The original list, agreed in 2001 (Decision 2455/2001/EC), contained 33 substances and formed part of the Priority Substances Directive (2008/105/EC), which is a Daughter Directive of the Water Framework Directive.
The Priority Substances Directive requires the progressive reduction or phasing out of these substances, referred to as 'priority substances', and those which are thought to pose the greatest threat are further identified as 'priority hazardous substances'.
Directive 2008/105/EC was subsequently amended by Directive 2013/39/EU. Directive 2013/39/EU revised a number of environmental standards for some current substances and added further additional substances to the original list.
Directives 2008/105/EC and 2013/39/EU are transposed in Northern Ireland through The Water Framework Directive (Classification, Priority Substances and Shellfish Waters) Regulations (Northern Ireland) 2015.