Theta radiation is a toxic by-product of warp reaction. Most space-faring species have learned to harness the radiation, converting it into energy which powers items like replicators, life support and other essential ship functions. The Malon, a species from the Delta Quadrant, hadn't discovered the ability to recycle theta radiation and, as such, had to dump the 6 billion iso-tonnes produced by their civilization on a daily basis.
Theta radiation had the ability to degrade both organic and inorganic matter, as the USS Voyager discovered in its frequent encounters with the Malon in its fifth and sixth year in the Delta Quadrant. (VOY episode: "Night")