Inorganic borates and boric acid (boron-oxygen compounds) enter the environment as contaminants from the manufacture and industrial and household use of boron-containing compounds; from borax mining; and from coal, oil, and geothermal power generation. In general, boron compounds are more toxic to freshwater invertebrates than to freshwater fishes, with 48-hr LC₅₀ values ranging from <52–226 mg B/L in the water flea (Daphnia magna) and 96-hr LC₅₀ values ranging from 332–979 mg B/L in fathead minnows (Pimephales promelas) and mosquitofish (Gambusia affinis). Data also indicate that boron compounds are more toxic to saltwater crustaceans and fish than to freshwater species, with a 24-hr LC₅₀ of 28.35 mg B/L in the saltwater isopod (Limnoria lignorum) and a 96-hr LC₅₀ of 74 mg B/L in dab (Limanda limanda). A comparison of the acute toxicity of boron-oxygen compounds to adult and early developmental stages of fishes and invertebrates indicates that most species are more sensitive to sodium borates (sodium perborate > borax) than to boric acid. This increased toxicity may be attributed to a pH shift into the alkaline range, which in fish results in increased mucus formation and crippling behavior.