The phrase water quality refers to a set of physical, biological, and chemical characteristics of water. In the case of rivers, water quality refers to physical variables (e.g., water temperature, the amount of oxygen in water), biological variables (e.g., the number of bacteria in water), and chemical variables (e.g., the mass of sediments, heavy metals, phosphorus, nitrogen, or pesticides in water). Those characteristics determine whether river water is suitable for aquatic life (e.g., fish, amphibians) or human use. Water quality is assessed by collecting water samples and testing them, either in the field or in a laboratory. The phrase water pollution is used when there are changes to water quality that have a negative impact on living organisms or human health.