Guidelines for Canadian drinking water quality boron: Exposure considerations

On this page

• Sources, uses and identity
• Environmental fate
• Exposure

Sources, uses and identity

Boron (Chemical Abstracts Service Registry No. 7440-42-8) originates from both natural and anthropogenic sources. It enters the environment naturally through weathering of boron-containing rocks and soils, seawater spray, volcanoes and geothermal ducts (Coughlin, 1996). Anthropogenic sources of boron include fossil fuel combustion, biomass burning, pesticide application, farming practices (for example, irrigation), wastewater from sewage treatment plants and industrial manufacturing and other human activities (for example, incineration) (CCME, 2009). The majority of the anthropogenic boron that enters drinking water sources originates from detergents and fertilizers (Parks and Edwards, 2005). However, anthropogenic inputs of boron to natural environments are considered smaller than inputs from natural processes (U.S. EPA, 2008).

Of the more than 200 minerals containing boron, only four (borax, kernite, colemite and ulexite) are commercially important and make up more than 90% of the borates used industrially worldwide (ATSDR, 2010). Borates and boric acids are used in the manufacturing of glass, soaps, detergents, flame retardants and as neutron absorbers for nuclear installations. Boric acid, borates and perborates are used in mild antiseptics, pharmaceuticals and natural health products, boron neutron capture therapy, antioxidants for soldering and agricultural fertilizers (Health Canada, 2007; CCME, 2009). Borates are used in adhesives and sealants, paints and coatings, personal care products, lubricants and greases, and food packaging (ECCC and Health Canada, 2016). Boric acid and its salts are used as insecticides and fungicides (Health Canada, 2012) and are conditionally permitted in cosmetic products at concentrations of less than 5% (Health Canada, 2018). Although boric acid occurs naturally in foods, and particularly plant-based foods (ATSDR, 2010), it cannot be imported, used or sold as a food ingredient in Canada (CFIA, 2013).

Boron has a molecular weight of 10.81 g/mol and does not exist in its elemental form in nature. In water, boron primarily exists in equilibrium as undissociated boric acid (H₃BO₄; CAS no. 10043-35-3) and other borates (for example, B(OH)₄^-) (CCME, 2009). Boric acid has a molecular weight of 61.83 g/mol, is highly soluble (49 g/L), has negligible vapour pressure and has an octanol/water partition coefficient (logKow) of 0.175 (ECOTOC, 1995; ECCC and Health Canada, 2016). The chemical and toxicological properties of boric acid and other borates (for example, borax) are anticipated to be similar on a molar boron equivalent basis when dissolved in water or biological fluids (WHO, 2009).

Environmental fate

Most environmentally relevant boron minerals are highly soluble in water (ATSDR, 2010) with their solubility increasing with temperature (Coughlin, 1996). Therefore, if present in water, boron compounds are not easily removed from solution by natural mechanisms (Butterwick et al., 1988). Their distribution in the environment depends on geology, rainfall, evaporation rates, and aquifer type (Coughlin, 1996). At acidic pH, boron exists in solution mainly as undissociated boric acid, whereas at alkaline pH (that is, above the pKa of 9.2), it is mainly present as borate ions; both forms are highly soluble and stable, and further degradation is not expected to occur (CCME, 2009). In calcium enriched hard waters, the speciation of boron might be affected by the formation of ion pairs between calcium and B(OH)₄^- (Butterwick et al., 1989). The only significant mechanism expected to influence the fate of boron in water is adsorption-desorption reactions with soil and sediment (CCME, 2009), the extent of which depends on the pH of the water, concentration of boron in solution and the chemical composition of the soil (CCME, 2009; ATSDR, 2010). The greatest adsorption is observed in water with a pH in the range of 7.5 to 9.0 (ATSDR, 2010). Further pH increases lower the adsorption of boron (Goldberg and Su, 2007) as the species of boron and the charges on the adsorbing surfaces become less favourable (U.S. EPA, 2008). In soil, primary boron adsorbing surfaces include aluminium and iron oxides, clay minerals, calcium carbonate and organic matter (Goldberg and Su, 2007) with amorphous aluminium oxide likely the most important (ATSDR, 2010). Typical forms of boron have low volatility and are expected to be emitted to air as particulate matter (ECCC and Health Canada, 2016). However, monitoring studies have also detected boric acid in the gaseous phase (for example, Anderson et al., 1994; Cheng et al., 2009). Airborne boron is anticipated to be deposited by wet (rain and snow) and dry deposition (Anderson et al., 1994; Fogg and Duce, 1985; Kot, 2009; Zhao and Liu, 2010).

Exposure

The average total daily boron intake from environmental media, food and drinking water for the general Canadian population ranges from 3 to 92 μg/kg body weight (bw) per day, depending on the age group, with an estimated 3% to 16% of total dietary intake attributable to drinking water (ECCC and Health Canada, 2016). Food is the main source of boron exposure, with fruits and vegetables contributing 40% to 60% of dietary intake (ECCC and Health Canada, 2016). Consumer products can also contribute significantly to total daily intake, with an upper bound exposure estimate of 2819 μg/kg bw per event for direct ingestion of modelling clay by children (ECCC and Health Canada, 2016). Soil, air and dust are likely negligible sources of boron exposure (WHO, 2009; ECCC and Health Canada, 2016). The large range in total daily intake results from the variability of boron in foods, drinking water, and consumer products and their use patterns, and from emissions into the environment from natural weathering and human activities (Becking and Chen, 1998; ECCC and Health Canada, 2016).

Most of the world's boron is in the oceans with an average concentration of 4.5 mg/L in seawater (CCME, 2009) while levels in Canadian coastal waters range from 3.7 to 4.3 mg/L (Moss and Nagpal, 2003). The amount of boron in fresh water varies with the geochemical nature of the drainage area, proximity to marine coastal regions and inputs from industrial/municipal effluents (Butterwick et al., 1989). The concentration of boron in surface water (rivers and streams) can vary seasonally with higher concentrations observed in winter and spring and lower concentrations in summer and fall (Hall Jr. et al., 2004). While boron is present in both surface and groundwater, the average concentrations in groundwater tend to be higher than in surface water (Frey et al., 2004).

Water monitoring data are available from the provinces and territories and the National Drinking Water Survey (Table 1), as well as from select river basins from Environment and Climate Change Canada (ECCC) (Appendix A). Total boron is detected in all water types across Canada. However, mean concentrations vary between provinces and territories with higher concentrations found in Manitoba, Saskatchewan and Ontario. Median, mean and 90th percentile concentrations are below 1 mg/L for raw, treated and distributed water for both surface and groundwater. Overall Canadian data (calculated as the weighted mean of the data from provinces and territories) show that the mean concentrations of boron across Canada in all types of municipal water supplies (that is, distributed and treated water, from ground and surface water) are below 0.1 mg/L.

Additional analysis of the higher concentrations presented in Table 1 and information from published literature indicates that elevated boron concentrations (> 1 mg/L) occur in groundwater from certain aquifers in Ontario, Manitoba, Saskatchewan and Alberta. In most cases, boron concentrations in these sources are below 5 mg/L, although the range of upper concentrations reported in various studies is 7 to 8 mg/L (Lemay, 2002; Desbarats, 2009; Government of Manitoba, 2010; Hamilton, 2015). Impact statements provided by provinces and territories indicate that these elevated concentrations of boron likely only occur in a limited number of drinking water systems in Canada.

Blood boron levels from a study in Alberta are considered to adequately represent average boron exposure in the Canadian population (Alberta Health and Wellness, 2008; Government of Alberta, 2010; ECCC and Health Canada, 2016). Based on this data, ECCC and Health Canada (2016) derived an average daily intake of 10 µg/kg bw per day.