PPM stands for Parts Per Million, which is a unit of measurement used to express the concentration of a substance in water. It is commonly used in water chemistry to measure the amounts of minerals and gases dissolved in water. For example, TDS (Total Dissolved Solids) is measured in PPM or mg/L (milligrams per liter) of water. A TDS level of 500 PPM is the recommended maximum amount for drinking water according to the EPA. PPM can be measured by running chemical tests or calculated using conversion equations.