What does 2.5mg/kg mean?
Whenever you have a measurement of mass over another mass, such as milligrams per kilogram, (mg/kg) it would mean that you would be dealing with to substances in a mixture. In this case substance A would have a ratio of 2.5 milligrams for every 1 kilogram of substance B. The milligram is 1/1000th of a gram and the kilogram represents 1000 grams.