In this project, we study the disguised pollution of industrial firms in China. We find that sulfur dioxide (SO2) readings increase by 15.9 per cent in monitoring stations four hours after sunset, relative to four hours before, in high factory density areas, controlling for station-day and city-hour fixed effects. We also find significantly increased readings for PM2.5 and PM10 in these areas after sunset. Next, using daily hospital visits data, we document that such disguised pollution behaviour has unforeseen health consequences: cumulative exposure to SO2 above 40 ?g/m3 for three days increases hospital visits on the entire population’s respiratory diseases by 15.6 per cent. Inspections by the Ministry of Environmental Protection can only temporarily reduce pollution and hospital visits, and local economic growth pressure is likely to encourage disguised pollution.