Parts per million

Percentage and molarity, and even molality, are convenient units for the solutions that chemists routinely make in the lab or the solutions commonly found in nature. However, if you begin to examine the concentrations of certain pollutants in the environment, you find that those concentrations are very, very small. Percentage and molarity work when you’re measuring solutions in the environment, but they’re not very convenient.

To express the concentrations of very dilute solutions, scientists have developed another concentration unit — parts per million.

Percentage is parts per hundred, or grams solute per 100 grams of solution. Parts per million (ppm) is grams of solute per 1 million grams of solution — or as it’s most commonly expressed, milligrams of solute per kilogram of solution, which is the same ratio. It’s expressed this way because chemists can easily weigh out milligrams or even tenths of milligrams, and if you’re talking about aqueous solutions, a kilogram of solution is the same as a liter of solution. (The density of water is 1 gram per milliliter, or 1 kilogram per liter. The weight of the solute in these solutions is so very small that it’s negligible when converting from the mass of the solution to the volume.)

By law, the maximum contamination level of lead in drinking water is 0.05 ppm. This number corresponds to 0.05 milligrams of lead per liter of water. That’s pretty dilute. But mercury is regulated at the 0.002 ppm level. Sometimes, even this unit isn’t sensitive enough, so environmentalists have resorted to the parts per billion (ppb) or parts per trillion (ppt) concentration units. Some neurotoxins are deadly at the parts per billion level.