If it's anything like some radioactive material then it will degrade over a huge amount of years.... or if it's like any typical poison it will never degrade!
The couple, believed to be Charlie Rowley and Dawn Sturgess, are in a critical condition having been found unconscious at a house on Saturday.
Police say no on else has presented with the same symptoms.
There was "nothing in their background" to suggest the pair were targeted, say the Met Police."