, 66,1–9.
USEPA (United States Environmental Protection Agency) (2008a). Drinking Water Health Advisory for Boron. Health and Ecological Criteria Division, Office of Science and Technology, Office of Water, USEPA Document Number: 822-R-08-013, May 2008.
USEPA (United States Environmental Protection Agency) (2008b). Health Effects Support Document for Boron, USEPA, Office of Water, Health and Ecological Criteria Division, EPA-822-R-08-002, January 2008.
WHO (World Health Organization) (2003). Boron in drinking-water. Background document for preparation of WHO Guidelines for drinking-water quality. WHO, Geneva (WHO/SDE/WSH/03.04/54).
WHO (World Health Organization) (2004a). Boron in Drinking Water. Background document for development of WHO Guidelines for Drinking-water Quality, WHO, Geneva, Switzerland. WHO/SDE/WSH/03.04/54.
WHO (World Health Organization) (2004b). Guidelines for Drinking-water Quality. First addendum to 3rd edition. Volume 1. Recommendations. WHO, Geneva, Switzerland.
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