Boron is a naturally occurring element that is found in nature in the form of borates in the oceans, sedimentary rocks, coal, shale, and some soils. Natural sources of borates released into the environment are the oceans, geothermal steam, and natural weathering of clay-rich sedimentary rocks. Boron is also released from anthropogenic sources to a lesser extent.
Boron is an essential micronutrient for higher plants, with interspecies differences in the levels required for optimum growth. Boron deficiency in terrestrial plants has been observed in many countries throughout the world. There is a small range between deficiency and toxicity in some plants.
Comparison of the environmental no-effect concentration (1 mg/litre) with the general ambient environmental levels of boron indicates that the risk of adverse effects of boron on the aquatic ecosystem is low. In a few boron-rich environments, natural levels will be higher. It is reasonable to assume that aquatic organisms in such habitats may be adapted to the local conditions.
For humans, boron exposure occurs primarily through the diet and drinking-water. The mean global boron concentration in drinking-water was considered to be between 0.1 and 0.3 mg boron/litre.
For the general population, the greatest boron exposure comes from the oral intake of food. The mean daily intake of boron in the diet is about 1.2 mg.
In humans and animals, boric acid and borate are absorbed from the gastrointestinal and respiratory tracts. More than 90% of administered doses of these compounds are absorbed, as evidenced by excretion in the urine, which is rapid, occurring over a few to several days.
Animal experiments have shown that boron in the form of boric acid and borate demonstrates reproductive and developmental toxicity at levels that are approximately 100- to 1000-fold greater than normal exposure levels. There is a lack of sufficient toxicity data on humans. The tolerable intake (TI) of boron was set as 0.4 mg/kg body weight per day. The allocation of the TI in various media should be based on the exposure data of individual countries.
a) Water and food guideline values should be based on the TI provided by this document.
b) The TI should be applied with the understanding that boron may provide a physiological benefit for human health.
c) It should be recognized in applying standards that boron is essential for some constituents of the environment (e.g. boron is an essential micronutrient for higher plants).
d) Dietary supplements that exceed the TI should be avoided.