Two mainland workers died last night and another was fighting for his life after they were overcome by deadly fumes inside a floating dock at a Yau Tong shipyard. On another black day for industrial safety in Hong Kong, the dead men were among four killed at work yesterday. A 19-year-old engineer fell four floors to his death as he installed an air conditioner in Tai Po, while a 47-year-old man died in San Po Kong after plunging five floors when an excavator hit scaffolding on which he was working. Last night's tragedy occurred when Cheung Ying-kwong, 38, entered the dock at Universal Shipyard Ltd to check whether it was leaking. When he failed to come out, his colleagues Sze Cho-kui, 44, and Cheung Shu-yiu, 36, went to investigate. A fourth worker, Wu Wing-wah, 44, alerted police after being overcome by fumes as he also went in search of the missing men. Firemen donned breathing apparatus during the rescue, but Cheung Ying-kwong and Sze were dead on arrival at United Christian Hospital. Cheung Shu-yiu was in critical condition early this morning, while Mr Wu was reported as 'fair'. Firemen took air samples from inside the dock as they tried to determine what overpowered the men. The Acting Commissioner for Labour, Alfred Chan Wing-kit, visited the shipyard in Ko Fai Road and was briefed on the tragedy. He promised a full investigation. The Labour Department is already inquiring into the deaths of six men last week on the airport railway. A platform being used on the Rambler Channel Bridge project plunged 20 metres, killing four men working on it and two on the ground.