Parts per million (PPM) is a unit of measurement expressing the concentration of one substance within another, indicating how many parts of a substance exist per million parts of the total mixture. It's commonly used in environmental science to measure pollutants in air or water. For example, atmospheric carbon dioxide levels are tracked in PPM (currently over 420 PPM), and drinking water contaminants are regulated using PPM thresholds. One PPM equals one milligram per litre in water or one milligram per kilogram in solids. PPM provides a standardized way to discuss very small but significant concentrations that affect environmental and public health.