Parts per million (ppm) is a common unit of measurement used to express very low concentrations of substances in various mediums. One ppm is equivalent to one part of a substance per one million parts of the medium in which it is present.

For example, if there are 10 ppm of a certain chemical in a solution, it means that there are 10 parts of the chemical for every million parts of the solution. This unit is often used to express the concentration of pollutants in air, water, and soil.

PPM is an important measurement in a wide range of fields, including environmental science, chemistry, and engineering. In environmental science, it is used to measure the concentration of pollutants in air and water. In chemistry, it is used to express the purity of a substance, while in engineering it is used to describe the precision of manufacturing processes.

To calculate ppm, the amount of the substance being measured is divided by the total amount of the medium and multiplied by 1 million. For example, if you have a solution that contains 50 milligrams of a certain chemical in 1 liter of water, the concentration of the chemical in ppm can be calculated as follows:

(50 mg / 1 L) x 1,000,000 = 50,000 ppm

PPM is a small unit of measurement, and concentrations in ppm are often expressed in scientific notation or with a prefix, such as micrograms per liter (μg/L) or milligrams per cubic meter (mg/m³).

It is important to note that ppm is a relative measurement and not an absolute measurement. The accuracy of the measurement is dependent on the accuracy of the instruments used to measure both the substance being analyzed and the medium in which it is present.

In summary, ppm is a useful unit of measurement for expressing very low concentrations of substances in various mediums. It is widely used in environmental science, chemistry, and engineering, and is calculated by dividing the amount of the substance being measured by the total amount of the medium and multiplying by 1 million.