A lost cargo-load of poison was last night threatening to cause widespread pollution after a container ship sank off Tuen Mun. Experts fear the missing container of ammonium chloride could break open and release its contents into an area off Butterfly Bay. The chemical can burn skin and eyes and kill marine life. As the Chinese ship, the Guan Hang, was hauled out of the water yesterday, environmental experts were taking samples from the sea for traces of the chemical. They said that, so far, 'no trace whatsoever of pollution' had been detected. But the container of ammonium chloride had not been found, apparently drifting along with seven others containing rattan and household batteries. Salvagers discovered nine containers still inside the hold as they brought the ship to the surface a kilometre from Tuen Mun ferry pier, supervised by three Marine Department launches. The Guan Hang had been due to unload its cargo in Hong Kong to be shipped on to other ports when it sank on Tuesday night. With the Marine Department investigating the ship's loading and safety practices, the inquiries into whether the Guan Hang had breached any environmental laws would continue, said Environment Protection Department deputy director Mike Stokoe. Butterfly Bay, Golden Beach, Old and New Cafeteria and Kadoorie Beach were closed as a precaution, but many swimmers ignored the warnings. The Environmental Protection Department tested for ammonia, heavy metals such as lead, cadmium and zinc, and alkalinity levels. 'It was all below the unsafe levels and there was nothing abnormal,' said Mr Stokoe. But ecology lecturer Dr Gordon Maxwell, from the Open University of Hong Kong, said that even low concentrations of these chemicals could still prove harmful and cause minor tumours in fish or even mammals such as dolphins. 'At a sub-lethal level they can be hard to detect but can still kill organisms and Hong Kong's waters are already well supplied with heavy metals.' It is the third time since January that chemical containers have been lost in the waters around Hong Kong.