Boron in drinking-water
Background document for development of WHO Guidelines for Drinking-water Quality
Environmental levels and human exposure
Boron is not present in the atmosphere at significant levels (Sprague, 1972). Because borates exhibit low volatility, boron would not be expected to be present to a significant degree as a vapour in the atmosphere. Atmospheric emissions of borates and boric acid in a particulate (<1–45 µm in size) or vapour form occur as a result of volatilization of boric acid from the sea, volcanic activity, mining operations, glass and ceramic manufacturing, the application of agricultural chemicals, and coal-fired power plants.
The natural borate content of groundwater and surface water is usually small. The borate content of surface water can be significantly increased as a result of wastewater discharges, because borate compounds are ingredients of domestic washing agents (ISO, 1990). Naturally occurring boron is present in groundwater primarily as a result of leaching from rocks and soils containing borates and borosilicates. Concentrations of boron in groundwater throughout the world range widely, from <0.3 to >100 mg/litre. In general, concentrations of boron in Europe were greatest in southern Europe (Italy, Spain) and least in northern Europe (Denmark, France, Germany, the Netherlands, and the United Kingdom). For Italy and Spain, mean boron concentrations ranged from 0.5 to 1.5 mg/litre. Values ranged up to approximately 0.6 mg/litre in the Netherlands and the United Kingdom, and approximately 90% of samples in Denmark, France, and Germany were found to contain boron at concentrations below 0.3, 0.3, and 0.1 mg/litre, respectively (WHO, in press). Monthly mean values of boron in the Ruhr River, Germany, ranged from 0.31 to 0.37 mg/litre in a survey conducted during 1992–1995 (Haberer, 1996).
The majority of the Earths boron occurs in the oceans, with an average concentration of 4.5 mg/litre (Weast et al., 1985). The amount of boron in fresh water depends on such factors as the geochemical nature of the drainage area, proximity to marine coastal regions, and inputs from industrial and municipal effluents (Butterwick et al., 1989).
Boron concentrations in fresh surface water range from <0.001 to 2 mg/litre in Europe, with mean values typically below 0.6 mg/litre. Similar concentration ranges have been reported for water bodies within Pakistan, Russia, and Turkey, from 0.01 to 7 mg/litre, with most values below 0.5 mg/litre. Concentrations ranged up to 0.01 mg/litre in Japan and up to 0.3 mg/litre in South African surface waters. Samples taken in surface waters from two South American rivers (Rio Arenales, Argentina, and Loa River, Chile) contained boron at concentrations ranging between 4 and 26 mg/litre in areas rich in boron-containing soils. In other areas, the Rio Arenales contained less than 0.3 mg of boron per litre. Concentrations of boron in surface waters of North America (Canada, USA) ranged from 0.02 mg/litre to as much as 360 mg/litre, indicative of boron-rich deposits. However, typical boron concentrations were less than 0.1 mg/litre, with a 90th-percentile boron concentration of approximately 0.4 mg/litre.
Concentrations of boron found in drinking-water from Chile, Germany, the United Kingdom, and the USA ranged from 0.01 to 15.0 mg/litre, with most values clearly below 0.4 mg/litre. These values are consistent with ranges and means observed for groundwater and surface waters. This consistency is supported by two factors: (i) boron concentrations in water are largely dependent on the leaching of boron from the surrounding geology and wastewater discharges, and (ii) boron is not removed by conventional drinking-water treatment methods.
The general population obtains the greatest amount of boron through food intake. Concentrations of boron reported in food after 1985 have more validity because of the use of more adequate analytical methods.
The richest sources of boron are fruits, vegetables, pulses, legumes, and nuts. Dairy products, fish, meats, and most grains are poor sources of boron. Based on the recent analyses of foods and food products, estimations of daily intakes of various age/sex groups have been made (WHO, in press). The estimated median, mean, and 95th-percentile daily intakes of boron were 0.75, 0.93, and 2.19 mg/day, respectively, for all groups, and 0.79, 0.98 and 2.33 mg/day, respectively, for adults aged 17 and older. Using food included in US Food and Drug Administration Total Diet Studies, Iyengar et al. (1988) determined the mean adult male daily intake of boron to be 1.52 mg/day, whereas Anderson et al. (1994) determined the intake to be 1.21 mg/day. Based on the United Kingdom National Food Survey (MAFF, 1991), the dietary intake of boron in the United Kingdom ranges from 0.8 to 1.9 mg/day. It should be noted that increased consumption of specific foods with high boron content will increase boron intake significantly; for example, one serving of wine or avocado provides 0.42 and 1.11 mg, respectively (Anderson et al., 1994).
Estimated total exposure and relative contribution of drinking-water
The mean daily intake of boron in the diet is judged to be near 1.2 mg/day (Anderson et al., 1994). Concentrations of boron in drinking-water have wide ranges, depending on the source of the drinking-water, but for most of the world the range is judged to be between 0.1 and 0.3 mg/litre. Based on usage data, consumer products have been estimated to contribute a geometric mean of 0.1 mg/day to the estimate of total boron exposure (WHO, in press). The contribution of boron intake from air is negligible. The total daily intake can therefore be estimated from mean concentrations and concentration ranges to be between 1.5 and 2 mg.