Not exact matches
Europium is element number 63 on the Periodic Table, and it's a somewhat hard, silvery metal that reacts with
oxygen and water —
so it's never found in
pure form.
Though very dense, these objects, made of either a mixture of carbon and
oxygen or nearly
pure helium, cool to temperatures
so low that fusion reactions can no longer occur.
Clearly... breathing
pure oxygen for an extended period of time can have unintended side effects...
so it too will eventually be declared a pollutant.