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The LD50 is the Lethal dose at which 50% of organisms exposed to, or injected with, a substance die.

If, for example, ten lab mice (that weigh 250g each) were injected with 100 micrograms of chemical, and half of these mice died within five days, the LD50 would be 400 micrograms of chemical per kilo of mouse tissue or 400 parts per billion.