Defined by the U.S. Occupational Health and Safety Administration OSHA in Appendix A as a chemical that falls in any of these three categories: (1) A chemical that has a median lethal dose (LD50) of 50 milligrams or less per kilogram of body weight when administered orally to albino rats weighing between 200 and 300 gram seach. (2) A chemical that has a median lethal dose (LD50) of 200 milligrams or less per kilogram of body weight when administered by continuous contact for 24 hours (or less if death occurs within 24 hours) with the bare skin of albino rabbits weighing between two and three kilograms each. A chemical that has a median lethal concentration (LC50) in air of 200 parts per million by volume or less of gas or vapor, or 2 milligrams per liter or less of mist, fume, or dust, when administered by continuous inhalation for one hour (or less if death occurs within one hour) to albino rats weighing between 200 and 300 grams each. Additional Info: This is the greatest level of toxicity defined in the OSHA Hazard Communication Standard (OSHA does not have an "Extremely toxic" ranking). A separate OSHA standard, Process Safety Management of Highly Hazardous Chemicals", 29 CFR 1910.119, applies to workplaces where highly toxic materials are used. See: Further Reading below for resources on this standard. MSDS Relevance: Accidental release or exposure to a highly toxic chemical can cause serious injury or death. If you can not avoid using highly toxic materials in the first place, then proper personal protective equipment (gloves, safety goggles, respirators etc). and engineering controls such as fume hoods are an absolute necessity. It is critical that you develop and enforce written standard operating procedures (SOP's) for working with each highly toxic material. In 1997, a professor at Dartmouth University (an experienced researcher and expert on the toxicology of heavy metal poisoning) died after spilling just a few drops of highly toxic dimethylmercury on her latex gloves (the wrong choice of gloves, obviously). In 2009, a worker in Canada died when he performed a chemical reaction in a fume hood that had been deactivated for maintenance; following an SOP to test all hoods before use would have saved his life. Part of your experiment or occupational procedure planning must include knowing the physical properties of the material as well as symptoms of exposure and first aid procedures. Never work with highly toxic materials while alone. If one exists, have the proper antidote on hand. See also: LC50, LD50, Poison, Toxic