To better understand how toxic TCDD is, one needs to understand how it and other toxins are measured. In toxicology, the median lethal dose is identified as LD₅₀ (abbreviation for “lethal dose, 50%”). LC50 is the abbreviation of lethal concentration, 50%. These are used to measure the lethal dose of a toxin, radiation, or pathogen.  The value of LD₅₀ for a substance is the dose required to kill half the members of a tested population after a specified amount of time. LD₅₀ figures are often used as a general indicator of a substance’s acute toxicity.

For example:  The lethal dose of lead (or LD₅₀)  for humans is 450 mg/kg of body weight. Using the EPA’s 1997 carcinogenicity benchmark  dose of TCDD as 0.0000156 mg/kg of body weight, TCDD would be approximately 28,846,154 times more toxic than lead.