Mercury is a naturally-occurring element which is present in various ores. The major mined source in Europe has been in Spain (Almadén). Mercury has been used for 100 years in electrolytic chlorine production, is gradually being phased out in favour of membrane technology.

Voluntary conversion of all mercury cells was to be expected to have been completed by 2020, but recent (2014) European regulation now stipulates that the use of mercury based technology must be phased out before December 2017.