Parts Per Billion is a 2014 romantic drama written and directed by Brian Horiuchi. It stars an ensemble cast comprising Josh Hartnett, Rosario Dawson, Teresa Palmer, Penn Badgley, Gena Rowlands, and Frank Langella as three loosely connected couples who must deal with a looming biological attack.

In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement. Commonly used are ppm (parts-per-million, ), ppb (parts-per-billion, ), ppt (parts-per-trillion, ) and ppq (parts-per-quadrillion, ).