Time taken by material or substance to lose every one-half of its quantity in the environment, or that of its concentration in a particular body organ. For example, the biochemical half-life of the pesticide DDT in the environment is 15 years: half of any quantity (whether one milligram or one ton) of DDT will be biodegraded (see biodegradation) in 15 years. The one-half of the remaining half will also take 15 years, and so on ... until the entire amount is biodegraded. Half life of a substance in human body varies from organ to organ. For example, organic mercury's half life in liver is 50 days and, while it averages 70 days in other organs, it is 150 days in the brain.