This way of measuring concentrations can be applied to volumes and weights as well as to numbers of molecules. 

The metric system makes expressing parts per million simple, because 1 million milliliters = 1 kiloliter (volume) and 1 million milligrams = 1 kilogram (mass or weight). 

So if 1 ml of benzene is present in 1,000 liters of water, the concentration of benzene is 1 ppm by volume.  And if 0.25 mg of particulates is found in 1 kg of air, the particulate concentration (reading) is 0.25 ppm by weight.