Lethal Dose 50% (LD50)

Testing a poison on animals until all of them die would take forever and waste most of the data, since the first and last deaths happen at wildly different doses. The midpoint is the sturdiest number to measure: it sits on the steepest, most consistent part of the dose-response curve, where a small change in dose produces a clear change in deaths. That makes LD50 reproducible across labs, which is why it became the standard way to compare toxicity between chemicals.

Picture a mortality curve with dose on the x-axis and percent of test animals killed on the y-axis. The curve is flat near zero at low doses, climbs steeply through the middle, then flattens near 100 percent at high doses. To read LD50, find 50 percent on the y-axis, trace across to where it meets the curve, then drop straight down to the x-axis. That dose is the LD50, usually reported in milligrams of chemical per kilogram of body weight.

Because LD50 is a rate per body mass, you scale it up by multiplying by the subject's actual weight to estimate a lethal dose for that individual. But LD50 only describes acute, single-dose death risk in a tested population, so a high LD50 does not mean a chemical is safe: it says nothing about chronic damage from repeated low doses, sublethal harm like organ or nervous system damage, or buildup in the body over time through bioaccumulation. In chronic exposure studies, researchers instead track lowest observed effect levels over repeated doses, connecting LD50 to the broader dose-response ideas used for long-term pollution exposure.