An environmental quality standard (EQS) is defined under the Water Framework Direct (WFD) as "the concentration of a particular pollutant or group of pollutants in water, sediment, or biota which should not be exceeded in order to protect human health and the environment.” While most EQS are derived as limits in the ambient environment, they can also be translated into controls at the point of emission, i.e., discharge permits. EQS are either set to protect against adverse effects after long-term (chronic) exposure (Annual Average EQS), or short-term (acute) intermittent chemical exposures [Maximum Admissible (or Allowable) Concentrations]. In the European Union (EU), Annual Average (AA) EQS have a range of uses including WFD surface water classification (compliance assessment) and permitting of industrial discharges (see section What Does This Mean and How Should this EQS be Used). Maximum Admissible Concentrations (MAC) are generally used to investigate pollution incidents or intermittent discharges of chemicals and are not routinely used by Member States. The focus of this fact sheet is the nickel AA EQS. Where the term EQS is used, it refers to the freshwater AA EQS. The EQS is derived from relevant toxicity data and should be protective of all relevant human health and environmental endpoints across all surface waters in the EU.