Chenium was a highly toxic and corrosive chemical. It was a byproduct of the manufacture of Qualon 37.
In 2050, Inspector Drake's attempts to replicate Mede technology made chenium leak into the River Thames, polluting it. When Starkey and K9 discovered fish which had died from the pollution, they took one to Gryffen Manor and analysed it. K9 (who was damaged by chenium exposure), Jorjie and Darius traced the source of the pollution, but it was June Turner who put a stop to the experiments. After she freed the Medes, they cleaned up the pollution before leaving. (TV: Alien Avatar)