ORP stands for oxidation-reduction potential, which is a measure of a substances ability to either oxidize or reduce another substance. In water, ORP measures the ability of the water to cleanse itself or break down waste products, such as contaminants and dead plants and animals. When the ORP value is high, there is lots of oxygen present in the water, which means that bacteria that decompose dead tissue and contaminants can work more efficiently. ORP is measured in millivolts (mV) using an ORP meter, and a positive reading on an ORP meter means that the substance is an oxidizing agent, while a negative reading indicates that the substance is a reducing agent. ORP is integral to water testing because it indicates how sanitized or contaminated water is based on its oxidation and reduction properties. ORP can be used to track the metallic pollution in groundwater or surface water or to determine the chlorine content of wastewater effluent. In cooling water treatment, ORP can be used as an estimation of oxidizing biocide residuals and activity.