The number of milligrams per kilogram is exactly the same as the number of parts per million. In other words, 1 mg/kg = 1 ppm.
According to Engineering Toolbox, parts per million, or ppm, is a term used to measure a small number of particles in air, water or bodily fluids. For example, a value of 10,000 parts per million refers to 1 percent of the solution, as 10,000 is 1 percent of 1 million. Similarly, 10,000 mg/kg x 1 g/1000 mg x 1 kg/1000 g = .01, showing that 10,000 mg/kg also represents 1 percent of the total solution. There is a one-to-one ratio between the number of parts per million and mg/kg.