ppm i.e. parts per million is a unit of concentration.
It is mentioned that the FDA limit of methylmercury in fish is 1 ppm
This means that the allowed level is 1 mg of methyl mercury for every 1 kg of the sample of fish
1 ppm = 1 mg/kg
For a 100 g i.e. 0.1 kg sample of fish the allowed levels of methyl mercury would be = 0.1 kg * 1 mg/1 kg = 0.1 mg
