You can't tell from Mr Wu's appearance, but the 34-year-old is gravely ill. He is suffering from silicosis, a preventable lung disease for which there is no cure and which authorities estimate affects more than 600,000 mainland workers. It is caused by the long-term inhalation of tiny particles and characterised by progressive fibrosis and a chronic shortness of breath. Without proper treatment it can deteriorate rapidly, ending in death.