Core laborer

From Memory Alpha, the free Star Trek reference.

Core laborers were Malon workers aboard waste export vessels who worked very close to the theta storage tanks. As a result of their job hazards, they were exposed to the most theta radiation, among the crew. This high exposure meant that only three out of every ten core laborers survived a typical six month long export mission. Core laborers, however, were very highly paid for the risks they took, usually earning more than what most Malon earned in a lifetime, the money usually went to their families if they did not survive.

Very rarely, a core laborer would survive severe poisoning, and mutated to become resistant to theta radiation. In the few cases where this happened, the mutated crewmember became unstable and violent, leading to the development of the Vihaar myth. (VOY: "Juggernaut")