Boron is a metalloid chemical element with symbol B and atomic number 5. Boron is concentrated on Earth by the water-solubility of its more common naturally occurring compounds, the borate minerals. Chemically uncombined boron is found in small amounts in meteoroids but is not found naturally on Earth.
Although boron is a relatively rare element in the Earth's crust, representing only 0.001% of the crust mass, it can be highly concentrated by the action of water, in which many borates are soluble. It is found naturally combined in compounds such as borax and boric acid (sometimes found in volcanic spring waters).
Elemental boron, boron oxide, boric acid, borates, and many organoboron compounds are nontoxic to humans and animals (with toxicity similar to that of table salt). An intake of 4 g/day of boric acid was reported without incident, but more than this is considered toxic in more than a few doses. Intakes of more than 0.5 grams per day for 50 days cause minor digestive and other problems suggestive of toxicity.
The boranes (boron hydrogen compounds) and similar gaseous compounds are quite poisonous. As usual, it is not an element that is intrinsically poisonous, but their toxicity depends on structure. The boranes are toxic as well as highly flammable and require special care when handling. Sodium borohydride presents a fire hazard owing to its reducing nature and the liberation of hydrogen on contact with acid. Boron halides are corrosive.