Parts per million ("ppm") denotes one particle of a given substance for every 999,999 other particles. This is roughly equivalent to one drop of ink in a 150 litre (40 gallon) drum of water, or one second per 280 hours. One part in 106 � a precision of 0.0001%.

A milligram is a thousandth of a thousandth, or a millionth of a kilogram. A milligram is one part per million of a kilogram thus, one part per million (ppm) by mass is the same as one milligram per kilogram. Just as part per million is abbreviated as ppm, a milligram per kilogram has its own symbolic form -- mg/kg, which unlike ppm is unambiguous.

http://en.wikipedia.org/wiki/Parts_per_million