Denatonium, usually available as denatonium benzoate (under trade names such as Bitrex or Aversion) and as denatonium saccharide, is the bitterest chemical compound known; with bitterness thresholds of 0.05 ppm for the benzoate and 0.01 ppm for the saccharide. It was discovered in 1958 during research on local anesthetics by Macfarlan Smith of Edinburgh, Scotland, and registered under the trademark Bitrex. Dilutions of as little as 10 ppm are unbearably bitter to most humans. Denatonium salts are usually colorless and odorless solids but are often traded as solutions. They are used as aversive agents to prevent accidental ingestion. Denatonium is used in denatured antifreeze, nail biting preventions, respirator mask fit-testing, animal repellents, liquid soaps, and shampoos. It is not known to pose any long-term health risks.