This Safety Alert serves as a reminder to Coast Guard (CG) personnel and the maritime community of potential dangers during sampling of cargo tanks. During a recent Port State Control (PSC) tank vessel examination, a CG member was exposed to a dangerous concentration of Hydrogen Sulfide (H2S) gas and suffered a serious injury. The exposure occurred during a tank vessel exam onboard a tank vessel carrying Grade E Sour Crude. A PSC team requested a ship's crew member check cargo tank oxygen levels using the ship’s portable gas meter.