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In soils, boron occurs at concentrations ranging from 10 mg/kg of soil up to 300 mg/kg of soil. The average boron concentration is approximately 30 mg/kg of soil. The boron concentration depends on the type of soil, the amount of organic matter, which contains boron, and the amount of rainfall, which can remove boron from the soil.
In surface water, concentrations of boron depend on the amount of boron present in the soils and rocks of the drainage area, and on the proximity of the drainage area to the ocean. The ocean provides boron both by the deposition of vaporised boric acid on the drainage area, and by infiltration of boron-containing seawater in tidal regions and estuaries. Surface waters can also receive boron inputs from effluent discharges, both from industrial processes and from municipal sewage treatment. Concentrations of boron in surface water range widely, from 0.001 mg/litre to as much as 360 mg/litre. However, average boron concentrations are typically well below 0.6 mg/litre (see table below).
| Regions of the world | Representative boron concentrations in surface water |
|---|---|
| Europe, Pakistan, Russia, and Turkey | Mean concentrations below 0.6 mg/litre |
| Japan, South Africa, and South America | Generally below 0.3 mg/litre |
| North America | Typical concentrations below 0.1 mg/litre (with only 10% above 0.4 mg/litre) |
In ambient air, boron concentrations range from less than 0.5 to approximately 80 ng/m3 of air, with an average for air laying over continental land masses of 20 ng/m3 of air.
Boron accumulates in aquatic and terrestrial plants but does not increase in concentration through the food-chain. Thus animals which eat plants or predators which eat these plant-eating animals do not build up increasing concentrations of boron in their bodies. Concentrations of boron in plants depend on their environment (see table below).
| Type of plant | Boron concentrations in plant material |
|---|---|
| Submerged aquatic freshwater plants | 26 to 382 mg/kg |
| Freshwater vegetation in which some parts of the plant are above the water surface | 11.3 to 57 mg/kg |
| Terrestrial plants | 2.3 to 94.7 (dry weight) |
In marine invertebrate animals and fish, boron concentrations are similar to the boron levels found in the oceans, i.e. between 0.5 and 4 mg/kg (wet weight). Two freshwater fish species have been found to take up very little boron, as the concentrations in their bodies only reached 30% of the level found in the water in which they live. More...
Humans are exposed to boron through diet, from drinking water, and from some consumer products including soaps and detergents, body building supplements, bottled water, fertilizers, pesticides, preservatives, and cosmetic, oral hygiene, eye care, and deodorant products. They may also, to a much smaller extent, ingest boron from the soil or breathe it in with the air.
| Sources of human exposure to boron | Average daily intake of boron |
|---|---|
| Diet | 1.2 mg per day |
| Drinking-water | 0.2 - 0.6 mg per day |
| Consumer products | 0.1 mg per day |
| Soil | 0.0005 mg per day |
| Air | 0.00044 mg per day |
| All sources combined | 1.5 - 1.9 mg per day |
Within the diet, the richest sources of boron are fruits, vegetables, pulses, legumes, and nuts. Dairy products, fish, meats and most grains contain very little boron.
Click here for a table showing "Boron content of some common foods"
When groundwater and fresh surface water are treated for drinking-water purposes, boron is not removed in the treatment.
Workers in borax mines and refining plants, or in industries which use borates to manufacture other products, may be exposed to boron during their work. These industries include the manufacture of fibreglass and other glass products, and the production of cleaning and laundry products, fertilizers, pesticides, and cosmetics.
Inhalation of dusts which contain boron is the most significant route of exposure at work for workers in these industries. Dermal absorption of boron may also occur if damaged skin is in contact with boron compounds. More...
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