When a deadly explosion destroyed BP’s Deepwater Horizon drilling rig in the Gulf of Mexico, 134 million gallons of crude erupted into the sea over the next three months — and tens of thousands of ordinary people were hired to help clean up environmental devastation from the biggest offshore oil spill in U.S. history.These workers were exposed to crude oil and the chemical dispersant Corexit while picking up tar balls along the shoreline, laying booms from fishing boats to soak up slicks and rescuing oil …