Sulphur hexafluoride (SF₆) is an extremely effective electrical insulant and has very significant advantages over alternative materials. It is non-flammable, a critical requirement in the high-voltage applications for which we use it, and because of its effectiveness, takes up less volume than an equivalent insulating volume of an oil alternative. Equipment filled with SF₆ can leak and requires replacement.

SF₆ is a greenhouse gas. In the UK, in its Guidelines for Company Reporting on Greenhouse Gas Emissions, the Government's Department of Environment, Food and Rural Affairs (DEFRA) defines one tonne as equivalent to approximately 23,900 tonnes of carbon dioxide.

Reported gas loss, as an absolute amount and percentage of inventory, increased to 18.85 tonnes (3.5%) in 2007/08 compared with 15.94 tonnes (3.3%) in 2006/07. The increase was due to several significant leaks in the UK.

Note:gas loss and gas loss as a percentage of inventory restated from originally reported 19.6 tonnes (3.9%) to reflect correction to UK data noted on graphs below.