PM2.5 is a measurement of small particulate matter in the air, and until recently its omission from official air quality readings has been a major hurdle in solving China's air pollution crisis. Lung cancer is now the most common cancer in China, and heart disease is the country's second-biggest cause of death. Smog hangs heavy over Beijing, Shanghai and Hong Kong, where children grow up with asthma and other respiratory illnesses. Every year, Hong Kong loses over HK$2 billion due to health costs and lost productivity.