The term parts per thousand described a concentration ratio of density used for measuring small concentrations of a substance within a mass/volume of a much larger one.

The supply of ryetalyn Doctor Leonard McCoy acquired from Flint was useless because it contained nearly one part per thousand of irillium, a substance that rendered the antitoxin inert and useless. (TOS: "Requiem for Methuselah")

The cure against the macrovirus released aboard USS Voyager in 2372 was dispersed through the ship's environmental controls, with the dispersal nodes set to 1 part per thousand. (VOY: "Macrocosm")

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